33 SWIG and Python

This chapter describes SWIG's support of Python. SWIG is compatible with all recent Python versions (Python 2.7 and Python >= 3.5). Python 3.3 and 3.4 may still work but we are no longer regularly testing with them. SWIG 4.0.x supported Python 3.2. SWIG 3.0.x supported older Python 2.x and 3.x.

This chapter covers most SWIG features, but certain low-level details are covered in less depth than in earlier chapters. At the very least, make sure you read the "SWIG Basics" chapter.

33.1 Overview

To build Python extension modules, SWIG uses a layered approach in which parts of the extension module are defined in C and other parts are defined in Python. The C layer contains low-level wrappers whereas Python code is used to define high-level features.

This layered approach recognizes the fact that certain aspects of extension building are better accomplished in each language (instead of trying to do everything in C or C++). Furthermore, by generating code in both languages, you get a lot more flexibility since you can enhance the extension module with support code in either language.

In describing the Python interface, this chapter starts by covering the basics of configuration, compiling, and installing Python modules. Next, the Python interface to common C and C++ programming features is described. Advanced customization features such as typemaps are then described followed by a discussion of low-level implementation details.

33.2 Preliminaries

33.2.1 Running SWIG

Suppose that you defined a SWIG module such as the following:

/* File: example.i */
%module example

%{
#include "example.h"
%}

int fact(int n);

This .i file wraps the following simple C file:

/* File: example.c */

#include "example.h"

int fact(int n) {
  if (n < 0) { /* This should probably return an error, but this is simpler */
    return 0;
  }
  if (n == 0) {
    return 1;
  } else {
    /* testing for overflow would be a good idea here */
    return n * fact(n-1);
  }
}

With the header file:

/* File: example.h */

int fact(int n);

To build a Python module, run SWIG using the -python option:

$ swig -python example.i

If building a C++ extension, add the -c++ option:

$ swig -c++ -python example.i

This creates two different files; a C/C++ source file example_wrap.c or example_wrap.cxx and a Python source file example.py. The generated C source file contains the low-level wrappers that need to be compiled and linked with the rest of your C/C++ application to create an extension module. The Python source file contains high-level support code. This is the file that you will import to use the module.

The name of the wrapper file is derived from the name of the input file. For example, if the input file is example.i, the name of the wrapper file is example_wrap.c. To change this, you can use the -o option. The name of the Python file is derived from the module name specified with %module. If the module name is example, then a file example.py is created.

The following sections have further practical examples and details on how you might go about compiling and using the generated files.

33.2.2 Using distutils

The preferred approach to building an extension module for Python is to compile it with distutils, which comes with all recent versions of Python (Distutils Docs).

Distutils takes care of making sure that your extension is built with all the correct flags, headers, etc. for the version of Python it is run with. Distutils will compile your extension into a shared object file or DLL (.so on Linux, .pyd on Windows, etc). In addition, distutils can handle installing your package into site-packages, if that is desired. A configuration file (conventionally called: setup.py) describes the extension (and related Python modules). The distutils will then generate all the right compiler directives to build it for you.

Here is a sample setup.py file for the above example:

#!/usr/bin/env python3

"""
setup.py file for SWIG example
"""

from distutils.core import setup, Extension


example_module = Extension('_example',
                           sources=['example_wrap.c', 'example.c'],
                           )

setup (name = 'example',
       version = '0.1',
       author      = "SWIG Docs",
       description = """Simple swig example from docs""",
       ext_modules = [example_module],
       py_modules = ["example"],
       )

In this example, the line: example_module = Extension(....) creates an Extension module object, defining the name as _example, and using the source code files: example_wrap.c, generated by swig, and example.c, your original c source. The swig (and other Python extension modules) tradition is for the compiled extension to have the name of the Python portion, prefixed by an underscore. If the name of your Python module is "example.py", then the name of the corresponding object file will be"_example.so"

The setup call then sets up distutils to build your package, defining some meta data, and passing in your extension module object. Once this is saved as setup.py, you can build your extension with these commands:

$ swig -python example.i
$ python setup.py build_ext --inplace

And a .so, or .pyd or... will be created for you. It will build a version that matches the Python that you run the command with. Taking apart the command line:

• python -- the version of Python you want to build for
• setup.py -- the name of your setup script (it can be called anything, but setup.py is the tradition)
• build_ext -- telling distutils to build extensions
• --inplace -- this tells distutils to put the extension lib in the current dir. Otherwise, it will put it inside a build hierarchy, and you'd have to move it to use it.

The distutils have many other features, consult the Python distutils docs for details.

This same approach works on all platforms if the appropriate compiler is installed. (it can even build extensions to the standard Windows Python using MingGW)

33.2.3 Hand compiling a dynamic module

While the preferred approach to building an extension module is to use the distutils, some people like to integrate building extensions with a larger build system, and thus may wish to compile their modules without the distutils. To do this, you need to compile your program using commands like this (shown for Linux):

$ swig -python example.i
$ gcc -O2 -fPIC -c example.c
$ gcc -O2 -fPIC -c example_wrap.c -I/usr/local/include/python2.5
$ gcc -shared example.o example_wrap.o -o _example.so

The exact commands for doing this vary from platform to platform. However, SWIG tries to guess the right options when it is installed. Therefore, you may want to start with one of the examples in the SWIG/Examples/python directory. If that doesn't work, you will need to read the man-pages for your compiler and linker to get the right set of options. You might also check the SWIG Wiki for additional information.

When linking the module, the name of the output file has to match the name of the module prefixed by an underscore. If the name of your module is "example", then the name of the corresponding object file should be "_example.so" or "_examplemodule.so". The name of the module is specified using the %module directive or the -module command line option.

Compatibility Note: In SWIG-1.3.13 and earlier releases, module names did not include the leading underscore. This is because modules were normally created as C-only extensions without the extra Python support file (instead, creating Python code was supported as an optional feature). This has been changed in SWIG-1.3.14 and is consistent with other Python extension modules. For example, the socket module actually consists of two files; socket.py and _socket.so. Many other built-in Python modules follow a similar convention.

33.2.4 Static linking

An alternative approach to dynamic linking is to rebuild the Python interpreter with your extension module added to it. In the past, this approach was sometimes necessary due to limitations in dynamic loading support on certain machines. However, the situation has improved greatly over the last few years and you should not consider this approach unless there is really no other option.

The usual procedure for adding a new module to Python involves finding the Python source, adding an entry to the Modules/Setup file, and rebuilding the interpreter using the Python Makefile. However, newer Python versions have changed the build process. You may need to edit the 'setup.py' file in the Python distribution instead.

In earlier versions of SWIG, the embed.i library file could be used to rebuild the interpreter. For example:

%module example

%inline %{
extern int fact(int);
extern int mod(int, int);
extern double My_variable;
%}

%include "embed.i"       // Include code for a static version of Python

The embed.i library file includes supporting code that contains everything needed to rebuild Python. To rebuild the interpreter, you simply do something like this:

$ swig -python -lembed.i example.i
$ gcc example.c example_wrap.c \
        -Xlinker -export-dynamic \
        -DHAVE_CONFIG_H -I/usr/include/python2.7 \
        -I/usr/lib/python2.7/config-x86_64-linux-gnu \
        -I/usr/lib/python2.7/config \
        -L/usr/lib/python2.7/config -lpython2.7 -lm -ldl \
        -o mypython

You will need to supply the same libraries that were used to build Python the first time. This may include system libraries such as -lsocket, -lnsl, and -lpthread. Assuming this actually works, the new version of Python should be identical to the default version except that your extension module will be a built-in part of the interpreter.

Comment: In practice, you should probably try to avoid static linking if possible. Some programmers may be inclined to use static linking in the interest of getting better performance. However, the performance gained by static linking tends to be rather minimal in most situations (and quite frankly not worth the extra hassle in the opinion of this author).

Compatibility note: The embed.i library file is deprecated and has not been actively maintained for many years. Even though it appears to "work" with Python 2.7, no future support is guaranteed. If using static linking, you might want to rely on a different approach (perhaps using distutils).

33.2.5 Using your module

To use your module, simply use the Python import statement. If all goes well, you will be able to run this:

$ python
>>> import example
>>> example.fact(4)
24
>>>

A common error received by first-time users is the following:

>>> import example
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "example.py", line 2, in ?
    import _example
ImportError: No module named _example

If you get this message, it means that you either forgot to compile the wrapper code into an extension module or you didn't give the extension module the right name. Make sure that you compiled the wrappers into a module called _example.so. And don't forget the leading underscore (_).

Another possible error is the following:

>>> import example
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ImportError: dynamic module does not define init function (init_example)
>>>

This error is almost always caused when a bad name is given to the shared object file. For example, if you created a file example.so instead of _example.so you would get this error. Alternatively, this error could arise if the name of the module is inconsistent with the module name supplied with the %module directive. Double-check the interface to make sure the module name and the shared object filename match. Another possible cause of this error is forgetting to link the SWIG-generated wrapper code with the rest of your application when creating the extension module.

Another common error is something similar to the following:

Traceback (most recent call last):
  File "example.py", line 3, in ?
    import example
ImportError: ./_example.so: undefined symbol: fact

This error usually indicates that you forgot to include some object files or libraries in the linking of the shared library file. Make sure you compile both the SWIG wrapper file and your original program into a shared library file. Make sure you pass all of the required libraries to the linker.

Sometimes unresolved symbols occur because a wrapper has been created for a function that doesn't actually exist in a library. This usually occurs when a header file includes a declaration for a function that was never actually implemented or it was removed from a library without updating the header file. To fix this, you can either edit the SWIG input file to remove the offending declaration or you can use the %ignore directive to ignore the declaration.

Finally, suppose that your extension module is linked with another library like this:

$ gcc -shared example.o example_wrap.o -L/home/beazley/projects/lib -lfoo \
      -o _example.so

If the foo library is compiled as a shared library, you might encounter the following problem when you try to use your module:

>>> import example
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ImportError: libfoo.so: cannot open shared object file: No such file or directory
>>>

This error is generated because the dynamic linker can't locate the libfoo.so library. When shared libraries are loaded, the system normally only checks a few standard locations such as /usr/lib and /usr/local/lib. To fix this problem, there are several things you can do. First, you can recompile your extension module with extra path information. For example, on Linux you can do this:

$ gcc -shared example.o example_wrap.o -L/home/beazley/projects/lib -lfoo \
      -Xlinker -rpath /home/beazley/projects/lib  \
      -o _example.so

Alternatively, you can set the LD_LIBRARY_PATH environment variable to include the directory with your shared libraries. If setting LD_LIBRARY_PATH, be aware that setting this variable can introduce a noticeable performance impact on all other applications that you run. To set it only for Python, you might want to do this instead:

$ env LD_LIBRARY_PATH=/home/beazley/projects/lib python

Finally, you can use a command such as ldconfig (Linux) or crle (Solaris) to add additional search paths to the default system configuration (this requires root access and you will need to read the man pages).

33.2.6 Compilation of C++ extensions

Compilation of C++ extensions has traditionally been a tricky problem. Since the Python interpreter is written in C, you need to take steps to make sure C++ is properly initialized and that modules are compiled correctly. This should be a non-issue if you're using distutils, as it takes care of all that for you. The following is included for historical reasons, and in case you need to compile on your own.

On most machines, C++ extension modules should be linked using the C++ compiler. For example:

$ swig -c++ -python example.i
$ g++ -O2 -fPIC -c example.cxx
$ g++ -O2 -fPIC -c example_wrap.cxx -I/usr/local/include/python2.5
$ g++ -shared example.o example_wrap.o -o _example.so

The -fPIC option tells GCC to generate position-independent code (PIC) which is required for most architectures (it's not vital on x86, but still a good idea as it allows code pages from the library to be shared between processes). Other compilers may need a different option specified instead of -fPIC.

In addition to this, you may need to include additional library files to make it work. For example, if you are using the Sun C++ compiler on Solaris, you often need to add an extra library -lCrun like this:

$ swig -c++ -python example.i
$ CC -c example.cxx
$ CC -c example_wrap.cxx -I/usr/local/include/python2.5
$ CC -G example.o example_wrap.o -L/opt/SUNWspro/lib -o _example.so -lCrun

Of course, the extra libraries to use are completely non-portable---you will probably need to do some experimentation.

Sometimes people have suggested that it is necessary to relink the Python interpreter using the C++ compiler to make C++ extension modules work. In the experience of this author, this has never actually appeared to be necessary. Relinking the interpreter with C++ really only includes the special run-time libraries described above---as long as you link your extension modules with these libraries, it should not be necessary to rebuild Python.

If you aren't entirely sure about the linking of a C++ extension, you might look at an existing C++ program. On many Unix machines, the ldd command will list library dependencies. This should give you some clues about what you might have to include when you link your extension module. For example:

$ ldd swig
        libstdc++-libc6.1-1.so.2 => /usr/lib/libstdc++-libc6.1-1.so.2 (0x40019000)
        libm.so.6 => /lib/libm.so.6 (0x4005b000)
        libc.so.6 => /lib/libc.so.6 (0x40077000)
        /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)

As a final complication, a major weakness of C++ is that it does not define any sort of standard for binary linking of libraries. This means that C++ code compiled by different compilers will not link together properly as libraries nor is the memory layout of classes and data structures implemented in any kind of portable manner. In a monolithic C++ program, this problem may be unnoticed. However, in Python, it is possible for different extension modules to be compiled with different C++ compilers. As long as these modules are self-contained, this probably won't matter. However, if these modules start sharing data, you will need to take steps to avoid segmentation faults and other erratic program behavior. If working with lots of software components, you might want to investigate using a more formal standard such as COM.

33.2.7 Compiling for 64-bit platforms

On platforms that support 64-bit applications (Solaris, Irix, etc.), special care is required when building extension modules. On these machines, 64-bit applications are compiled and linked using a different set of compiler/linker options. In addition, it is not generally possible to mix 32-bit and 64-bit code together in the same application.

To utilize 64-bits, the Python executable will need to be recompiled as a 64-bit application. In addition, all libraries, wrapper code, and every other part of your application will need to be compiled for 64-bits. If you plan to use other third-party extension modules, they will also have to be recompiled as 64-bit extensions.

If you are wrapping commercial software for which you have no source code, you will be forced to use the same linking standard as used by that software. This may prevent the use of 64-bit extensions. It may also introduce problems on platforms that support more than one linking standard (e.g., -o32 and -n32 on Irix).

On the Linux x86_64 platform (Opteron or EM64T), besides of the required compiler option -fPIC discussed above, you will need to be careful about the libraries you link with or the library path you use. In general, a Linux distribution will have two set of libraries, one for native x86_64 programs (under /usr/lib64), and another for 32 bits compatibility (under /usr/lib). Also, the compiler options -m32 and -m64 allow you to choose the desired binary format for your Python extension.

33.2.8 Building Python extensions under Windows

Building a SWIG extension to Python under Windows is roughly similar to the process used with Unix. Using the distutils, it is essentially identical. If you have the same version of the MS compiler that Python was built with (the python2.4 and python2.5 distributed by python.org are built with Visual Studio 2003), the standard python setup.py build should just work.

As of python2.5, the distutils support building extensions with MingGW out of the box. Following the instruction here: Building Python extensions for Windows with only free tools should get you started.

If you need to build it on your own, the following notes are provided:

You will need to create a DLL that can be loaded into the interpreter. This section briefly describes the use of SWIG with Microsoft Visual C++. As a starting point, many of SWIG's examples include project files (.dsp files) for Visual C++ 6. These can be opened by more recent versions of Visual Studio. You might want to take a quick look at these examples in addition to reading this section.

In Developer Studio, SWIG should be invoked as a custom build option. This is usually done as follows:

• Open up a new workspace and use the AppWizard to select a DLL project.
• Add both the SWIG interface file (the .i file), any supporting C files, and the name of the wrapper file that will be created by SWIG (ie. example_wrap.c). Note : If using C++, choose a different suffix for the wrapper file such as example_wrap.cxx. Don't worry if the wrapper file doesn't exist yet--Developer Studio keeps a reference to it.
• Select the SWIG interface file and go to the settings menu. Under settings, select the "Custom Build" option.
• Enter "SWIG" in the description field.
• Enter "swig -python -o $(ProjDir)\$(InputName)_wrap.c $(InputPath)" in the "Build command(s) field"
• Enter "$(ProjDir)\$(InputName)_wrap.c" in the "Output files(s) field".
• Next, select the settings for the entire project and go to "C++:Preprocessor". Add the include directories for your Python installation under "Additional include directories".
• Define the symbol __WIN32__ under preprocessor options.
• Finally, select the settings for the entire project and go to "Link Options". Add the Python library file to your link libraries. For example "python27.lib". Also, set the name of the output file to match the name of your Python module, i.e. _example.pyd
• Build your project.

If all went well, SWIG will be automatically invoked whenever you build your project. Any changes made to the interface file will result in SWIG being automatically executed to produce a new version of the wrapper file.

To run your new Python extension, simply run Python and use the import command as normal. For example :

$ python
>>> import example
>>> print(example.fact(4))
24
>>>

If you get an ImportError exception when importing the module, you may have forgotten to include additional library files when you built your module. If you get an access violation or some kind of general protection fault immediately upon import, you have a more serious problem. This is often caused by linking your extension module against the wrong set of Win32 debug or thread libraries. You will have to fiddle around with the build options of project to try and track this down.

A 'Debug' build of the wrappers requires a debug build of the Python interpreter. This normally requires building the Python interpreter from source, which is not a job for the feint-hearted. Alternatively you can use the 'Release' build of the Python interpreter with a 'Debug' build of your wrappers by defining the SWIG_PYTHON_INTERPRETER_NO_DEBUG symbol under the preprocessor options. Or you can ensure this macro is defined at the beginning of the wrapper code using the following in your interface file, where _MSC_VER ensures it is only used by the Visual Studio compiler:

%begin %{
#ifdef _MSC_VER
#define SWIG_PYTHON_INTERPRETER_NO_DEBUG
#endif
%}

Some users have reported success in building extension modules using Cygwin and other compilers. However, the problem of building usable DLLs with these compilers tends to be rather problematic. For the latest information, you may want to consult the SWIG Wiki.

33.2.9 Additional Python commandline options

The following table lists the additional commandline options available for the Python module. They can also be seen by using:

swig -python -help

Many of these options are covered later on and their use should become clearer by the time you have finished reading this section on SWIG and Python.

33.3 A tour of basic C/C++ wrapping

By default, SWIG tries to build a very natural Python interface to your C/C++ code. Functions are wrapped as functions, classes are wrapped as classes, and so forth. This section briefly covers the essential aspects of this wrapping.

33.3.1 Modules

The SWIG %module directive specifies the name of the Python module. If you specify `%module example', then everything is wrapped into a Python 'example' module. Underneath the covers, this module consists of a Python source file example.py and a low-level extension module _example.so. When choosing a module name, make sure you don't use the same name as a built-in Python command or standard module name.

33.3.2 Functions

Global functions are wrapped as new Python built-in functions. For example,

%module example
int fact(int n);

creates a built-in function example.fact(n) that works exactly like you think it does:

>>> import example
>>> print(example.fact(4))
24
>>>

33.3.3 Global variables

C/C++ global variables are fully supported by SWIG. However, the underlying mechanism is somewhat different than you might expect due to the way that Python assignment works. When you type the following in Python

a = 3.4

"a" becomes a name for an object containing the value 3.4. If you later type

b = a

then "a" and "b" are both names for the object containing the value 3.4. Thus, there is only one object containing 3.4 and "a" and "b" are both names that refer to it. This is quite different than C where a variable name refers to a memory location in which a value is stored (and assignment copies data into that location). Because of this, there is no direct way to map variable assignment in C to variable assignment in Python.

To provide access to C global variables, SWIG creates a special object called `cvar' that is added to each SWIG generated module. Global variables are then accessed as attributes of this object. For example, consider this interface

// SWIG interface file with global variables
%module example
...
%inline %{
extern int My_variable;
extern double density;
%}
...

Now look at the Python interface:

>>> import example
>>> # Print out value of a C global variable
>>> print(example.cvar.My_variable)
4
>>> # Set the value of a C global variable
>>> example.cvar.density = 0.8442
>>> # Use in a math operation
>>> example.cvar.density = example.cvar.density*1.10

If you make an error in variable assignment, you will receive an error message. For example:

>>> example.cvar.density = "Hello"
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: C variable 'density (double )'
>>>

If a variable is declared as const, it is wrapped as a read-only variable. Attempts to modify its value will result in an error.

To make ordinary variables read-only, you can use the %immutable directive. For example:

%{
extern char *path;
%}
%immutable;
extern char *path;
%mutable;

The %immutable directive stays in effect until it is explicitly disabled or cleared using %mutable. See the Creating read-only variables section for further details.

If you just want to make a specific variable immutable, supply a declaration name. For example:

%{
extern char *path;
%}
%immutable path;
...
extern char *path;      // Read-only (due to %immutable)

If you would like to access variables using a name other than "cvar", it can be changed using the -globals option :

$ swig -python -globals myvar example.i

Some care is in order when importing multiple SWIG modules. If you use the "from <file> import *" style of importing, you will get a name clash on the variable `cvar' and you will only be able to access global variables from the last module loaded. To prevent this, you might consider renaming cvar or making it private to the module by giving it a name that starts with a leading underscore. SWIG does not create cvar if there are no global variables in a module.

33.3.4 Constants and enums

C/C++ constants are installed as Python objects containing the appropriate value. To create a constant, use #define, enum, or the %constant directive. For example:

#define PI 3.14159
#define VERSION "1.0"

enum Beverage { ALE, LAGER, STOUT, PILSNER };

%constant int FOO = 42;
%constant const char *path = "/usr/local";

For enums, make sure that the definition of the enumeration actually appears in a header file or in the wrapper file somehow---if you just stick an enum in a SWIG interface without also telling the C compiler about it, the wrapper code won't compile.

Note: declarations declared as const are wrapped as read-only variables and will be accessed using the cvar object described in the previous section. They are not wrapped as constants. For further discussion about this, see the SWIG Basics chapter.

Constants are not guaranteed to remain constant in Python---the name of the constant could be accidentally reassigned to refer to some other object. Unfortunately, there is no easy way for SWIG to generate code that prevents this. You will just have to be careful.

33.3.5 Pointers

C/C++ pointers are fully supported by SWIG. Furthermore, SWIG has no problem working with incomplete type information. Here is a rather simple interface:

%module example

FILE *fopen(const char *filename, const char *mode);
int fputs(const char *, FILE *);
int fclose(FILE *);

When wrapped, you will be able to use the functions in a natural way from Python. For example:

>>> import example
>>> f = example.fopen("junk", "w")
>>> example.fputs("Hello World\n", f)
>>> example.fclose(f)

If this makes you uneasy, rest assured that there is no deep magic involved. Underneath the covers, pointers to C/C++ objects are simply represented as opaque values using an especial Python container object:

>>> print(f)
<Swig Object of type 'FILE *' at 0xb7d6f470>

This pointer value can be freely passed around to different C functions that expect to receive an object of type FILE *. The only thing you can't do is dereference the pointer from Python. Of course, that isn't much of a concern in this example.

In older versions of SWIG (1.3.22 or older), pointers were represented using a plain string object. If you have an old package that still requires that representation, or you just feel nostalgic, you can always retrieve it by casting the pointer object to a string:

>>> print(str(f))
_c0671108_p_FILE

Also, if you need to pass the raw pointer value to some external Python library, you can do it by casting the pointer object to an integer:

>>> print(int(f))
135833352

However, the inverse operation is not possible, i.e., you can't build a SWIG pointer object from a raw integer value.

Note also that the '0' or NULL pointer is always represented by None, no matter what type swig is addressing. In the previous example, you can call:

>>> example.fclose(None)

and that will be equivalent to the following, but not really useful, C code:

FILE *f = NULL;
fclose(f);

As much as you might be inclined to modify a pointer value directly from Python, don't. The hexadecimal encoding is not necessarily the same as the logical memory address of the underlying object. Instead it is the raw byte encoding of the pointer value. The encoding will vary depending on the native byte-ordering of the platform (i.e., big-endian vs. little-endian). Similarly, don't try to manually cast a pointer to a new type by simply replacing the type-string. This may not work like you expect, it is particularly dangerous when casting C++ objects. If you need to cast a pointer or change its value, consider writing some helper functions instead. For example:

%inline %{
/* C-style cast */
Bar *FooToBar(Foo *f) {
  return (Bar *) f;
}

/* C++-style cast */
Foo *BarToFoo(Bar *b) {
  return dynamic_cast<Foo*>(b);
}

Foo *IncrFoo(Foo *f, int i) {
  return f+i;
}
%}

Also, if working with C++, you should always try to use the new C++ style casts. For example, in the above code, the C-style cast may return a bogus result whereas as the C++-style cast will return None if the conversion can't be performed.

33.3.6 Structures

If you wrap a C structure, it is wrapped by a Python class. This provides a very natural interface. For example,

struct Vector {
  double x, y, z;
};

is used as follows:

>>> v = example.Vector()
>>> v.x = 3.5
>>> v.y = 7.2
>>> print(v.x, v.y, v.z)
3.5 7.2 0.0
>>>

Similar access is provided for unions and the data members of C++ classes.

If you print out the value of v in the above example, you will see something like this:

>>> print(v)
<C Vector instance at _18e31408_p_Vector>

This object is actually a Python instance that has been wrapped around a pointer to the low-level C structure. This instance doesn't actually do anything--it just serves as a proxy. The pointer to the C object can be found in the .this attribute. For example:

>>> print(v.this)
_18e31408_p_Vector
>>>

Further details about the Python proxy class are covered a little later.

const members of a structure are read-only. Data members can also be forced to be read-only using the %immutable directive. For example:

struct Foo {
  ...
  %immutable;
  int x;        /* Read-only members */
  char *name;
  %mutable;
  ...
};

When char * members of a structure are wrapped, the contents are assumed to be dynamically allocated using malloc or new (depending on whether or not SWIG is run with the -c++ option). When the structure member is set, the old contents will be released and a new value created. If this is not the behavior you want, you will have to use a typemap (described later).

If a structure contains arrays, access to those arrays is managed through pointers. For example, consider this:

struct Bar {
  int  x[16];
};

If accessed in Python, you will see behavior like this:

>>> b = example.Bar()
>>> print(b.x)
_801861a4_p_int
>>>

This pointer can be passed around to functions that expect to receive an int * (just like C). You can also set the value of an array member using another pointer. For example:

>>> c = example.Bar()
>>> c.x = b.x             # Copy contents of b.x to c.x

For array assignment, SWIG copies the entire contents of the array starting with the data pointed to by b.x. In this example, 16 integers would be copied. Like C, SWIG makes no assumptions about bounds checking---if you pass a bad pointer, you may get a segmentation fault or access violation.

When a member of a structure is itself a structure, it is handled as a pointer. For example, suppose you have two structures like this:

struct Foo {
  int a;
};

struct Bar {
  Foo f;
};

Now, suppose that you access the f attribute of Bar like this:

>>> b = Bar()
>>> x = b.f

In this case, x is a pointer that points to the Foo that is inside b. This is the same value as generated by this C code:

Bar b;
Foo *x = &b->f;       /* Points inside b */

Because the pointer points inside the structure, you can modify the contents and everything works just like you would expect. For example:

>>> b = Bar()
>>> b.f.a = 3               # Modify attribute of structure member
>>> x = b.f
>>> x.a = 3                 # Modifies the same structure

Note that there is a limitation with structs within structs that will cause a problem if the outer struct is not a named variable in Python. The following will cause a segfault:

Bar().f.a = 3

because the unnamed Python proxy class for Bar() has its reference count decremented by the Python interpreter after f has been obtained from it and before f is used to obtain a. This results in the underlying Bar instance being deleted, which of course also deletes f inside it. Hence the pointer to f points to deleted memory and use of it results in a segfault or some sort of other undefined behaviour.

33.3.7 C++ classes

C++ classes are wrapped by Python classes as well. For example, if you have this class,

class List {
public:
  List();
  ~List();
  int  search(char *item);
  void insert(char *item);
  void remove(char *item);
  char *get(int n);
  int  length;
};

you can use it in Python like this:

>>> l = example.List()
>>> l.insert("Ale")
>>> l.insert("Stout")
>>> l.insert("Lager")
>>> l.get(1)
'Stout'
>>> print(l.length)
3
>>>

Class data members are accessed in the same manner as C structures.

Static class members present a special problem for Python. Prior to Python-2.2, Python classes had no support for static methods and no version of Python supports static member variables in a manner that SWIG can utilize. Therefore, SWIG generates wrappers that try to work around some of these issues. To illustrate, suppose you have a class like this:

class Spam {
public:
  static void foo();
  static int bar;
};

In Python, the static member can be accessed in three different ways:

>>> s = example.Spam()
>>> s.foo()               # Spam::foo() via an instance
>>> example.Spam.foo()    # Spam::foo() using class method
>>> example.Spam_foo()    # Spam::foo() "flattened" name

The last technique is only available when using the -flatstaticmethod option. This option is not recommended, it is only available for backwards compatibility as ancient versions of Python did not have Python class methods.

Static member variables are currently accessed as global variables. This means, they are accessed through cvar or via an instance property:

>>> example.cvar.Spam_bar # Spam::bar
7
>>> s = example.Spam()
>>> s.bar                 # Spam::bar via an instance property
7

The -builtin option uses a metaclass to additionally provide access as follows:

>>> example.Spam.bar      # Spam::bar using -builtin option only
7

33.3.8 C++ inheritance

SWIG is fully aware of issues related to C++ inheritance. Therefore, if you have classes like this

class Foo {
...
};

class Bar : public Foo {
...
};

those classes are wrapped into a hierarchy of Python classes that reflect the same inheritance structure. All of the usual Python utility functions work normally:

>>> b = Bar()
>>> instance(b, Foo)
1
>>> issubclass(Bar, Foo)
1
>>> issubclass(Foo, Bar)
0

Furthermore, if you have functions like this

void spam(Foo *f);

then the function spam() accepts Foo * or a pointer to any class derived from Foo.

It is safe to use multiple inheritance with SWIG.

33.3.9 Pointers, references, values, and arrays

In C++, there are many different ways a function might receive and manipulate objects. For example:

void spam1(Foo *x);      // Pass by pointer
void spam2(Foo &x);      // Pass by reference
void spam3(const Foo &x);// Pass by const reference
void spam4(Foo x);       // Pass by value
void spam5(Foo x[]);     // Array of objects

In Python, there is no detailed distinction like this--specifically, there are only "objects". There are no pointers, references, arrays, and so forth. Because of this, SWIG unifies all of these types together in the wrapper code. For instance, if you actually had the above functions, it is perfectly legal to do this:

>>> f = Foo()           # Create a Foo
>>> spam1(f)            # Ok. Pointer
>>> spam2(f)            # Ok. Reference
>>> spam3(f)            # Ok. Const reference
>>> spam4(f)            # Ok. Value.
>>> spam5(f)            # Ok. Array (1 element)

Similar behavior occurs for return values. For example, if you had functions like this,

Foo *spam6();
Foo &spam7();
Foo  spam8();
const Foo &spam9();

then all three functions will return a pointer to some Foo object. Since the third function (spam8) returns a value, newly allocated memory is used to hold the result and a pointer is returned (Python will release this memory when the return value is garbage collected). The fourth case (spam9) which returns a const reference, in most of the cases will be treated as a returning value, and it will follow the same allocation/deallocation process.

33.3.10 C++ overloaded functions

C++ overloaded functions, methods, and constructors are mostly supported by SWIG. For example, if you have two functions like this:

void foo(int);
void foo(char *c);

You can use them in Python in a straightforward manner:

>>> foo(3)           # foo(int)
>>> foo("Hello")     # foo(char *c)

Similarly, if you have a class like this,

class Foo {
public:
  Foo();
  Foo(const Foo &);
  ...
};

you can write Python code like this:

>>> f = Foo()          # Create a Foo
>>> g = Foo(f)         # Copy f

Overloading support is not quite as flexible as in C++. Sometimes there are methods that SWIG can't disambiguate. For example:

void spam(int);
void spam(short);

or

void foo(Bar *b);
void foo(Bar &b);

If declarations such as these appear, you will get a warning message like this:

example.i:12: Warning 509: Overloaded method spam(short) effectively ignored,
example.i:11: Warning 509: as it is shadowed by spam(int).

To fix this, you either need to ignore or rename one of the methods. For example:

%rename(spam_short) spam(short);
...
void spam(int);
void spam(short);   // Accessed as spam_short

or

%ignore spam(short);
...
void spam(int);
void spam(short);   // Ignored

SWIG resolves overloaded functions and methods using a disambiguation scheme that ranks and sorts declarations according to a set of type-precedence rules. The order in which declarations appear in the input does not matter except in situations where ambiguity arises--in this case, the first declaration takes precedence.

Please refer to the "SWIG and C++" chapter for more information about overloading.

33.3.11 C++ operators

Certain C++ overloaded operators can be handled automatically by SWIG. For example, consider a class like this:

class Complex {
private:
  double rpart, ipart;
public:
  Complex(double r = 0, double i = 0) : rpart(r), ipart(i) { }
  Complex(const Complex &c) : rpart(c.rpart), ipart(c.ipart) { }
  Complex &operator=(const Complex &c);

  Complex operator+=(const Complex &c) const;
  Complex operator+(const Complex &c) const;
  Complex operator-(const Complex &c) const;
  Complex operator*(const Complex &c) const;
  Complex operator-() const;

  double re() const { return rpart; }
  double im() const { return ipart; }
};

When wrapped, it works like you expect:

>>> c = Complex(3, 4)
>>> d = Complex(7, 8)
>>> e = c + d
>>> e.re()
10.0
>>> e.im()
12.0
>>> c += d
>>> c.re()
10.0
>>> c.im()
12.0

One restriction with operator overloading support is that SWIG is not able to fully handle operators that aren't defined as part of the class. For example, if you had code like this

class Complex {
...
friend Complex operator+(double, const Complex &c);
...
};

then SWIG ignores it and issues a warning. You can still wrap the operator, but you may have to encapsulate it in a special function. For example:

%rename(Complex_add_dc) operator+(double, const Complex &);

There are ways to make this operator appear as part of the class using the %extend directive. Keep reading.

Also, be aware that certain operators don't map cleanly to Python. For instance, overloaded assignment operators don't map to Python semantics and will be ignored.

Operator overloading is implemented in the pyopers.swg library file. In particular overloaded operators are marked with the python:maybecall feature, also known as %pythonmaybecall. This feature forces SWIG to generate code that returns an instance of Python's NotImplemented instead of raising the usual TypeError exception when an incorrect type is passed to a SWIG wrapped method. This follows the guidelines in PEP 207 - Rich Comparisons and NotImplemented Python constant.

33.3.12 C++ namespaces

SWIG is aware of C++ namespaces, but namespace names do not appear in the module nor do namespaces result in a module that is broken up into submodules or packages. For example, if you have a file like this,

%module example

namespace foo {
  int fact(int n);
  struct Vector {
    double x, y, z;
  };
};

it works in Python as follows:

>>> import example
>>> example.fact(3)
6
>>> v = example.Vector()
>>> v.x = 3.4
>>> print(v.y)
0.0
>>>

If your program has more than one namespace, name conflicts (if any) can be resolved using %rename For example:

%rename(Bar_spam) Bar::spam;

namespace Foo {
  int spam();
}

namespace Bar {
  int spam();
}

If you have more than one namespace and your want to keep their symbols separate, consider wrapping them as separate SWIG modules. For example, make the module name the same as the namespace and create extension modules for each namespace separately. If your program utilizes thousands of small deeply nested namespaces each with identical symbol names, well, then you get what you deserve.

33.3.13 C++ templates

C++ templates don't present a huge problem for SWIG. However, in order to create wrappers, you have to tell SWIG to create wrappers for a particular template instantiation. To do this, you use the %template directive. For example:

%module example
%{
#include "pair.h"
%}

template<class T1, class T2>
struct pair {
  typedef T1 first_type;
  typedef T2 second_type;
  T1 first;
  T2 second;
  pair();
  pair(const T1&, const T2&);
 ~pair();
};

%template(pairii) pair<int, int>;

In Python:

>>> import example
>>> p = example.pairii(3, 4)
>>> p.first
3
>>> p.second
4

Obviously, there is more to template wrapping than shown in this example. More details can be found in the SWIG and C++ chapter. Some more complicated examples will appear later.

33.3.14 C++ Smart Pointers

33.3.14.1 The shared_ptr Smart Pointer

The C++11 standard provides std::shared_ptr which was derived from the Boost implementation, boost::shared_ptr. Both of these are available for Python in the SWIG library and usage is outlined in the shared_ptr smart pointer library section.

33.3.14.2 Generic Smart Pointers

In certain C++ programs, it is common to use classes that have been wrapped by so-called "smart pointers." Generally, this involves the use of a template class that implements operator->() like this:

template<class T> class SmartPtr {
  ...
  T *operator->();
  ...
}

Then, if you have a class like this,

class Foo {
public:
  int x;
  int bar();
};

A smart pointer would be used in C++ as follows:

SmartPtr<Foo> p = CreateFoo();   // Created somehow (not shown)
...
p->x = 3;                        // Foo::x
int y = p->bar();                // Foo::bar

To wrap this in Python, simply tell SWIG about the SmartPtr class and the low-level Foo object. Make sure you instantiate SmartPtr using %template if necessary. For example:

%module example
...
%template(SmartPtrFoo) SmartPtr<Foo>;
...

Now, in Python, everything should just "work":

>>> p = example.CreateFoo()          # Create a smart-pointer somehow
>>> p.x = 3                          # Foo::x
>>> p.bar()                          # Foo::bar

If you ever need to access the underlying pointer returned by operator->() itself, simply use the __deref__() method. For example:

>>> f = p.__deref__()     # Returns underlying Foo *

33.3.15 C++ reference counted objects

The C++ reference counted objects section contains Python examples of memory management using referencing counting.

33.4 Further details on the Python class interface

In the previous section, a high-level view of Python wrapping was presented. A key component of this wrapping is that structures and classes are wrapped by Python proxy classes. This provides a very natural Python interface and allows SWIG to support a number of advanced features such as operator overloading. However, a number of low-level details were omitted. This section provides a brief overview of how the proxy classes work.

New in SWIG version 2.0.4: The use of Python proxy classes has performance implications that may be unacceptable for a high-performance library. The new -builtin option instructs SWIG to forego the use of proxy classes, and instead create wrapped types as new built-in Python types. When this option is used, the following section ("Proxy classes") does not apply. Details on the use of the -builtin option are in the Built-in Types section.

33.4.1 Proxy classes

In the "SWIG basics" and "SWIG and C++" chapters, details of low-level structure and class wrapping are described. To summarize those chapters, if you have a class like this

class Foo {
public:
  int x;
  int spam(int);
  ...

then SWIG transforms it into a set of low-level procedural wrappers. For example:

Foo *new_Foo() {
  return new Foo();
}
void delete_Foo(Foo *f) {
  delete f;
}
int Foo_x_get(Foo *f) {
  return f->x;
}
void Foo_x_set(Foo *f, int value) {
  f->x = value;
}
int Foo_spam(Foo *f, int arg1) {
  return f->spam(arg1);
}

These wrappers can be found in the low-level extension module (e.g., _example).

Using these wrappers, SWIG generates a high-level Python proxy class (also known as a shadow class) like this (shown for Python 2.2):

import _example

class Foo(object):
    def __init__(self):
        self.this = _example.new_Foo()
        self.thisown = True
    def __del__(self):
        if self.thisown:
            _example.delete_Foo(self.this)
    def spam(self, arg1):
        return _example.Foo_spam(self.this, arg1)
    x = property(_example.Foo_x_get, _example.Foo_x_set)

This class merely holds a pointer to the underlying C++ object (.this) and dispatches methods and member variable access to that object using the low-level accessor functions. From a user's point of view, it makes the class work normally:

>>> f = example.Foo()
>>> f.x = 3
>>> y = f.spam(5)

The fact that the class has been wrapped by a real Python class offers certain advantages. For instance, you can attach new Python methods to the class and you can even inherit from it (something not supported by Python built-in types until Python 2.2).

33.4.2 Built-in Types

The -builtin option provides a significant performance improvement in the wrapped code. To understand the difference between proxy classes and built-in types, let's take a look at what a wrapped object looks like under both circumstances.

When proxy classes are used, each wrapped object in Python is an instance of a pure Python class. As a reminder, here is what the __init__ method looks like in a proxy class:

class Foo(object):
    def __init__(self):
        self.this = _example.new_Foo()
        self.thisown = True

When a Foo instance is created, the call to _example.new_Foo() creates a new C++ Foo instance; wraps that C++ instance inside an instance of a Python built-in type called SwigPyObject; and stores the SwigPyObject instance in the 'this' field of the Python Foo object. Did you get all that? So, the Python Foo object is composed of three parts:

• The Python Foo instance, which contains...
• ... an instance of struct SwigPyObject, which contains...
• ... a C++ Foo instance

When -builtin is used, the pure Python layer is stripped off. Each wrapped class is turned into a new Python built-in type which inherits from SwigPyObject, and SwigPyObject instances are returned directly from the wrapped methods. For more information about Python built-in extensions, please refer to the Python documentation:

https://docs.python.org/3/extending/newtypes.html

33.4.2.1 Limitations

Use of the -builtin option implies a couple of limitations:

• Some legacy syntax is no longer supported; in particular:

  • The functional interface is no longer exposed. For example, you may no longer call Whizzo.new_CrunchyFrog(). Instead, you must use Whizzo.CrunchyFrog().
  • Static member variables are no longer accessed through the 'cvar' field (e.g., Dances.cvar.FishSlap). They are instead accessed in the idiomatic way (Dances.FishSlap).

• Wrapped types may not be raised as Python exceptions. Here's why: the Python internals expect that all sub-classes of Exception will have this struct layout:

  typedef struct {
    PyObject_HEAD
    PyObject *dict;
    PyObject *args;
    PyObject *message;
  } PyBaseExceptionObject;
  

  But swig-generated wrappers expect that all swig-wrapped classes will have this struct layout:

  typedef struct {
    PyObject_HEAD
    void *ptr;
    swig_type_info *ty;
    int own;
    PyObject *next;
    PyObject *dict;
  } SwigPyObject;
  

  There are workarounds for this. For example, if you wrap this class:

  class MyException {
  public:
    MyException (const char *msg_);
    ~MyException ();
  
    const char *what () const;
  
  private:
    char *msg;
  };
  

  ... you can define this Python class, which may be raised as an exception:

  class MyPyException(Exception):
      def __init__(self, msg, *args):
          Exception.__init__(self, *args)
          self.myexc = MyException(msg)
      def what(self):
          return self.myexc.what()
  

• Reverse binary operators (e.g., __radd__) are not supported.

  To illustrate this point, if you have a wrapped class called MyString, and you want to use instances of MyString interchangeably with native Python strings, you can define an 'operator+ (const char*)' method :

  class MyString {
  public:
    MyString (const char *init);
    MyString operator+ (const char *other) const;
    ...
  };
  

  SWIG will automatically create an operator overload in Python that will allow this:

  from MyModule import MyString
  
  mystr = MyString("No one expects")
  episode = mystr + " the Spanish Inquisition"
  

  This works because the first operand (mystr) defines a way to add a native string to itself. However, the following will not work:

  from MyModule import MyString
  
  mystr = MyString("Parrot")
  episode = "Dead " + mystr
  

  The above code fails, because the first operand -- a native Python string -- doesn't know how to add an instance of MyString to itself.

• If you have multiple SWIG modules that share type information (more info), the -builtin option requires a bit of extra discipline to ensure that base classes are initialized before derived classes. Specifically:

  • There must be an unambiguous dependency graph for the modules.

  • Module dependencies must be explicitly stated with %import statements in the SWIG interface file.

  As an example, suppose module A has this interface in A.i :

  %module "A";
  
  class Base {
  ...
  };
  

  If you want to wrap another module containing a class that inherits from A, this is how it would look :

  %module "B";
  
  %import "A.i"
  
  class Derived : public Base {
  ...
  };
  

  The import "A.i" statement is required, because module B depends on module A.

  As long as you obey these requirements, your Python code may import the modules in any order :

  import B
  import A
  
  assert(issubclass(B.Derived, A.Base))
  

• Python annotations are not supported.

33.4.2.2 Operator overloads and slots -- use them!

The entire justification for the -builtin option is improved performance. To that end, the best way to squeeze maximum performance out of your wrappers is to use operator overloads. Named method dispatch is slow in Python, even when compared to other scripting languages. However, Python built-in types have a large number of "slots", analogous to C++ operator overloads, which allow you to short-circuit named method dispatch for certain common operations.

By default, SWIG will translate most C++ arithmetic operator overloads into Python slot entries. For example, suppose you have this class:

class Twit {
public:
  Twit operator+ (const Twit& twit) const;

  // Forward to operator+
  Twit add (const Twit& twit) const {
    return *this + twit;
  }
};

SWIG will automatically register operator+ as a Python slot operator for addition. You may write Python code like this:

from MyModule import Twit

nigel = Twit()
emily = Twit()
percival = nigel + emily
percival = nigel.add(emily)

The last two lines of the Python code are equivalent, but the line that uses the '+' operator is much faster.

In-place operators (e.g., operator+=) and comparison operators (operator==, operator<, etc.) are also converted to Python slot operators. For a complete list of C++ operators that are automatically converted to Python slot operators, refer to the file python/pyopers.swg in the SWIG library.

Read about all of the available Python slots here: https://docs.python.org/3/c-api/typeobj.html

There are two ways to define a Python slot function: dispatch to a statically defined function; or dispatch to a method defined on the operand.

To dispatch to a statically defined function, use %feature("python:<slot>"), where <slot> is the name of a field in a PyTypeObject, PyNumberMethods, PyMappingMethods, PySequenceMethods or PyBufferProcs. You may override (almost) all of these slots.

Let's consider an example setting the tp_hash slot for the MyClass type. This is akin to providing a __hash__ method (for non-builtin types) to make a type hashable. The hashable type can then for example be added to a Python dict.

%feature("python:tp_hash") MyClass "myHashFunc";

class MyClass {
public:
  long field1;
  long field2;
  ...
};

%{
#if PY_VERSION_HEX >= 0x03020000
  static Py_hash_t myHashFunc(PyObject *pyobj)
#else
  static long myHashFunc(PyObject *pyobj)
#endif
  {
    MyClass *cobj;
    // Convert pyobj to cobj
    return (cobj->field1 * (cobj->field2 << 7));
  }
%}

If you examine the generated code, the supplied hash function will now be the function callback in the tp_hash slot for the builtin type for MyClass:

static PyHeapTypeObject SwigPyBuiltin__MyClass_type = {
  ...
  (hashfunc) myHashFunc,       /* tp_hash */
  ...

NOTE: It is the responsibility of the programmer (that's you!) to ensure that a statically defined slot function has the correct signature, the hashfunc typedef in this case.

If, instead, you want to dispatch to an instance method, you can use %feature("python:slot"). For example:

%feature("python:slot", "tp_hash", functype="hashfunc") MyClass::myHashFunc;

#if PY_VERSION_HEX < 0x03020000
  #define Py_hash_t long
#endif

class MyClass {
  public:
    Py_hash_t myHashFunc() const;
    ...
};

NOTE: Some Python slots use a method signature which does not match the signature of SWIG-wrapped methods. For those slots, SWIG will automatically generate a "closure" function to re-marshal the arguments before dispatching to the wrapped method. Setting the "functype" attribute of the feature enables SWIG to generate the chosen closure function.

There is further information on %feature("python:slot") in the file python/pyopers.swg in the SWIG library.

33.4.3 Memory management

NOTE: Although this section refers to proxy objects, everything here also applies when the -builtin option is used.

Associated with proxy object, is an ownership flag thisown. The value of this flag determines who is responsible for deleting the underlying C++ object. If set to True, the Python interpreter will destroy the C++ object when the proxy class is garbage collected. If set to False (or if the attribute is missing), then the destruction of the proxy class has no effect on the C++ object.

When an object is created by a constructor or returned by value, Python automatically takes ownership of the result. For example:

class Foo {
public:
  Foo();
  ~Foo();
};
Foo CreateFoo();

In Python:

>>> f = Foo()
>>> f.thisown
True
>>> g = CreateFoo()
>>> g.thisown
True

The type information is a Python class, a proxy of the underlying C++ type:

>>> type(f)
<class 'example.Foo'>
>>> f
<example.Foo; proxy of <Swig Object of type 'Foo *' at 0x7f5a86d48600> >
True

Unlike a C or C++ compiler, SWIG can work with incomplete type information and generate wrapper code even if the definition of class Foo is not provided to SWIG. However, this has negative consequences in the form of a memory leak.

>>> h = CreateFoo()
>>> type(h)
<class 'SwigPyObject'>
>>> h
<Swig Object of type 'Foo *' at 0x7fc341b4d6f0>
>>> quit()
swig/python detected a memory leak of type 'Foo *', no destructor found.

Note that the type is no longer a proxy Python class, but is instead a generic SwigPyObject type. The memory leak warning is shown as SWIG has no way of knowing how to call the destructor with this generic type. The recommendation is to fix the leak by providing the missing type information. However, for those who don't care about memory leaks, the warning can also be suppressed by by defining SWIG_PYTHON_SILENT_MEMLEAK when compiling the generated C/C++ wrapper code, such as passing -DSWIG_PYTHON_SILENT_MEMLEAK to the compiler or getting SWIG to define this macro at the beginning of the generated wrapper code by adding the following to the interface file using:

%begin %{
#define SWIG_PYTHON_SILENT_MEMLEAK
%}

The generic SwigPyObject type also has a disown() method which can be called to deactivate the warning.

Next, let's consider returning raw pointers even when the full type information is available to SWIG. When pointers or references are returned to Python, there is often no way to know where they came from and how to manage the memory that they point to. Therefore, the ownership is set to False. For example:

class Foo {
public:
  Foo();
  ~Foo();
};
Foo *PointerCreate();
Foo &ReferenceCreate();

>>> p = PointerCreate()
>>> p.thisown
False
>>> r = ReferenceCreate()
>>> r.thisown
False

This behavior is especially important for classes that act as containers. For example, if a method returns a pointer to an object that is contained inside another object, you definitely don't want Python to assume ownership and destroy it!

A good way to indicate that ownership should be set for a returned pointer is to use the %newobject directive.

Related to containers, ownership issues can arise whenever an object is assigned to a member or global variable. For example, consider this interface:

%module example

struct Foo {
  int  value;
  Foo  *next;
};

Foo *head = 0;

When wrapped in Python, careful observation will reveal that ownership changes whenever an object is assigned to a global variable. For example:

>>> f = example.Foo()
>>> f.thisown
True
>>> example.cvar.head = f
>>> f.thisown
False
>>>

In this case, C is now holding a reference to the object---you probably don't want Python to destroy it. Similarly, this occurs for members. For example:

>>> f = example.Foo()
>>> g = example.Foo()
>>> f.thisown
True
>>> g.thisown
True
>>> f.next = g
>>> g.thisown
False
>>>

For the most part, memory management issues remain hidden. However, there are occasionally situations where you might have to manually change the ownership of an object. For instance, consider code like this:

class Node {
  Object *value;
public:
  void set_value(Object *v) { value = v; }
  ...
};

Now, consider the following Python code:

>>> v = Object()           # Create an object
>>> n = Node()             # Create a node
>>> n.set_value(v)         # Set value
>>> v.thisown
True
>>> del v

In this case, the object n is holding a reference to v internally. However, SWIG has no way to know that this has occurred. Therefore, Python still thinks that it has ownership of the object. Should the proxy object be destroyed, then the C++ destructor will be invoked and n will be holding a stale-pointer. If you're lucky, you will only get a segmentation fault.

To work around this, it is always possible to flip the ownership flag. For example,

>>> v.thisown = False

It is also possible to deal with situations like this using typemaps--an advanced topic discussed later.

33.5 Cross language polymorphism

Proxy classes provide a more natural, object-oriented way to access extension classes. As described above, each proxy instance has an associated C++ instance, and method calls to the proxy are passed to the C++ instance transparently via C wrapper functions.

This arrangement is asymmetric in the sense that no corresponding mechanism exists to pass method calls down the inheritance chain from C++ to Python. In particular, if a C++ class has been extended in Python (by extending the proxy class), these extensions will not be visible from C++ code. Virtual method calls from C++ are thus not able access the lowest implementation in the inheritance chain.

Changes have been made to SWIG 1.3.18 to address this problem and make the relationship between C++ classes and proxy classes more symmetric. To achieve this goal, new classes called directors are introduced at the bottom of the C++ inheritance chain. The job of the directors is to route method calls correctly, either to C++ implementations higher in the inheritance chain or to Python implementations lower in the inheritance chain. The upshot is that C++ classes can be extended in Python and from C++ these extensions look exactly like native C++ classes. Neither C++ code nor Python code needs to know where a particular method is implemented: the combination of proxy classes, director classes, and C wrapper functions takes care of all the cross-language method routing transparently.

33.5.1 Enabling directors

The director feature is disabled by default. To use directors you must make two changes to the interface file. First, add the "directors" option to the %module directive, like this:

%module(directors="1") modulename

Without this option no director code will be generated. Second, you must use the %feature("director") directive to tell SWIG which classes and methods should get directors. The %feature directive can be applied globally, to specific classes, and to specific methods, like this:

// generate directors for all classes that have virtual methods
%feature("director");

// generate directors for the virtual methods in class Foo
%feature("director") Foo;

You can use the %feature("nodirector") directive to turn off directors for specific classes or methods. So for example,

%feature("director") Foo;
%feature("nodirector") Foo::bar;

will generate directors for the virtual methods of class Foo except bar().

Directors can also be generated implicitly through inheritance. In the following, class Bar will get a director class that handles the methods one() and two() (but not three()):

%feature("director") Foo;
class Foo {
public:
  Foo(int foo);
  virtual ~Foo();
  virtual void one();
  virtual void two();
};

class Bar: public Foo {
public:
  virtual void three();
};

then at the Python side you can define

import mymodule

class MyFoo(mymodule.Foo):
    def __init__(self, foo):
        mymodule.Foo.__init__(self, foo)
#       super().__init__(foo) # Alternative construction for Python3

    def one(self):
        print("one from Python")

33.5.2 Director classes

For each class that has directors enabled, SWIG generates a new class that derives from both the class in question and a special Swig::Director class. These new classes, referred to as director classes, can be loosely thought of as the C++ equivalent of the Python proxy classes. The director classes store a pointer to their underlying Python object and handle various issues related to object ownership. Indeed, this is quite similar to the "this" and "thisown" members of the Python proxy classes.

For simplicity let's ignore the Swig::Director class and refer to the original C++ class as the director's base class. By default, a director class extends all virtual methods in the inheritance chain of its base class (see the preceding section for how to modify this behavior). Virtual methods that have a final specifier are unsurprisingly excluded. Thus the virtual method calls, whether they originate in C++ or in Python via proxy classes, eventually end up in at the implementation in the director class. The job of the director methods is to route these method calls to the appropriate place in the inheritance chain. By "appropriate place" we mean the method that would have been called if the C++ base class and its extensions in Python were seamlessly integrated. That seamless integration is exactly what the director classes provide, transparently skipping over all the messy extension API glue that binds the two languages together.

In reality, the "appropriate place" is one of only two possibilities: C++ or Python. Once this decision is made, the rest is fairly easy. If the correct implementation is in C++, then the lowest implementation of the method in the C++ inheritance chain is called explicitly. If the correct implementation is in Python, the Python API is used to call the method of the underlying Python object (after which the usual virtual method resolution in Python automatically finds the right implementation).

Now how does the director decide which language should handle the method call? The basic rule is to handle the method in Python, unless there's a good reason not to. The reason for this is simple: Python has the most "extended" implementation of the method. This assertion is guaranteed, since at a minimum the Python proxy class implements the method. If the method in question has been extended by a class derived from the proxy class, that extended implementation will execute exactly as it should. If not, the proxy class will route the method call into a C wrapper function, expecting that the method will be resolved in C++. The wrapper will call the virtual method of the C++ instance, and since the director extends this the call will end up right back in the director method. Now comes the "good reason not to" part. If the director method were to blindly call the Python method again, it would get stuck in an infinite loop. We avoid this situation by adding special code to the C wrapper function that tells the director method to not do this. The C wrapper function compares the pointer to the Python object that called the wrapper function to the pointer stored by the director. If these are the same, then the C wrapper function tells the director to resolve the method by calling up the C++ inheritance chain, preventing an infinite loop.

One more point needs to be made about the relationship between director classes and proxy classes. When a proxy class instance is created in Python, SWIG creates an instance of the original C++ class and assigns it to .this. This is exactly what happens without directors and is true even if directors are enabled for the particular class in question. When a class derived from a proxy class is created, however, SWIG then creates an instance of the corresponding C++ director class. The reason for this difference is that user-defined subclasses may override or extend methods of the original class, so the director class is needed to route calls to these methods correctly. For unmodified proxy classes, all methods are ultimately implemented in C++ so there is no need for the extra overhead involved with routing the calls through Python.

33.5.3 Ownership and object destruction

Memory management issues are slightly more complicated with directors than for proxy classes alone. Python instances hold a pointer to the associated C++ director object, and the director in turn holds a pointer back to the Python object. By default, proxy classes own their C++ director object and take care of deleting it when they are garbage collected.

This relationship can be reversed by calling the special __disown__() method of the proxy class. After calling this method, the thisown flag is set to False, and the director class increments the reference count of the Python object. When the director class is deleted it decrements the reference count. Assuming no outstanding references to the Python object remain, the Python object will be destroyed at the same time. This is a good thing, since directors and proxies refer to each other and so must be created and destroyed together. Destroying one without destroying the other will likely cause your program to segfault.

To help ensure that no references to the Python object remain after calling __disown__(), this method returns a weak reference to the Python object. Here is an example:

class Foo {
public:
  ...
};
class FooContainer {
public:
  void addFoo(Foo *);
  ...
};

>>> c = FooContainer()
>>> a = Foo().__disown__()
>>> c.addFoo(a)
>>> b = Foo()
>>> b = b.__disown__()
>>> c.addFoo(b)
>>> c.addFoo(Foo().__disown__())

In this example, we are assuming that FooContainer will take care of deleting all the Foo pointers it contains at some point. Note that no hard references to the Foo objects remain in Python.

33.5.4 Exception unrolling

With directors routing method calls to Python, and proxies routing them to C++, the handling of exceptions is an important concern. By default, the directors ignore exceptions that occur during method calls that are resolved in Python. To handle such exceptions correctly, it is necessary to temporarily translate them into C++ exceptions. This can be done with the %feature("director:except") directive. The following code should suffice in most cases:

%feature("director:except") {
  if ($error != NULL) {
    throw Swig::DirectorMethodException();
  }
}

This code will check the Python error state after each method call from a director into Python, and throw a C++ exception if an error occurred. This exception can be caught in C++ to implement an error handler. Currently no information about the Python error is stored in the Swig::DirectorMethodException object, but this will likely change in the future.

It may be the case that a method call originates in Python, travels up to C++ through a proxy class, and then back into Python via a director method. If an exception occurs in Python at this point, it would be nice for that exception to find its way back to the original caller. This can be done by combining a normal %exception directive with the director:except handler shown above. Here is an example of a suitable exception handler:

%exception {
  try { $action }
  catch (Swig::DirectorException &e) { SWIG_fail; }
}

The class Swig::DirectorException used in this example is actually a base class of Swig::DirectorMethodException, so it will trap this exception. Because the Python error state is still set when Swig::DirectorMethodException is thrown, Python will register the exception as soon as the C wrapper function returns.

33.5.5 Overhead and code bloat

Enabling directors for a class will generate a new director method for every virtual method in the class' inheritance chain. This alone can generate a lot of code bloat for large hierarchies. Method arguments that require complex conversions to and from target language types can result in large director methods. For this reason it is recommended that you selectively enable directors only for specific classes that are likely to be extended in Python and used in C++.

Compared to classes that do not use directors, the call routing in the director methods does add some overhead. In particular, at least one dynamic cast and one extra function call occurs per method call from Python. Relative to the speed of Python execution this is probably completely negligible. For worst case routing, a method call that ultimately resolves in C++ may take one extra detour through Python in order to ensure that the method does not have an extended Python implementation. This could result in a noticeable overhead in some cases.

Although directors make it natural to mix native C++ objects with Python objects (as director objects) via a common base class pointer, one should be aware of the obvious fact that method calls to Python objects will be much slower than calls to C++ objects. This situation can be optimized by selectively enabling director methods (using the %feature directive) for only those methods that are likely to be extended in Python.

33.5.6 Typemaps

Typemaps for input and output of most of the basic types from director classes have been written. These are roughly the reverse of the usual input and output typemaps used by the wrapper code. The typemap operation names are 'directorin', 'directorout', and 'directorargout'. The director code does not currently use any of the other kinds of typemaps. It is not clear at this point which kinds are appropriate and need to be supported.

33.5.7 Thread safety

The director implementation uses a std::map to manage C++ pointer ownership. This code is not thread-safe by default. See Thread safety for turning on thread-safety. This defines a macro called SWIG_THREADS which forces the C++ code to then use a mutex for managing the pointer ownership, which in turn makes the directors implementation thread-safe.

Please see the director_guard.swg Library file for details of the implementation.

33.5.8 Miscellaneous

Director typemaps for STL classes are in place, and hence you should be able to use std::vector, std::string, etc., as you would any other type.

Note: The director typemaps for return types based on const references, such as

class Foo {
…
  virtual const int& bar();
…
};

will work only for simple call scenarios. Usually the resulting code is neither thread nor reentrant safe. Hence, the user is advised to avoid returning const references in director methods. For example, the user could modify the method interface to use return by value semantics instead.

class Foo {
…
  virtual int bar();
…
};

If that is not possible, the user should avoid enabling the director feature for reentrant, recursive or threaded member methods that return const references.

33.6 Common customization features

The last section presented the absolute basics of C/C++ wrapping. If you do nothing but feed SWIG a header file, you will get an interface that mimics the behavior described. However, sometimes this isn't enough to produce a nice module. Certain types of functionality might be missing or the interface to certain functions might be awkward. This section describes some common SWIG features that are used to improve your the interface to an extension module.

33.6.1 C/C++ helper functions

Sometimes when you create a module, it is missing certain bits of functionality. For example, if you had a function like this

void set_transform(Image *im, double m[4][4]);

it would be accessible from Python, but there may be no easy way to call it. For example, you might get errors like this:

>>> a = [
...   [1, 0, 0, 0],
...   [0, 1, 0, 0],
...   [0, 0, 1, 0],
...   [0, 0, 0, 1]]
>>> set_transform(im, a)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: Type error. Expected _p_a_4__double

The problem here is that there is no easy way to construct and manipulate a suitable double [4][4] value to use. To fix this, you can write some extra C helper functions. Just use the %inline directive. For example:

%inline %{
/* Note: double[4][4] is equivalent to a pointer to an array double (*)[4] */
double (*new_mat44())[4] {
  return (double (*)[4]) malloc(16*sizeof(double));
}
void free_mat44(double (*x)[4]) {
  free(x);
}
void mat44_set(double x[4][4], int i, int j, double v) {
  x[i][j] = v;
}
double mat44_get(double x[4][4], int i, int j) {
  return x[i][j];
}
%}

From Python, you could then write code like this:

>>> a = new_mat44()
>>> mat44_set(a, 0, 0, 1.0)
>>> mat44_set(a, 1, 1, 1.0)
>>> mat44_set(a, 2, 2, 1.0)
...
>>> set_transform(im, a)
>>>

Admittedly, this is not the most elegant looking approach. However, it works and it wasn't too hard to implement. It is possible to clean this up using Python code, typemaps, and other customization features as covered in later sections.

33.6.2 Adding additional Python code

If writing support code in C isn't enough, it is also possible to write code in Python. This code gets inserted in to the .py file created by SWIG. One use of Python code might be to supply a high-level interface to certain functions. For example:

/* Rename the SWIG-generated wrapper. */
%rename _set_transform set_transform;

...

void set_transform(Image *im, double x[4][4]);

...

/* Rewrite the high level interface to set_transform */
%pythoncode %{
def set_transform(im, x):
    a = new_mat44()
    for i in range(4):
        for j in range(4):
            mat44_set(a, i, j, x[i][j])
    _example._set_transform(im, a)
    free_mat44(a)
%}

In this example, set_transform() provides a high-level Python interface built on top of low-level helper functions. For example, this code now seems to work:

>>> a = [
...   [1, 0, 0, 0],
...   [0, 1, 0, 0],
...   [0, 0, 1, 0],
...   [0, 0, 0, 1]]
>>> set_transform(im, a)
>>>

Admittedly, this whole scheme for wrapping the two-dimension array argument is rather ad-hoc. Besides, shouldn't a Python list or a Numeric Python array just work normally? We'll get to those examples soon enough. For now, think of this example as an illustration of what can be done without having to rely on any of the more advanced customization features.

There is also %pythonbegin which is another directive very similar to %pythoncode, but generates the given Python code at the beginning of the .py file. This directive works in the same way as %pythoncode, except the code is copied just after the SWIG banner (comment) at the top of the file, before any real code. This provides an opportunity to add your own description in a comment near the top of the file as well as Python imports that have to appear at the top of the file, such as "from __future__ import" statements.

The following example for Python 2.x shows how to insert code into the generated wrapper to enable print to be used as a Python3-compatible function instead of a statement:

%pythonbegin %{
# This module provides wrappers to the Whizz Bang library
%}

%pythonbegin %{
from __future__ import print_function
print("Loading", "Whizz", "Bang", sep=' ... ')
%}

The insert code can be seen at the start of the generated .py file:

# This file was automatically generated by SWIG (https://www.swig.org).
# Version 4.0.0
#
# Do not make changes to this file unless you know what you are doing--modify
# the SWIG interface file instead.

# This module provides wrappers to the Whizz Bang library

from __future__ import print_function
print("Loading", "Whizz", "Bang", sep=' ... ')

When using %pythoncode and %pythonbegin you generally want to make sure that the block is delimited by %{ and %}. If you delimit it with { and } then any lines with a leading # will be handled by SWIG as preprocessor directives, when you probably meant them as Python comments. Prior to SWIG 3.0.3, invalid preprocessor directives were silently ignored, so generally using the wrong delimiters resulted in such comments not appearing in the generated output (though a comment starting with a valid preprocessor directive could cause problems, for example: # error handling). SWIG 3.0.3 and later report an error for invalid preprocessor directives, so you may have to update existing interface files to delimit blocks of Python code correctly.

As an alternative to providing a block containing Python code, you can include Python code from a file. The code is inserted exactly as in the file, so this avoids any issues with the SWIG preprocessor. It's a good approach if you have a non-trivial chunk of Python code to insert. To use this feature you specify a filename in double quotes, for example:

%pythoncode "somecode.py"

Sometimes you may want to replace or modify the wrapper function that SWIG creates in the proxy .py file. The Python module in SWIG provides some features that enable you to do this. First, to entirely replace a proxy function you can use %feature("shadow"). For example:

%module example

// Rewrite bar() Python code

%feature("shadow") Foo::bar(int) %{
def bar(*args):
    #do something before
    $action
    #do something after
%}

class Foo {
public:
  int bar(int x);
};

where $action will be replaced by the call to the C/C++ proper method. Note that this does not include the arguments to the C/C++ method.

Often the proxy function created by SWIG is fine, but you simply want to add code to it without touching the rest of the generated function body. For these cases SWIG provides the pythonprepend and pythonappend features which do exactly as their names suggest. The pythonprepend feature will insert its value at the beginning of the proxy function, and pythonappend will insert code at the end of the proxy, just before the return statement.

%module example

// Add Python code to bar()

%feature("pythonprepend") Foo::bar(int) %{
    #do something before C++ call
%}

%feature("pythonappend") Foo::bar(int) %{
    #do something after C++ call
    #the 'val' variable holds the return value
%}


class Foo {
public:
  int bar(int x);
};

Notes: Usually the pythonappend and pythonprepend features are safer to use than the shadow feature. Also, from SWIG version 1.3.28 you can use the directive forms %pythonappend and %pythonprepend as follows:

%module example

// Add Python code to bar()

%pythonprepend Foo::bar(int) %{
    #do something before C++ call
%}

%pythonappend Foo::bar(int) %{
    #do something after C++ call
    #the 'val' variable holds the return value
%}


class Foo {
public:
  int bar(int x);
};

Note that when the underlying C++ method is overloaded, there is only one proxy Python method for multiple C++ methods. In this case, only one of parsed methods is examined for the feature. You are better off specifying the feature without the argument list to ensure it will get used, as it will then get attached to all the overloaded C++ methods. For example:

%module example

// Add Python code to bar()

%pythonprepend Foo::bar %{
    #do something before C++ call
%}

%pythonappend Foo::bar %{
    #do something after C++ call
%}


class Foo {
public:
  int bar(int x);
  int bar();
};

The same applies for overloaded constructors.

33.6.3 Class extension with %extend

One of the more interesting features of SWIG is that it can extend structures and classes with new methods--at least in the Python interface. Here is a simple example:

%module example
%{
#include "someheader.h"
%}

struct Vector {
  double x, y, z;
};

%extend Vector {
  char *__str__() {
    static char tmp[1024];
    sprintf(tmp, "Vector(%g, %g, %g)", $self->x, $self->y, $self->z);
    return tmp;
  }
  Vector(double x, double y, double z) {
    Vector *v = (Vector *) malloc(sizeof(Vector));
    v->x = x;
    v->y = y;
    v->z = z;
    return v;
  }
};

Now, in Python

>>> v = example.Vector(2, 3, 4)
>>> print(v)
Vector(2, 3, 4)
>>>

%extend can be used for many more tasks than this. For example, if you wanted to overload a Python operator, you might do this:

%extend Vector {
  Vector __add__(Vector *other) {
    Vector v;
    v.x = $self->x + other->x;
    v.y = $self->y + other->y;
    v.z = $self->z + other->z;
    return v;
  }
};

Use it like this:

>>> import example
>>> v = example.Vector(2, 3, 4)
>>> w = example.Vector(10, 11, 12)
>>> print(v+w)
Vector(12, 14, 16)
>>>

%extend works with both C and C++ code. It does not modify the underlying object in any way---the extensions only show up in the Python interface.

33.6.4 Exception handling with %exception

If a C or C++ function throws an error, you may want to convert that error into a Python exception. To do this, you can use the %exception directive. %exception simply lets you rewrite part of the generated wrapper code to include an error check.

In C, a function often indicates an error by returning a status code (a negative number or a NULL pointer perhaps). Here is a simple example of how you might handle that:

%exception malloc {
  $action
  if (!result) {
    PyErr_SetString(PyExc_MemoryError, "Not enough memory");
    SWIG_fail;
  }
}
void *malloc(size_t nbytes);

In Python,

>>> a = example.malloc(2000000000)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
MemoryError: Not enough memory
>>>

If a library provides some kind of general error handling framework, you can also use that. For example:

%exception {
  $action
  if (err_occurred()) {
    PyErr_SetString(PyExc_RuntimeError, err_message());
    SWIG_fail;
  }
}

No declaration name is given to %exception, it is applied to all wrapper functions.

C++ exceptions are also easy to handle. For example, you can write code like this:

%exception getitem {
  try {
    $action
  } catch (std::out_of_range &e) {
    PyErr_SetString(PyExc_IndexError, const_cast<char*>(e.what()));
    SWIG_fail;
  }
}

class Base {
public:
  Foo *getitem(int index);      // Exception handled added
  ...
};

When raising a Python exception from C, use the PyErr_SetString() function as shown above followed by SWIG_fail. The following exception types can be used as the first argument.

PyExc_ArithmeticError
PyExc_AssertionError
PyExc_AttributeError
PyExc_EnvironmentError
PyExc_EOFError
PyExc_Exception
PyExc_FloatingPointError
PyExc_ImportError
PyExc_IndexError
PyExc_IOError
PyExc_KeyError
PyExc_KeyboardInterrupt
PyExc_LookupError
PyExc_MemoryError
PyExc_NameError
PyExc_NotImplementedError
PyExc_OSError
PyExc_OverflowError
PyExc_RuntimeError
PyExc_StandardError
PyExc_SyntaxError
PyExc_SystemError
PyExc_TypeError
PyExc_UnicodeError
PyExc_ValueError
PyExc_ZeroDivisionError

SWIG_fail is a C macro which when called within the context of SWIG wrapper function, will jump to the error handler code. This will call any cleanup code (freeing any temp variables) and then return from the wrapper function so that the Python interpreter can raise the Python exception. This macro should always be called after setting a Python error in code snippets, such as typemaps and %exception, that are ultimately generated into the wrapper function.

The language-independent exception.i library file can also be used to raise exceptions. See the SWIG Library chapter.

33.6.5 Optimization options

33.6.5.1 -fastproxy

The -fastproxy command line option enables faster method calling as the call is made directly into the C/C++ layer rather than going through a method wrapper.

Consider wrapping a C++ class:

struct Go {
  void callme0() {}
  void callme4(int a, int b, int c, int d) {}
  void callme8(double a, double b, double c, double d, double e, double f, double g, double i) {}
};

The default generated proxy class is:

class Go(object):
    def callme0(self):
        return _example.Go_callme0(self)

    def callme4(self, a, b, c, d):
        return _example.Go_callme4(self, a, b, c, d)

    def callme8(self, a, b, c, d, e, f, g, i):
        return _example.Go_callme8(self, a, b, c, d, e, f, g, i)
    ...

Each method in the Python class contains a Python proxy method which passes the arguments on to the underlying function in the low-level C/C++ module (_example in this case). The generated proxy class when using -fastproxy is:

%module example
class Go(object):
    callme0 = _swig_new_instance_method(_example.Go_callme0)
    callme4 = _swig_new_instance_method(_example.Go_callme4)
    callme8 = _swig_new_instance_method(_example.Go_callme8)
    ...

where _swig_new_instance_method adds the method to the proxy class via C API calls for direct access to the underlying function in the low-level C/C++ module. Note that for some methods it is not possible to generate the direct access call and so -fastproxy is ignored. This happens, for example, when adding additional code to Python proxy methods, such as using %pythonprepend.

The overhead calling into C/C++ from Python is reduced slightly using -fastproxy. Below are some timings in microseconds calling the 3 functions in the example above. Also included in the table for comparison is using the -builtin option covered in the Built-in Types.

Although the -fastproxy option results in faster code over the default, the generated proxy code is not as user-friendly as docstring/doxygen comments, Python annotations and functions with default values are not visible in the generated Python proxy class. The -olddefs option can rectify this.

The generated proxy class for the example above when using -fastproxy -olddefs is:

class Go(object):
    def callme0(self):
        return _example.Go_callme0(self)
    callme0 = _swig_new_instance_method(_example.Go_callme0)

    def callme4(self, a, b, c, d):
        return _example.Go_callme4(self, a, b, c, d)
    callme4 = _swig_new_instance_method(_example.Go_callme4)

    def callme8(self, a, b, c, d, e, f, g, i):
        return _example.Go_callme8(self, a, b, c, d, e, f, g, i)
    callme8 = _swig_new_instance_method(_example.Go_callme8)
    ...

The class defines each method in two different ways. The first definition is replaced by the second definition and so the second definition is the one used when the method is called. While this possibly provides the best of both worlds, the time to import the module will be slightly slower when the class is defined due to the additional method definitions.

The command line options mentioned above also apply to wrapped C/C++ global functions, not just class methods.

33.6.6 Stable ABI

By default, the version of Python used to compile the wrappers needs to be the same as that used during runtime. Alternatvely, the Python Stable ABI enables a single compiled binary to be used by different versions of Python. This is enabled by defining Py_LIMITED_API during the compilation of the C/C++ wrapper code and setting this macro to a particular minimum version of Python that one wants to support.

SWIG supports the stable ABI, but only version 3.4 of Python and later is supported. There are two recommended approaches for using SWIG and the stable ABI and both require setting the Py_LIMITED_API macro to be set to 0x03040000 as a minimum value (Python 3.4). Either set this using %begin by adding the following into your interface file so that this macro appears at the beginning of the generated C/C++ code:

%begin %{
#define Py_LIMITED_API 0x03040000
%}

or simply define the macro using your C/C++ compiler's -D command line option, for example, -DPy_LIMITED_API=0x03040000.

The default SWIG command line options generate code that enable the limited API/stable ABI. Some options, such as -builtin, -fast (used by -O) do not use the limited API and hence when Py_LIMITED_API is defined there will be missing symbols during compilation. Compatibility of user's custom typemaps is of course dependent on the Python APIs used in the typemaps.

As discussed in the official Python Stable ABI documentation, there is no guarantee that code conforms to the Limited API or Stable ABI when defining Py_LIMITED_API. There are a number of recommendations and caveats detailed there which are worth studying. The Stable ABI is also evolving and being modified with each Python release. In fact there is a Python C API working group, capi-workgroup, which was setup in the first half of 2023. The capi-workgroup/problems issues is very enlightening reading for anyone using the Stable ABI. PEP 733 came out of the working group and is an evaluation of Python's C API.

There is a lot of information to digest in this space, but a simple approach would be to follow the offical recommendation of testing an extension with all minor Python versions you want to support, and to build with the lowest supported version. SWIG supports at least Python 3.4 for the Stable ABI so this would be a good version to compile a SWIG extension against. There is also a useful tool abi3audit which can be used to check the compiled extension for Stable ABI conformance. It can be easily installed with pip and then used to check a SWIG generated shared object or wheel even. Note that if you are using it to check a shared object, the shared object must be renamed to have abi3 in the name. For example, below is the output on Linux for the class example shipped with SWIG, if Py_LIMITED_API is set as described earlier:

~/swig/Examples/python/class $ mv _example.so _example.abi3.so
~/swig/Examples/python/class $ abi3audit --assume-minimum-abi3 3.4 _example.abi3.so
[22:12:18] WARNING  no wheel to infer abi3 version from; assuming (3.4)
[22:12:18] _example.abi3.so: 1 extensions scanned; 0 ABI version mismatches and 0 ABI violations found

If Py_LIMITED_API is not set, the check fails, but this should be expected:

~/swig/Examples/python/class $ mv _example.so _example.abi3.so
~/swig/Examples/python/class $ abi3audit --report --assume-minimum-abi3 3.4 _example.abi3.so
[22:22:27] WARNING  no wheel to infer abi3 version from; assuming (3.4)
[22:22:27] _example.abi3.so: 1 extensions scanned; 1 ABI version mismatches and 0 ABI violations found
{"specs": {"_example.abi3.so": {"kind": "object", "object": {"name": "_example.abi3.so",
"result": {"is_abi3": true, "is_abi3_baseline_compatible": false, "baseline": "3.4", "computed": "3.10",
"non_abi3_symbols": [], "future_abi3_objects": {"PyUnicode_AsUTF8AndSize": "3.10"}}}}}}

Note that ABI conformance as measured by abi3audit is far from simple and can depend on function inlining. There are functions in Python.h, which if inlined, become ABI compliant. Your chances of your C or C++ compiler inlining functions is greater with higher compiler optimisation levels. So merely by changing your compiler optimisation level may switch your Python extension from being identified as ABI compliant or not.

Compatibility Note: Support for the stable ABI was added in SWIG-4.2.0.

33.7 Tips and techniques

Although SWIG is largely automatic, there are certain types of wrapping problems that require additional user input. Examples include dealing with output parameters, strings, binary data, and arrays. This chapter discusses the common techniques for solving these problems.

33.7.1 Input and output parameters

A common problem in some C programs is handling parameters passed as simple pointers. For example:

void add(int x, int y, int *result) {
  *result = x + y;
}

or perhaps

int sub(int *x, int *y) {
  return *x-*y;
}

The easiest way to handle these situations is to use the typemaps.i file. For example:

%module example
%include "typemaps.i"

void add(int, int, int *OUTPUT);
int  sub(int *INPUT, int *INPUT);

In Python, this allows you to pass simple values. For example:

>>> a = add(3, 4)
>>> print(a)
7
>>> b = sub(7, 4)
>>> print(b)
3
>>>

Notice how the INPUT parameters allow integer values to be passed instead of pointers and how the OUTPUT parameter creates a return result.

If you don't want to use the names INPUT or OUTPUT, use the %apply directive. For example:

%module example
%include "typemaps.i"

%apply int *OUTPUT { int *result };
%apply int *INPUT  { int *x, int *y};

void add(int x, int y, int *result);
int  sub(int *x, int *y);

If a function mutates one of its parameters like this,

void negate(int *x) {
  *x = -(*x);
}

you can use INOUT like this:

%include "typemaps.i"
...
void negate(int *INOUT);

In Python, a mutated parameter shows up as a return value. For example:

>>> a = negate(3)
>>> print(a)
-3
>>>

Note: Since most primitive Python objects are immutable, it is not possible to perform in-place modification of a Python object passed as a parameter.

The most common use of these special typemap rules is to handle functions that return more than one value. For example, sometimes a function returns a result as well as a special error code:

/* send message, return number of bytes sent, along with success code */
int send_message(char *text, int *success);

To wrap such a function, simply use the OUTPUT rule above. For example:

%module example
%include "typemaps.i"
%apply int *OUTPUT { int *success };
...
int send_message(char *text, int *success);

When used in Python, the function will return multiple values.

bytes, success = send_message("Hello World")
if not success:
    print("Whoa!")
else:
    print("Sent", bytes)

Another common use of multiple return values are in query functions. For example:

void get_dimensions(Matrix *m, int *rows, int *columns);

To wrap this, you might use the following:

%module example
%include "typemaps.i"
%apply int *OUTPUT { int *rows, int *columns };
...
void get_dimensions(Matrix *m, int *rows, *columns);

Now, in Python:

>>> r, c = get_dimensions(m)

Be aware that the primary purpose of the typemaps.i file is to support primitive datatypes. Writing a function like this

void foo(Bar *OUTPUT);

may not have the intended effect since typemaps.i does not define an OUTPUT rule for Bar.

33.7.2 Simple pointers

If you must work with simple pointers such as int * or double * and you don't want to use typemaps.i, consider using the cpointer.i library file. For example:

%module example
%include "cpointer.i"

%inline %{
extern void add(int x, int y, int *result);
%}

%pointer_functions(int, intp);

The %pointer_functions(type, name) macro generates five helper functions that can be used to create, destroy, copy, assign, and dereference a pointer. In this case, the functions are as follows:

int  *new_intp();
int  *copy_intp(int *x);
void  delete_intp(int *x);
void  intp_assign(int *x, int value);
int   intp_value(int *x);

In Python, you would use the functions like this:

>>> result = new_intp()
>>> print(result)
_108fea8_p_int
>>> add(3, 4, result)
>>> print(intp_value(result))
7
>>>

If you replace %pointer_functions() by %pointer_class(type, name), the interface is more class-like.

>>> result = intp()
>>> add(3, 4, result)
>>> print(result.value())
7

See the SWIG Library chapter for further details.

33.7.3 Unbounded C Arrays

Sometimes a C function expects an array to be passed as a pointer. For example,

int sumitems(int *first, int nitems) {
  int i, sum = 0;
  for (i = 0; i < nitems; i++) {
    sum += first[i];
  }
  return sum;
}

To wrap this into Python, you need to pass an array pointer as the first argument. A simple way to do this is to use the carrays.i library file. For example:

%include "carrays.i"
%array_class(int, intArray);

The %array_class(type, name) macro creates wrappers for an unbounded array object that can be passed around as a simple pointer like int * or double *. For instance, you will be able to do this in Python:

>>> a = intArray(10000000)         # Array of 10-million integers
>>> for i in xrange(10000):        # Set some values
...     a[i] = i
>>> sumitems(a, 10000)
49995000
>>>

The array "object" created by %array_class() does not encapsulate pointers inside a special array object. In fact, there is no bounds checking or safety of any kind (just like in C). Because of this, the arrays created by this library are extremely low-level indeed. You can't iterate over them nor can you even query their length. In fact, any valid memory address can be accessed if you want (negative indices, indices beyond the end of the array, etc.). Needless to say, this approach is not going to suit all applications. On the other hand, this low-level approach is extremely efficient and well suited for applications in which you need to create buffers, package binary data, etc.

33.7.4 String handling

If a C function has an argument of char *, then a Python string can be passed as input. For example:

// C
void foo(char *s);

# Python
>>> foo("Hello")

When a Python string is passed as a parameter, the C function receives a pointer to the raw data contained in the string. Since Python strings are immutable, it is illegal for your program to change the value. In fact, doing so will probably crash the Python interpreter.

If your program modifies the input parameter or uses it to return data, consider using the cstring.i library file described in the SWIG Library chapter.

When functions return a char *, it is assumed to be a NULL-terminated string. Data is copied into a new Python string and returned.

If your program needs to work with binary data, you can use a typemap to expand a Python string into a pointer/length argument pair. As luck would have it, just such a typemap is already defined. Just do this:

%apply (char *STRING, size_t LENGTH) { (char *data, size_t size) };
...
int parity(char *data, size_t size, int initial);

Now in Python:

>>> parity("e\x09ffss\x00\x00\x01\nx", 0)

If you need to return binary data, you might use the cstring.i library file. The cdata.i library can also be used to extra binary data from arbitrary pointers.

33.7.5 Default arguments

C++ default argument code generation is documented in the main Default arguments section. There is also an optional Python specific feature that can be used called the python:cdefaultargs feature flag. By default, SWIG attempts to convert C++ default argument values into Python values and generates code into the Python layer containing these values. For example:

struct CDA {
  int fff(int a = 1, bool b = false);
};

From Python this can be called as follows:

>>> CDA().fff()        # C++ layer receives a=1 and b=false
>>> CDA().fff(2)       # C++ layer receives a=2 and b=false
>>> CDA().fff(3, True) # C++ layer receives a=3 and b=true

The default code generation in the Python layer is:

class CDA(object):
    ...
    def fff(self, a=1, b=False):
        return _default_args.CDA_fff(self, a, b)

Adding the feature:

%feature("python:cdefaultargs") CDA::fff;
struct CDA {
  int fff(int a = 1, bool b = false);

results in identical behaviour when called from Python, however, it results in different code generation:

class CDA(object):
    ...
    def fff(self, *args):
        return _default_args.CDA_fff(self, *args)

The default arguments are obtained in the C++ wrapper layer instead of the Python layer. Some code generation modes are quite different, eg -builtin and -fastproxy, and are unaffected by python:cdefaultargs as the default values are always obtained from the C++ layer.

Note that not all default arguments can be converted into a Python equivalent. When SWIG does not convert them, it will generate code to obtain them from the C++ layer as if python:cdefaultargs was specified. This will happen if just one argument cannot be converted into a Python equivalent. This occurs typically when the argument is not fully numeric, such as int(1):

struct CDA {
  int fff(int a = int(1), bool b = false);
};

Compatibility Note: SWIG-3.0.6 introduced the python:cdefaultargs feature. Versions of SWIG prior to this varied in their ability to convert C++ default values into equivalent Python default argument values.

33.8 Typemaps

This section describes how you can modify SWIG's default wrapping behavior for various C/C++ datatypes using the %typemap directive. This is an advanced topic that assumes familiarity with the Python C API as well as the material in the "Typemaps" chapter.

Before proceeding, it should be stressed that typemaps are not a required part of using SWIG---the default wrapping behavior is enough in most cases. Typemaps are only used if you want to change some aspect of the primitive C-Python interface or if you want to elevate your guru status.

33.8.1 What is a typemap?

A typemap is nothing more than a code generation rule that is attached to a specific C datatype. For example, to convert integers from Python to C, you might define a typemap like this:

%module example

%typemap(in) int {
  $1 = (int) PyLong_AsLong($input);
  printf("Received an integer : %d\n", $1);
}
%inline %{
extern int fact(int n);
%}

Typemaps are always associated with some specific aspect of code generation. In this case, the "in" method refers to the conversion of input arguments to C/C++. The datatype int is the datatype to which the typemap will be applied. The supplied C code is used to convert values. In this code a number of special variable prefaced by a $ are used. The $1 variable is placeholder for a local variable of type int. The $input variable is the input object of type PyObject *.

When this example is compiled into a Python module, it operates as follows:

>>> from example import *
>>> fact(6)
Received an integer : 6
720

In this example, the typemap is applied to all occurrences of the int datatype. You can refine this by supplying an optional parameter name. For example:

%module example

%typemap(in) int nonnegative {
  $1 = (int) PyLong_AsLong($input);
  if ($1 < 0) {
    PyErr_SetString(PyExc_ValueError, "Expected a nonnegative value.");
    SWIG_fail;
  }
}
%inline %{
extern int fact(int nonnegative);
%}

In this case, the typemap code is only attached to arguments that exactly match int nonnegative.

The application of a typemap to specific datatypes and argument names involves more than simple text-matching--typemaps are fully integrated into the SWIG C++ type-system. When you define a typemap for int, that typemap applies to int and qualified variations such as const int. In addition, the typemap system follows typedef declarations. For example:

%typemap(in) int n {
  $1 = (int) PyLong_AsLong($input);
  printf("n = %d\n", $1);
}
%inline %{
typedef int Integer;
extern int fact(Integer n);    // Above typemap is applied
%}

Typemaps can also be defined for groups of consecutive arguments. For example:

%typemap(in) (char *str, int len) {
  $1 = PyString_AsString($input);
  $2 = PyString_Size($input);
};

int count(char c, char *str, int len);

When a multi-argument typemap is defined, the arguments are always handled as a single Python object. This allows the function to be used like this (notice how the length parameter is omitted):

>>> example.count('e', 'Hello World')
1
>>>

33.8.2 Python typemaps

The previous section illustrated an "in" typemap for converting Python objects to C. A variety of different typemap methods are defined by the Python module. For example, to convert a C integer back into a Python object, you might define an "out" typemap like this:

%typemap(out) int {
    $result = PyInt_FromLong((long) $1);
}

A detailed list of available methods can be found in the "Typemaps" chapter.

However, the best source of typemap information (and examples) is probably the Python module itself. In fact, all of SWIG's default type handling is defined by typemaps. You can view these typemaps by looking at the files in the SWIG library. Just take into account that in the latest versions of swig (1.3.22+), the library files are not very pristine clear for the casual reader, as they used to be. The extensive use of macros and other ugly techniques in the latest version produce a very powerful and consistent Python typemap library, but at the cost of simplicity and pedagogic value.

To learn how to write a simple or your first typemap, you better take a look at the SWIG library version 1.3.20 or so.

33.8.3 Typemap variables

Within typemap code, a number of special variables prefaced with a $ may appear. A full list of variables can be found in the "Typemaps" chapter. This is a list of the most common variables:

$1

$input

$result

$1_name

$1_type

$1_ltype

$symname

33.8.4 Useful Python Functions

When you write a typemap, you usually have to work directly with Python objects. The following functions may prove to be useful.

Python Integer Functions

PyObject *PyInt_FromLong(long l);
long      PyInt_AsLong(PyObject *);
int       PyInt_Check(PyObject *);

Python Floating Point Functions

PyObject *PyFloat_FromDouble(double);
double    PyFloat_AsDouble(PyObject *);
int       PyFloat_Check(PyObject *);

Python String Functions

PyObject *PyString_FromString(char *);
PyObject *PyString_FromStringAndSize(char *, lint len);
int       PyString_Size(PyObject *);
char     *PyString_AsString(PyObject *);
int       PyString_Check(PyObject *);

Python List Functions

PyObject *PyList_New(int size);
int       PyList_Size(PyObject *list);
PyObject *PyList_GetItem(PyObject *list, int i);
int       PyList_SetItem(PyObject *list, int i, PyObject *item);
int       PyList_Insert(PyObject *list, int i, PyObject *item);
int       PyList_Append(PyObject *list, PyObject *item);
PyObject *PyList_GetSlice(PyObject *list, int i, int j);
int       PyList_SetSlice(PyObject *list, int i, int , PyObject *list2);
int       PyList_Sort(PyObject *list);
int       PyList_Reverse(PyObject *list);
PyObject *PyList_AsTuple(PyObject *list);
int       PyList_Check(PyObject *);

Python Tuple Functions

PyObject *PyTuple_New(int size);
int       PyTuple_Size(PyObject *);
PyObject *PyTuple_GetItem(PyObject *, int i);
int       PyTuple_SetItem(PyObject *, int i, PyObject *item);
PyObject *PyTuple_GetSlice(PyObject *t, int i, int j);
int       PyTuple_Check(PyObject *);

Python Dictionary Functions

PyObject *PyDict_New();
int       PyDict_Check(PyObject *);
int       PyDict_SetItem(PyObject *p, PyObject *key, PyObject *val);
int       PyDict_SetItemString(PyObject *p, const char *key, PyObject *val);
int       PyDict_DelItem(PyObject *p, PyObject *key);
int       PyDict_DelItemString(PyObject *p, char *key);
PyObject* PyDict_Keys(PyObject *p);
PyObject* PyDict_Values(PyObject *p);
PyObject* PyDict_GetItem(PyObject *p, PyObject *key);
PyObject* PyDict_GetItemString(PyObject *p, const char *key);
int       PyDict_Next(PyObject *p, Py_ssize_t *ppos, PyObject **pkey, PyObject **pvalue);
Py_ssize_t PyDict_Size(PyObject *p);
int       PyDict_Update(PyObject *a, PyObject *b);
int       PyDict_Merge(PyObject *a, PyObject *b, int override);
PyObject* PyDict_Items(PyObject *p);

Python File Conversion Functions

PyObject *PyFile_FromFile(FILE *f);
FILE     *PyFile_AsFile(PyObject *);
int       PyFile_Check(PyObject *);

Abstract Object Interface

write me

33.9 Typemap Examples

This section includes a few examples of typemaps. For more examples, you might look at the files "python.swg" and "typemaps.i" in the SWIG library.

33.9.1 Converting a Python list to a char **

A common problem in many C programs is the processing of command line arguments, which are usually passed in an array of NULL terminated strings. SWIG provides typemaps which allow passing a Python list or tuple - see argcargv.i.

33.9.2 Expanding a Python object into multiple arguments

Suppose that you had a collection of C functions with arguments such as the following:

int foo(int argc, char **argv);

In the previous example, a typemap was written to pass a Python list as the char **argv. This allows the function to be used from Python as follows:

>>> foo(4, ["foo", "bar", "spam", "1"])

Although this works, it's a little awkward to specify the argument count. To fix this, a multi-argument typemap can be defined. This is not very difficult--you only have to make slight modifications to the previous example:

%typemap(in) (int argc, char **argv) {
  /* Check if is a list */
  if (PyList_Check($input)) {
    int i;
    $1 = PyList_Size($input);
    $2 = (char **) malloc(($1+1)*sizeof(char *));
    for (i = 0; i < $1; i++) {
      PyObject *o = PyList_GetItem($input, i);
      if (PyString_Check(o)) {
        $2[i] = PyString_AsString(PyList_GetItem($input, i));
      } else {
        PyErr_SetString(PyExc_TypeError, "list must contain strings");
        SWIG_fail;
      }
    }
    $2[i] = 0;
  } else {
    PyErr_SetString(PyExc_TypeError, "not a list");
    SWIG_fail;
  }
}

%typemap(freearg) (int argc, char **argv) {
  free((char *) $2);
}

When writing a multiple-argument typemap, each of the types is referenced by a variable such as $1 or $2. The typemap code simply fills in the appropriate values from the supplied Python object.

With the above typemap in place, you will find it no longer necessary to supply the argument count. This is automatically set by the typemap code. For example:

>>> foo(["foo", "bar", "spam", "1"])

If your function is overloaded in C++, for example:

int foo(int argc, char **argv);
int foo();

don't forget to also provide a suitable typecheck typemap for overloading such as:

%typecheck(SWIG_TYPECHECK_STRING_ARRAY) (int argc, char **argv) {
  $1 = PyList_Check($input) ? 1 : 0;
}

If you don't you'll get an error message along the lines of:

Traceback (most recent call last):
  File "runme.py", line 3, in >module<
    example.foo(["foo", "bar", "spam", "1"])
TypeError: Wrong number or type of arguments for overloaded function 'foo'.
  Possible C/C++ prototypes are:
    foo(int, char **)
    foo()

33.9.3 Using typemaps to return arguments

A common problem in some C programs is that values may be returned in arguments rather than in the return value of a function. For example:

/* Returns a status value and two values in out1 and out2 */
int spam(double a, double b, double *out1, double *out2) {
  ... Do a bunch of stuff ...
  *out1 = result1;
  *out2 = result2;
  return status;
}

A typemap can be used to handle this case as follows :

%module outarg

// This tells SWIG to treat an double * argument with name 'OutValue' as
// an output value.  We'll append the value to the current result which
// is guaranteed to be a List object by SWIG.

%typemap(argout) double *OutValue {
  PyObject *o, *o2, *o3;
  o = PyFloat_FromDouble(*$1);
  if ((!$result) || ($result == Py_None)) {
    $result = o;
  } else {
    if (!PyTuple_Check($result)) {
      PyObject *o2 = $result;
      $result = PyTuple_New(1);
      PyTuple_SetItem($result, 0, o2);
    }
    o3 = PyTuple_New(1);
    PyTuple_SetItem(o3, 0, o);
    o2 = $result;
    $result = PySequence_Concat(o2, o3);
    Py_DecRef(o2);
    Py_DecRef(o3);
  }
}

int spam(double a, double b, double *OutValue, double *OutValue);

The typemap works as follows. First, a check is made to see if any previous result exists. If so, it is turned into a tuple and the new output value is concatenated to it. Otherwise, the result is returned normally. For the sample function spam(), there are three output values--meaning that the function will return a 3-tuple of the results.

As written, the function must accept 4 arguments as input values, last two being pointers to doubles. If these arguments are only used to hold output values (and have no meaningful input value), an additional typemap can be written. For example:

%typemap(in, numinputs=0) double *OutValue(double temp) {
  $1 = &temp;
}

By specifying numinputs=0, the input value is ignored. However, since the argument still has to be set to some meaningful value before calling C, it is set to point to a local variable temp. When the function stores its output value, it will simply be placed in this local variable. As a result, the function can now be used as follows:

>>> a = spam(4, 5)
>>> print(a)
(0, 2.45, 5.0)
>>> x, y, z = spam(4, 5)
>>>

33.9.4 Mapping Python tuples into small arrays

In some applications, it is sometimes desirable to pass small arrays of numbers as arguments. For example :

extern void set_direction(double a[4]);       // Set direction vector

This too, can be handled used typemaps as follows :

// Grab a 4 element array as a Python 4-tuple
%typemap(in) double[4](double temp[4]) {   // temp[4] becomes a local variable
  int i;
  if (PyTuple_Check($input)) {
    if (!PyArg_ParseTuple($input, "dddd", temp, temp+1, temp+2, temp+3)) {
      PyErr_SetString(PyExc_TypeError, "tuple must have 4 elements");
      SWIG_fail;
    }
    $1 = &temp[0];
  } else {
    PyErr_SetString(PyExc_TypeError, "expected a tuple.");
    SWIG_fail;
  }
}

This allows our set_direction function to be called from Python as follows :

>>> set_direction((0.5, 0.0, 1.0, -0.25))

Since our mapping copies the contents of a Python tuple into a C array, such an approach would not be recommended for huge arrays, but for small structures, this approach works fine.

33.9.5 Mapping sequences to C arrays

Suppose that you wanted to generalize the previous example to handle C arrays of different sizes. To do this, you might write a typemap as follows:

// Map a Python sequence into any sized C double array
%typemap(in) double[ANY](double temp[$1_dim0]) {
  int i;
  if (!PySequence_Check($input)) {
    PyErr_SetString(PyExc_TypeError, "Expecting a sequence");
    SWIG_fail;
  }
  if (PyObject_Length($input) != $1_dim0) {
    PyErr_SetString(PyExc_ValueError, "Expecting a sequence with $1_dim0 elements");
    SWIG_fail;
  }
  for (i =0; i < $1_dim0; i++) {
    PyObject *o = PySequence_GetItem($input, i);
    if (!PyFloat_Check(o)) {
      Py_DecRef(o);
      PyErr_SetString(PyExc_ValueError, "Expecting a sequence of floats");
      SWIG_fail;
    }
    temp[i] = PyFloat_AsDouble(o);
    Py_DecRef(o);
  }
  $1 = &temp[0];
}

In this case, the variable $1_dim0 is expanded to match the array dimensions actually used in the C code. This allows the typemap to be applied to types such as:

void foo(double x[10]);
void bar(double a[4], double b[8]);

Since the above typemap code gets inserted into every wrapper function where used, it might make sense to use a helper function instead. This will greatly reduce the amount of wrapper code. For example:

%{
static int convert_darray(PyObject *input, double *ptr, int size) {
  int i;
  if (!PySequence_Check(input)) {
    PyErr_SetString(PyExc_TypeError, "Expecting a sequence");
    return 0;
  }
  if (PyObject_Length(input) != size) {
    PyErr_SetString(PyExc_ValueError, "Sequence size mismatch");
    return 0;
  }
  for (i =0; i < size; i++) {
    PyObject *o = PySequence_GetItem(input, i);
    if (!PyFloat_Check(o)) {
      Py_DecRef(o);
      PyErr_SetString(PyExc_ValueError, "Expecting a sequence of floats");
      return 0;
    }
    ptr[i] = PyFloat_AsDouble(o);
    Py_DecRef(o);
  }
  return 1;
}
%}

%typemap(in) double [ANY](double temp[$1_dim0]) {
  if (!convert_darray($input, temp, $1_dim0)) {
    SWIG_fail;
  }
  $1 = &temp[0];
}

33.9.6 Pointer handling

Occasionally, it might be necessary to convert pointer values that have been stored using the SWIG typed-pointer representation. Since there are several ways in which pointers can be represented, the following two functions are used to safely perform this conversion:

int SWIG_ConvertPtr(PyObject *obj, void **ptr, swig_type_info *ty, int flags)

PyObject *SWIG_NewPointerObj(void *ptr, swig_type_info *ty, int own)

Both of these functions require the use of a special SWIG type-descriptor structure. This structure contains information about the mangled name of the datatype, type-equivalence information, as well as information about converting pointer values under C++ inheritance. For a type of Foo *, the type descriptor structure is usually accessed as follows:

Foo *f;
if (!SWIG_IsOK(SWIG_ConvertPtr($input, (void **) &f, SWIGTYPE_p_Foo, 0))) {
  SWIG_exception_fail(SWIG_TypeError, "in method '$symname', expecting type Foo");
}

PyObject *obj;
obj = SWIG_NewPointerObj(f, SWIGTYPE_p_Foo, 0);

In a typemap, the type descriptor should always be accessed using the special typemap variable $1_descriptor. For example:

%typemap(in) Foo * {
  if (!SWIG_IsOK(SWIG_ConvertPtr($input, (void **) &$1, $1_descriptor, 0))) {
    SWIG_exception_fail(SWIG_TypeError, "in method '$symname', expecting type Foo");
  }
}

If necessary, the descriptor for any type can be obtained using the $descriptor() macro in a typemap. For example:

%typemap(in) Foo * {
  if (!SWIG_IsOK(SWIG_ConvertPtr($input, (void **) &$1, $descriptor(Foo *), 0))) {
    SWIG_exception_fail(SWIG_TypeError, "in method '$symname', expecting type Foo");
  }
}

Although the pointer handling functions are primarily intended for manipulating low-level pointers, both functions are fully aware of Python proxy classes. Specifically, SWIG_ConvertPtr() will retrieve a pointer from any object that has a this attribute. In addition, SWIG_NewPointerObj() can automatically generate a proxy class object (if applicable).

33.9.7 Memory management when returning references to member variables

This example shows how to prevent premature garbage collection of objects when the underlying C++ class returns a pointer or reference to a member variable. The example is a direct equivalent to this Java equivalent.

Consider the following C++ code:

#include <iostream>
struct Wheel {
  int size;
  Wheel(int sz = 0) : size(sz) {}
  ~Wheel() { std::cout << "~Wheel" << std::endl; }
};

class Bike {
  Wheel wheel;
public:
  Bike(int val) : wheel(val) {}
  Wheel& getWheel() { return wheel; }
};

and the following usage from Python after running the code through SWIG:

bike = Bike(10)
wheel = bike.getWheel()
print("wheel size: {}".format(wheel.size))

del bike  # Allow bike to be garbage collected
print("wheel size: {}".format(wheel.size))

Don't be surprised that if the resulting output gives strange results such as...

wheel size: 10
~Wheel
wheel size: 135019664

What has happened here is the garbage collector has collected the Bike instance as it doesn't think it is needed any more. The proxy instance, wheel, contains a reference to memory that was deleted when the Bike instance was collected. In order to prevent the garbage collector from collecting the Bike instance, a reference to the Bike must be added to the wheel instance.

You can do this by adding the reference when the getWheel() method is called using one of three approaches:

The easier, but less optimized, way is to use the %pythonappend directive (see Adding additional Python code):

%pythonappend getWheel %{
  # val is the Wheel proxy, self is the Bike instance
  val.__bike_reference = self
%}

The code gets appended to the Python code generated for the Bike::getWheel wrapper function, where we store the Bike proxy instance onto the Wheel proxy instance before it is returned to the caller as follows.

class Bike(object):
    ...
    def getWheel(self):
        val = _example.Bike_getWheel(self)

        # val is the Wheel proxy, self is the Bike instance
        val.__bike_reference = self

        return val

The second option, which performs better and is required if you use the -builtin option, is to set the reference in the CPython implementation:

%extend Wheel {
// A reference to the parent class is added to ensure the underlying C++
// object is not deleted while the item is in use
%typemap(ret) Wheel& getWheel {
  PyObject *bike_reference_string = SWIG_Python_str_FromChar("__bike_reference");
  PyObject_SetAttr($result, bike_reference_string, $self);
  Py_DecRef(bike_reference_string);
}
}

The third approach, shown below, is an optimization of the above approach and creates the "__bike_reference" Python string object just once. While this looks more complex, it is just a small variation on the above typemap plus a support function bike_reference() in a fragment called bike_reference_function. The bike_reference_init typemap generates code into the "init" section for an initial call to bike_reference() when the module is initialized and is done to create the "__bike_reference" Python string singleton in a thread-safe manner.

%fragment("bike_reference_init", "init") {
  // Thread-safe initialization - initialize during Python module initialization
  bike_reference();
}

%fragment("bike_reference_function", "header", fragment="bike_reference_init") {

static PyObject *bike_reference() {
  static PyObject *bike_reference_string = SWIG_Python_str_FromChar("__bike_reference");
  return bike_reference_string;
}

}

%extend Wheel {
// A reference to the parent class is added to ensure the underlying C++
// object is not deleted while the item is in use
%typemap(ret, fragment="bike_reference_function") Wheel& getWheel %{
  PyObject_SetAttr($result, bike_reference(), $self);
%}
}

33.10 Docstring Features

Using docstrings in Python code is becoming more and more important and more tools are coming on the scene that take advantage of them, everything from full-blown documentation generators to class browsers and popup call-tips in Python-aware IDEs. Given the way that SWIG generates the proxy code by default, your users will normally get something like "function_name(*args)" in the popup calltip of their IDE which is next to useless when the real function prototype might be something like this:

bool function_name(int x, int y, Foo* foo=NULL, Bar* bar=NULL);

The features described in this section make it easy for you to add docstrings to your modules, functions and methods that can then be used by the various tools out there to make the programming experience of your users much simpler.

33.10.1 Module docstring

Python allows a docstring at the beginning of the .py file before any other statements, and it is typically used to give a general description of the entire module. SWIG supports this by setting an option of the %module directive. For example:

%module(docstring="This is the example module's docstring") example

When you have more than just a line or so then you can retain the easy readability of the %module directive by using a macro. For example:

%define DOCSTRING
"The `XmlResource` class allows program resources defining menus,
layout of controls on a panel, etc. to be loaded from an XML file."
%enddef

%module(docstring=DOCSTRING) xrc

33.10.2 %feature("autodoc")

As alluded to above SWIG will generate all the function and method proxy wrappers with just "*args" (or "*args, **kwargs" if the -keyword option is used) for a parameter list and will then sort out the individual parameters in the C wrapper code. This is nice and simple for the wrapper code, but makes it difficult to be programmer and tool friendly as anyone looking at the .py file will not be able to find out anything about the parameters that the functions accept.

But since SWIG does know everything about the function it is possible to generate a docstring containing the parameter types, names and default values. Since many of the docstring tools are adopting a standard of recognizing if the first thing in the docstring is a function prototype then using that instead of what they found from introspection, then life is good once more.

SWIG's Python module provides support for the "autodoc" feature, which when attached to a node in the parse tree will cause a docstring to be generated that includes the name of the function, parameter names, default values if any, and return type if any. There are also four levels for autodoc controlled by the value given to the feature, %feature("autodoc", "level"). The four values for level are covered in the following sub-sections.

33.10.2.1 %feature("autodoc", "0")

When level "0" is used then the types of the parameters will not be included in the autodoc string. For example, given this function prototype:

%feature("autodoc", "0");
bool function_name(int x, int y, Foo* foo=NULL, Bar* bar=NULL);

Then Python code like this will be generated:

def function_name(*args, **kwargs):
    """function_name(x, y, foo=None, bar=None) -> bool"""
    ...

33.10.2.2 %feature("autodoc", "1")

When level "1" is used then the parameter types will be used in the autodoc string. In addition, an attempt is made to simplify the type name such that it makes more sense to the Python user. Pointer, reference and const info is removed if the associated type is has an associated Python type (%rename's are thus shown correctly). This works most of the time, otherwise a C/C++ type will be used. See the next section for the "docstring" feature for tweaking the docstrings to your liking. Given the example above, then turning on the parameter types with level "1" will result in Python code like this:

def function_name(*args, **kwargs):
    """function_name(int x, int y, Foo foo=None, Bar bar=None) -> bool"""
    ...

33.10.2.3 %feature("autodoc", "2")

Level "2" results in the function prototype as per level "0". In addition, a line of documentation is generated for each parameter using numpydoc style. Using the previous example, the generated code will be:

def function_name(*args, **kwargs):
    """
    function_name(x, y, foo=None, bar=None) -> bool

    Parameters
    ----------
    x: int
    y: int
    foo: Foo *
    bar: Bar *

    """
    ...

Note that the documentation for each parameter is sourced from the "doc" typemap which by default shows the C/C++ type rather than the simplified Python type name described earlier for level "1". Typemaps can of course change the output for any particular type, for example the int x parameter:

%feature("autodoc", "2");
%typemap("doc") int x "$1_name (C++ type: $1_type) -- Input $1_name dimension"
bool function_name(int x, int y, Foo* foo=NULL, Bar* bar=NULL);

resulting in

def function_name(*args, **kwargs):
    """
    function_name(x, y, foo=None, bar=None) -> bool

    Parameters
    ----------
    x (C++ type: int) -- Input x dimension
    y: int
    foo: Foo *
    bar: Bar *

    """

33.10.2.4 %feature("autodoc", "3")

Level "3" results in the function prototype as per level "1" but also contains the same additional line of documentation for each parameter as per level "2". Using our earlier example again, the generated code will be:

def function_name(*args, **kwargs):
    """
    function_name(int x, int y, Foo foo=None, Bar bar=None) -> bool

    Parameters
    ----------
    x: int
    y: int
    foo: Foo *
    bar: Bar *

    """
    ...

33.10.2.5 %feature("autodoc", "docstring")

Finally, there are times when the automatically generated autodoc string will make no sense for a Python programmer, particularly when a typemap is involved. So if you give an explicit value for the autodoc feature then that string will be used in place of the automatically generated string. For example:

%feature("autodoc", "GetPosition() -> (x, y)") GetPosition;
void GetPosition(int* OUTPUT, int* OUTPUT);

33.10.3 %feature("docstring")

In addition to the autodoc strings described above, you can also attach any arbitrary descriptive text to a node in the parse tree with the "docstring" feature. When the proxy module is generated then any docstring associated with classes, function or methods are output. If an item already has an autodoc string then it is combined with the docstring and they are output together. If the docstring is all on a single line then it is output like this::

"""This is the docstring"""

Otherwise, to aid readability it is output like this:

"""
This is a multi-line docstring
with more than one line.
"""

33.10.4 Doxygen comments

Please see the separate Doxygen chapter for information on making use of C++ Doxygen comments and translating them into Python docstring comments.

Note that when generating docstrings and Doxygen comments have also been turned on, the docstring feature will take precedence over a Doxygen comment. If the autodoc feature is also turned on, then it will be used in conjunction with the docstring feature. However, if there is no docstring feature present and there is a Doxygen comment, then the autodoc docstring will not be generated. The Doxygen comment alone will be used.

This way, if the autodoc feature is specified globally it will fill in any missing Doxygen documentation comments. Doxygen comments can be overridden by using the docstring feature.

33.11 Python Packages

Python has concepts of modules and packages. Modules are separate units of code and may be grouped together to form a package. Packages may be nested, that is they may contain subpackages. This leads to tree-like hierarchy, with packages as intermediate nodes and modules as leaf nodes.

The hierarchy of Python packages/modules follows the hierarchy of *.py files found in a source tree (or, more generally, in the Python path). Normally, the developer creates new module by placing a *.py file somewhere under Python path; the module is then named after that *.py file. A package is created by placing an __init__.py file within a directory; the package is then named after that directory. For example, the following source tree:

mod1.py
pkg1/__init__.py
pkg1/mod2.py
pkg1/pkg2/__init__.py
pkg1/pkg2/mod3.py

defines the following Python packages and modules:

pkg1            # package
pkg1.pkg2       # package
mod1            # module
pkg1.mod2       # module
pkg1.pkg2.mod3  # module

The purpose of an __init__.py file is two-fold. First, the existence of __init__.py in a directory informs the Python interpreter that this directory contains a Python package. Second, the code in __init__.py is loaded/executed automatically when the package is initialized (when it or its submodule/subpackage gets import'ed). By default, SWIG generates proxy Python code – one *.py file for each *.i interface. The __init__.py files, however, are not generated by SWIG. They should be created by other means. Both files (module *.py and __init__.py) should be installed in appropriate destination directories in order to obtain a desirable package/module hierarchy.

Python3 adds another option for packages with PEP 0420 (implicit namespace packages). Implicit namespace packages no longer use __init__.py files. SWIG generated Python modules support implicit namespace packages. See Implicit namespace packages for more information.

You can place a SWIG generated module into a Python package or keep as a global module, details are covered a little later in Location of modules.

The way Python defines its modules and packages impacts SWIG users. Some users may need to use special features such as the package option in the %module directive or import related command line options. These are explained in the following sections.

33.11.1 Setting the Python package

Using the package option in the %module directive allows you to specify a Python package that the module will be in when installed.

%module(package="wx") xrc

This is useful when the .i file is %imported by another .i file. By default SWIG will assume that the importer is able to find the importee with just the module name, but if they live in separate Python packages then this won't work. However if the importee specifies what its package is with the %module option then the Python code generated for the importer will use that package name when importing the other module and in base class declarations, etc..

SWIG assumes that the package option provided to %module together with the module name (that is, wx.xrc in the above example) forms a fully qualified (absolute) name of a module (in Python terms). This is important especially for Python 3, where absolute imports are used by default. It's up to you to place the generated module files (.py, .so) in appropriate subdirectories. For example, if you have an interface file foo.i with:

%module(package="pkg1.pkg2") foo

then the resulting directory layout should be

pkg1/
pkg1/__init__.py
pkg1/pkg2/__init__.py
pkg1/pkg2/foo.py        # (generated by SWIG)
pkg1/pkg2/_foo.so       # (shared library built from C/C++ code generated by SWIG)

33.11.2 Absolute and relative imports

Suppose, we have the following hierarchy of files:

pkg1/
pkg1/__init__.py
pkg1/mod2.py
pkg1/pkg2/__init__.py
pkg1/pkg2/mod3.py

Let the contents of pkg1/pkg2/mod3.py be

class M3: pass

We edit pkg1/mod2.py and want to import module pkg1/pkg2/mod3.py in order to derive from class M3. We can write appropriate Python code in several ways, for example:

1. Using "import <>" syntax with absolute package name:

   # pkg1/mod2.py
   import pkg1.pkg2.mod3
   class M2(pkg1.pkg2.mod3.M3): pass
   

2. Using "import <>" syntax with package name relative to pkg1 (only in Python 2.7 and earlier):

   # pkg1/mod2.py
   import pkg2.mod3
   class M2(pkg2.mod3.M3): pass
   

3. Using "from <> import <>" syntax (relative import syntax, only in Python 2.5 and later):

   # pkg1/mod2.py
   from .pkg2 import mod3
   class M2(mod3.M3): pass
   

4. Other variants, for example the following construction in order to have the pkg2.mod3.M3 symbol available in mod2 as in point 2 above (but now under Python 3):

   # pkg1/mod2.py
   from . import pkg2
   from .pkg2 import mod3
   class M2(pkg2.mod3.M3): pass
   

Now suppose we have mod2.i with

// mod2.i
%module (package="pkg1") mod2
%import "mod3.i"
// ...

and mod3.i with

// mod3.i
%module (package="pkg1.pkg2") mod3
// ...

By default, SWIG will generate mod2.py proxy file with import directive as in point 1. This can be changed with the -relativeimport command line option. The -relativeimport instructs SWIG to organize imports as in point 4 for Python 2.7.0 and newer.

Compatibility Note: Versions of SWIG prior to SWIG-4.0.0 supported Python < 2.7.0 and would organize the imports as in point 2 if an older version of Python was detected at runtime.

In short, if you have mod2.i and mod3.i as above, then without -relativeimport SWIG will write

import pkg1.pkg2.mod3

to mod2.py proxy file, and with -relativeimport it will write

from . import pkg2
from .pkg2 import mod3

You should avoid using relative imports and use absolute ones whenever possible. There are some cases, however, when relative imports may be necessary. The first example is, when some (legacy) Python code refers entities imported by proxy files generated by SWIG, and it assumes that the proxy file uses relative imports. Second case is, when one puts import directives in __init__.py to import symbols from submodules or subpackages and the submodule depends on other submodules (discussed later).

33.11.3 Enforcing absolute import semantics

As you may know, there is an incompatibility in import semantics (for the import <> syntax) between Python 2 and 3. In Python 2.4 and earlier it is not clear whether

import foo

refers to a top-level module or to another module inside the current package. In Python 3 it always refers to a top-level module (see PEP 328). To instruct Python 2.5 through 2.7 to use new semantics (that is import foo is interpreted as absolute import), one has to put the following line

from __future__ import absolute_import

at the very beginning of your proxy *.py file. In SWIG, it may be accomplished with %pythonbegin directive as follows:

%pythonbegin %{
from __future__ import absolute_import
%}

33.11.4 Importing from __init__.py

Imports in __init__.py are handy when you want to populate a package's namespace with names imported from other modules. In SWIG based projects this approach may also be used to split large pieces of code into smaller modules, compile them in parallel and then re-assemble everything at runtime by importing submodules' contents in __init__.py, for example.

Unfortunately import directives in __init__.py may cause problems, especially if they refer to a package's submodules. This is caused by the way Python initializes packages. If you spot problems with imports from __init__.py try using -relativeimport option. Below we explain in detail one issue, for which the -relativeimport workaround may be helpful.

Consider the following example (Python 3):

pkg1/__init__.py        # (empty)
pkg1/pkg2/__init__.py   # (imports something from bar.py)
pkg1/pkg2/foo.py
pkg1/pkg2/bar.py        # (imports foo.py)

If the file contents are:

• pkg1/pkg2/__init__.py:

  # pkg1/pkg2/__init__.py
  from .bar import Bar
  

• pkg1/pkg2/foo.py:

  # pkg1/pkg2/foo.py
  class Foo: pass
  

• pkg1/pkg2/bar.py:

  # pkg1/pkg2/bar.py
  import pkg1.pkg2.foo
  class Bar(pkg1.pkg2.foo.Foo): pass
  

Now if one simply used import pkg1.pkg2, it will usually fail:

>>> import pkg1.pkg2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "./pkg1/pkg2/__init__.py", line 2, in <module>
    from .bar import Bar
  File "./pkg1/pkg2/bar.py", line 3, in <module>
    class Bar(pkg1.pkg2.foo.Foo): pass
AttributeError: 'module' object has no attribute 'pkg2'

Surprisingly, if we execute the import pkg1.pkg2 directive for the second time, it succeeds. The reason seems to be following: when Python spots the from .bar import Bar directive in pkg1/pkg2/__init__.py it starts loading pkg1/pkg2/bar.py. This module imports pkg1.pkg2.foo in turn and tries to use pkg1.pkg2.foo.Foo, but the package pkg1 is not fully initialized yet (the initialization procedure is actually in progress) and it seems like the effect of the already seen import pkg1.pkg2.pkg3.foo is "delayed" or ignored. Exactly the same may happen to a proxy module generated by SWIG.

One workaround for this case is to use a relative import in pkg1/pkg2/bar.py. If we change bar.py to be:

from .pkg3 import foo
class Bar(foo.Foo): pass

or

from . import pkg3
from .pkg3 import foo
class Bar(pkg3.foo.Foo): pass

then the example works again. With SWIG, you need to enable the -relativeimport option in order to have the above workaround in effect (note, that the Python 2 case also needs the -relativeimport workaround).

33.11.5 Implicit namespace packages

Python 3.3 introduced PEP 0420 which implements implicit namespace packages. In a nutshell, implicit namespace packages remove the requirement of an __init__.py file and allow packages to be split across multiple PATH elements. For example:

/fragment1/pkg1/mod1.py
/fragment2/pkg1/mod2.py
/fragment3/pkg1/mod3.py

If PYTHONPATH is set to "/fragment1:/fragment2:/fragment3", then mod1, mod2 and mod3 will be part of pkg1. This allows for splitting of packages into separate pieces. This can be useful for SWIG generated wrappers in the following way.

Suppose you create a SWIG wrapper for a module called robin. The SWIG generated code consists of two files robin.py and _robin.so. You wish to make these modules part of a subpackage (brave.sir). With implicit namespace packages you can place these files in the following configurations:

Using PYTHONPATH="/some/path"

/some/path/brave/sir/robin.py
/some/path/brave/sir/_robin.so

Using PYTHONPATH="/some/path:/some/other/path"

/some/path/brave/sir/robin.py
/some/other/path/brave/sir/_robin.so

Finally suppose that your pure Python code is stored in a .zip file or some other way (database, web service connection, etc). Python can load the robin.py module using a custom importer. But the _robin.so module will need to be located on a file system. Implicit namespace packages make this possible. For example, using PYTHONPATH="/some/path/foo.zip:/some/other/path"

Contents of foo.zip

brave/
brave/sir/
brave/sir/robin.py

File system contents

/some/other/path/brave/sir/_robin.so

Support for implicit namespace packages was added to python-3.3. The zipimporter requires python-3.5.1 or newer to work with subpackages.

Compatibility Note: Support for implicit namespace packages was added in SWIG-3.0.9.

33.11.6 Location of modules

When SWIG creates wrappers from an interface file, say foo.i, two Python modules are created. There is a pure Python module (foo.py) and C/C++ code which is compiled and linked into a dynamically (or statically) loaded low-level module _foo (see the Preliminaries section for details). So, the interface file really defines two Python modules. How these two modules are loaded is covered next.

The pure Python module needs to load the C/C++ module in order to call the wrapped C/C++ methods. To do this it must make some assumptions about the location of the C/C++ module. There are two configurations that are supported by default.

1. Both modules in the same package

2. Both modules are global

Additional configurations are supported but require custom import code.

The following sub-sections look more closely at the two default configurations as well as some customized configurations. An input interface file, foo.i, results in the two modules foo.py and _foo.so for each of the configurations.

33.11.6.1 Both modules in the same package

In this configuration, the pure Python module, foo.py, tries to load the C/C++ module, _foo, from the same package foo.py is located in. The package name is determined from the __package__ attribute if available, see PEP 366. Otherwise it is derived from the __name__ attribute given to foo.py by the Python loader that imported foo.py. The interface file for this configuration would contain:

%module(package="mypackage") foo

The location of the files could be as follows:

/dir/mypackage/foo.py
/dir/mypackage/__init__.py
/dir/mypackage/_foo.so

Assuming /dir/ is in PYTHONPATH, the module can be imported using

from mypackage import foo

33.11.6.2 Both modules are global

In this configuration, there are no packages. If foo.py is not in a package, that is, it is a global module, then _foo is loaded as a global module. The interface file for this configuration would contain:

%module foo

The location of the files could be as follows:

/dir/foo.py
/dir/_foo.so

Assuming /dir/ is in PYTHONPATH, the module can be imported using

import foo

33.11.6.3 Split modules custom configuration

In this non-standard 'split module' configuration, the pure Python module is in a package and the low level C/C++ module is global. This configuration is not generally recommended and is not supported by default as it needs a custom configuration. The module import code customization required is via the moduleimport attribute in the %module directive. The next sub-section elaborates further on this. The interface file for this split module configuration would contain:

%module(package="mypackage", moduleimport="import _foo") foo

When using -builtin, use the following instead (the reasons are also covered shortly in the next sub-section):

%module(package="mypackage", moduleimport="from _foo import *") foo

The location of the files could be as follows:

/dir/mypackage/foo.py
/dir/mypackage/__init__.py
/dir/_foo.so

Assuming /dir/ is in PYTHONPATH, the module can be imported using

from mypackage import foo

Compatibility Note: Versions of SWIG prior to SWIG-4.0.0 supported split modules without the above customization. However, this had to be removed as the default import code often led to confusion due to obfuscation of genuine Python ImportError problems. Using one of the two default configurations is the recommended approach now.

33.11.6.4 More on customizing the module import code

The Python code implementing the default import logic is shown below. It supports the two configurations described earlier, that is, either both modules are in a package or loading both as global modules. The code is generated into the pure Python module, foo.py, and merely imports the low-level _foo module.

if __package__ or '.' in __name__:
    from . import _foo
else:
    import _foo

This import code implementation is non-trivial but it can be replaced with custom code providing opportunities to make it simpler and/or more flexible. This is not normally recommended though unless you have a good understanding of the intricacies of importing Python modules. The custom code can be specified by setting the moduleimport option of the %module directive with the appropriate import code. For example:

%module(moduleimport="import _foo") foo

This will replace the default import logic above and generate the following into the pure Python module, foo.py:

import _foo

In fact the above is a simplification customization for the configuration where both modules are global; it removes the logic for also handling the modules being in a package.

There is a special variable, $module, which is expanded into the low-level C/C++ module name, _foo in the case above. The identical output would be generated if instead the following had been used:

%module(moduleimport="import $module") foo

When you have many lines you can retain the easy readability of the %module directive by using a macro. For example:

%define MODULEIMPORT
"
print('Loading low-level module $module')
import $module
print('Module has loaded')
"
%enddef

%module(moduleimport=MODULEIMPORT) foo

This will of course generate the following into the pure Python module:

print('Loading low-level module $module')
import _foo
print('Module has loaded')

When using the -builtin option, the link between the pure Python module and the low-level C/C++ module is slightly different as all the objects from the low-level module are imported directly into the pure Python module. The default import loading code is thus different:

if __package__ or '.' in __name__:
    from ._foo import *
else:
    from _foo import *

Any customizations must import the code in a similar manner. The best way to support both with and without -builtin is to make use of the SWIGPYTHON_BUILTIN macro which is defined when -builtin is specified. The following will do this for the split modules case above.

#if defined(SWIGPYTHON_BUILTIN) /* defined when using -builtin */
%module(package="mypackage", moduleimport="from $module import *") foo
#else
%module(package="mypackage", moduleimport="import $module") foo
#endif

33.11.6.5 Statically linked C modules

It is strongly recommended to use dynamically linked modules for the C portion of your pair of Python modules. If for some reason you still need to link the C module of the pair of Python modules generated by SWIG into your interpreter, then this section provides some details on how this impacts the pure Python modules ability to locate the other part of the pair. Please also see the Static Linking section.

When Python is extended with C code the Python interpreter needs to be informed about details of the new C functions that have been linked into the executable. The code to do this is created by SWIG and is automatically called in the correct way when the module is dynamically loaded. However when the code is not dynamically loaded (because it is statically linked) Then the initialization method for the module created by SWIG is not called automatically and the Python interpreter has no idea that the new SWIG C module exists.

Before Python 3, one could simply call the init method created by SWIG which would have normally been called when the shared object was dynamically loaded. The specific name of this method is not given here because statically linked modules are not encouraged with SWIG (Static Linking). However one can find this init function in the C file generated by SWIG.

If you are really keen on static linking there are two ways to initialize the SWIG generated C module with the init method. Which way you use depends on what version of Python your module is being linked with. Python 2 and Python 3 treat this init function differently. And the way they treat it affects how the pure Python module will be able to locate the C module.

The details concerning this are covered completely in the documentation for Python itself. Links to the relevant sections follow:

• Extending in python2
• Extending in python3

There are two keys things to understand. The first is that in Python 2 the init() function returns void. In Python 3 the init() function returns a PyObject * which points to the new module. Secondly, when you call the init() method manually, you are the Python importer. So, you determine which package the C module will be located in.

So, if you are using Python 3 it is important that you follow what is described in the Python documentation linked above. In particular, you can't simply call the init() function generated by SWIG and cast the PyObject pointer it returns over the side. If you do then Python 3 will have no idea that your C module exists and the pure Python half of your wrapper will not be able to find it. You need to register your module with the Python interpreter as described in the Python docs.

With Python 2 things are somewhat more simple. In this case the init() function returns void. Calling it will register your new C module as a global module. The pure Python part of the SWIG wrapper will be able to find it because it tries both the pure Python module it is part of and the global module. If you wish not to have the statically linked module be a global module then you will either need to refer to the Python documentation on how to do this (remember you are now the Python importer) or use dynamic linking.

33.12 Python 3 Support

SWIG is able to support Python 3.x. The wrapper code generated by SWIG can be compiled with both Python 2.x or 3.x.

The list of known-to-be-broken features around Python 3 are:

• No more support for FILE* typemaps, because PyFile_AsFile has been dropped in Python 3.
• The -apply command line option is removed and generating code using apply() is no longer supported.

The following are Python 3 new features that are currently supported by SWIG.

33.12.1 Python function annotations and variable annotations

Python 3 supports function annotations as defined in PEP 3107. Python 3.6 and later additionally support variable annotations as defined in PEP 526. Note that currently there is no annotations support in SWIG for the -builtin nor the -fastproxy option. Annotations are added via the python:annotations %feature directives. SWIG currently supports one type of function annotation.

33.12.1.1 C/C++ annotation types

The %feature("python:annotations", "c") directive generates annotations containing C/C++ types. For example:

%feature("python:annotations", "c") global_ints;
int *global_ints(int &ri);

The generated code then contains function annotations containing the C++ types:

def global_ints(ri: "int &") -> "int *":
    return _example.global_ints(ri)

There are some limitations with function annotations support, for example, overloaded functions use *args or **kwargs when keyword arguments are enabled. The parameter names and types are then not shown. For example, with input:

int *global_overloaded(int &ri);
int *global_overloaded();

The generated Python function including annotations is shown below. Only the return type is annotated.

def global_overloaded(*args) -> "int *":
    return _example.global_overloaded(*args)

Below is an example demonstrating variable annotations.

%feature("python:annotations", "c");

struct V {
  float val;
};

The generated code contains a variable annotation containing the C float type:

class V(object):
    val: "float" = property(_example.V_val_get, _example.V_val_set)
    ...

Variable annotations are only supported from Python 3.6. If you need to support earlier versions of Python, you'll need to turn variable annotations off via the python:annotations:novar feature flag. It is quite easy to support function annotations but turn off variable annotations. The next example shows how to do this for all variables.

%feature("python:annotations", "c");  // Turn on function annotations and variable annotations globally
%feature("python:annotations:novar"); // Turn off variable annotations globally

struct V {
  float val;
  void vv(float *v) const;
};

The resulting code will work with versions older than Python 3.6 as the variable annotations are turned off:

class V(object):
    val = property(_example.V_val_get, _example.V_val_set)

    def vv(self, v: "float *") -> "void":
        return _example.V_vv(self, v)
    ...

Compatibility Note: SWIG-4.1.0 changed the way that function annotations are generated. Prior versions required the (now removed) -py3 option to generate function annotation support containing C/C++ types instead of supporting %feature("python:annotations", "c"). Variable annotations were also added in SWIG-4.1.0.

33.12.2 Buffer interface

SWIG has a series of typemaps to support buffer interfaces. These typemap macros are defined in pybuffer.i, which must be included in order to use them. By using these typemaps, your wrapped function will be able to accept any Python object that exposes a suitable buffer interface.

For example, the get_path() function puts the path string into the memory pointed to by its argument:

void get_path(char *s);

Then you can write a typemap like this: (the following example is applied to both Python 2 and 3, since the bytearray type was backported to 2.6.

%include <pybuffer.i>
%pybuffer_mutable_string(char *str);
void get_path(char *str);

And then on the Python side the wrapped get_path could be used in this way:

>>> p = bytearray(10)
>>> get_path(p)
>>> print(p)
bytearray(b'/Foo/Bar/\x00')

The macros defined in pybuffer.i are similar to those in cstring.i:

%pybuffer_mutable_binary(parm, size_parm)

%pybuffer_mutable_binary(char *str, size_t size);
...
int snprintf(char *str, size_t size, const char *format, ...);

>>> buf = bytearray(6)
>>> snprintf(buf, "Hello world!")
>>> print(buf)
bytearray(b'Hello\x00')
>>>

%pybuffer_mutable_string(parm)

%pybuffer_mutable_string(char *str);
...
size_t make_upper(char *str);

>>> buf = bytearray(b'foo\x00')
>>> make_upper(buf)
>>> print(buf)
bytearray(b'FOO\x00')
>>>

%pybuffer_binary(parm, size_parm)

%pybuffer_string(parm)

33.12.3 Abstract base classes

By including pyabc.i in your interface file, the proxy classes of the STL containers will automatically gain an appropriate abstract base class from the collections.abc module for Python 3.3 and later, otherwise from the collections module. For example, the following SWIG interface:

%include <pyabc.i>
%include <std_map.i>
%include <std_list.i>

namespace std {
  %template(Mapii) map<int, int>;
  %template(IntList) list<int>;
}

will generate a Python proxy class Mapii inheriting from collections.abc.MutableMap for Python 3.3 and later, or collections.MutableMap for earlier versions and a proxy class IntList inheriting from collections.abc.MutableSequence for Python 3.3 or later, or collections.MutableSequence for earlier versions.

pyabc.i also provides a macro %pythonabc that could be used to define an abstract base class for your own C++ class:

%pythonabc(MySet, collections.abc.MutableSet); # Python 3.3 and later
%pythonabc(MySet, collections.MutableSet);     # Prior to Python 3.3
%pythonabc(MySet, "collections.abc.MutableSet if _swig_python_version_info >= (3, 3) else collections.MutableSet"); # All Python versions

For details of abstract base class, please see PEP 3119.

Compatibility Note: SWIG-4.0.0 changed the base classes to use the collections.abc module instead of collections due to the deprecation of the classes in the collections module in Python 3.7. The collections.abc module was introduced in Python 3.3 and hence this feature requires Python 3.3 or later. SWIG-4.1.0 introduced the flexibility of using either the collections.abc module for Python 3.3 and later or the collections module for earlier Python versions.

33.12.4 Byte string output conversion

By default, any byte string (char* or std::string) returned from C or C++ code is decoded to text as UTF-8. This decoding uses the surrogateescape error handler under Python 3.1 or higher -- this error handler decodes invalid byte sequences to high surrogate characters in the range U+DC80 to U+DCFF. As an example, consider the following SWIG interface, which exposes a byte string that cannot be completely decoded as UTF-8:

%module example

%inline %{

const char * non_utf8_c_str(void) {
  return "h\xe9llo w\xc3\xb6rld";
}

void instring(const char *s) {
  ...
}

%}

Note that "\xe9" is an invalid UTF-8 encoding, but "\xc3\xb6" is valid. When this method is called from Python 3, the return value is the following text string:

>>> s = example.non_utf8_c_str()
>>> s
'h\udce9llo wörld'

Since the C string contains bytes that cannot be decoded as UTF-8, those raw bytes are represented as high surrogate characters that can be used to obtain the original byte sequence:

>>> b = s.encode('utf-8', errors='surrogateescape')
>>> b
b'h\xe9llo w\xc3\xb6rld'

One can then attempt a different encoding, if desired (or simply leave the byte string as a raw sequence of bytes for use in binary protocols):

>>> b.decode('latin-1')
'héllo wörld'

Note, however, that text strings containing surrogate characters are rejected with the default strict codec error handler. For example:

>>> with open('test', 'w') as f:
...     print(s, file=f)
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
UnicodeEncodeError: 'utf-8' codec can't encode character '\udce9' in position 1: surrogates not allowed

This requires the user to check most strings returned by SWIG bindings, but the alternative is for a non-UTF8 byte string to be completely inaccessible in Python 3 code.

For more details about the surrogateescape error handler, please see PEP 383.

When Python 3 strings are passed to the C/C++ layer, they are expected to be valid UTF8 Unicode strings too. For example, when the instring method above is wrapped and called, any invalid UTF8 Unicode code strings will result in a TypeError because the attempted conversion fails:

>>> example.instring('h\xe9llo')
>>> example.instring('h\udce9llo')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: in method 'instring', argument 1 of type 'char const *'

In some cases, users may wish to instead handle all byte strings as bytes objects in Python 3. This can be accomplished by adding SWIG_PYTHON_STRICT_BYTE_CHAR to the generated code:

%module char_to_bytes
%begin %{
#define SWIG_PYTHON_STRICT_BYTE_CHAR
%}

char *charstring(char *s) {
  return s;
}

This will modify the behavior so that only Python 3 bytes objects will be accepted and converted to a C/C++ string, and any string returned from C/C++ will be converted to a bytes object in Python 3:

>>> from char_to_bytes import *
>>> charstring(b"hi") # Byte string
b'hi'
>>> charstring("hi")  # Unicode string
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: in method 'charstring', argument 1 of type 'char *'

Note that in Python 2, defining SWIG_PYTHON_STRICT_BYTE_CHAR has no effect, since strings in Python 2 are equivalent to Python 3 bytes objects. However, there is a similar capability to force unicode-only handling for wide characters C/C++ strings (wchar_t * or std::wstring types) in Python 2. By default, in Python 2 both strings and unicode strings are converted to C/C++ wide strings, and returned wide strings are converted to a Python unicode string. To instead only convert unicode strings to wide strings, users can add SWIG_PYTHON_STRICT_UNICODE_WCHAR to the generated code:

%module wchar_to_unicode
%begin %{
#define SWIG_PYTHON_STRICT_UNICODE_WCHAR
%}

wchar_t *wcharstring(wchar_t *s) {
  return s;
}

This ensures that only unicode strings are accepted by wcharstring in both Python 2 and Python 3:

>>> from wchar_to_unicode import *
>>> wcharstring(u"hi") # Unicode string
u'hi'
>>> wcharstring(b"hi") # Byte string
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: in method 'charstring', argument 1 of type 'wchar_t *'

By defining both SWIG_PYTHON_STRICT_BYTE_CHAR and SWIG_PYTHON_STRICT_UNICODE_WCHAR, Python wrapper code can support overloads taking both std::string (as Python bytes) and std::wstring (as Python unicode).

33.12.5 Python 2 Unicode

A Python 3 string is a Unicode string so by default a Python 3 string that contains Unicode characters passed to C/C++ will be accepted and converted to a C/C++ string (char * or std::string types). A Python 2 string is not a unicode string by default and should a Unicode string be passed to C/C++ it will fail to convert to a C/C++ string (char * or std::string types). The Python 2 behavior can be made more like Python 3 by defining SWIG_PYTHON_2_UNICODE when compiling the generated C/C++ code. By default when the following is wrapped:

%module unicode_strings
char *charstring(char *s) {
  return s;
}

An error will occur when using Unicode strings in Python 2:

>>> from unicode_strings import *
>>> charstring("hi")
'hi'
>>> charstring(u"hi")
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: in method 'charstring', argument 1 of type 'char *'

When the SWIG_PYTHON_2_UNICODE macro is added to the generated code:

%module unicode_strings
%begin %{
#define SWIG_PYTHON_2_UNICODE
%}

char *charstring(char *s) {
  return s;
}

Unicode strings will be successfully accepted and converted from UTF-8, but note that they are returned as a normal Python 2 string:

>>> from unicode_strings import *
>>> charstring("hi")
'hi'
>>> charstring(u"hi")
'hi'
>>>

Note that defining both SWIG_PYTHON_2_UNICODE and SWIG_PYTHON_STRICT_BYTE_CHAR at the same time is not allowed, since the first is allowing unicode conversion and the second is explicitly prohibiting it.

33.13 Support for Multithreaded Applications

By default, SWIG does not enable support for multithreaded Python applications. More specifically, the Python wrappers generated by SWIG will not release the Python's interpreter's Global Interpreter Lock (GIL) when wrapped C/C++ code is entered. Hence, while any of the wrapped C/C++ code is executing, the Python interpreter will not be able to run any other threads, even if the wrapped C/C++ code is waiting in a blocking call for something like network or disk IO. Fortunately, SWIG does have the ability to enable multithreaded support and automatic release of the GIL either for all wrapped code in a module or on a more selective basis. The user interface for this is described in the next section.

33.13.1 UI for Enabling Multithreading Support

The user interface is as follows:

1. Module thread support can be enabled in two ways:

   • The -threads SWIG Python option at the command line (or in setup.py):

     $ swig -python -threads example.i

   • The threads module option in the *.i template file:

     %module(threads="1")

2. You can disable thread support for a given method:

   %feature("nothread") method;

   or

   %nothread method;

3. You can partially disable thread support for a given method:

   • To disable the C++/Python thread protection:

     %feature("nothreadblock") method;

     or

     %nothreadblock method;

   • To disable the Python/C++ thread protection

     %feature("nothreadallow") method;

     or

     %nothreadallow method;

33.13.2 Multithread Performance

For the curious about performance, here are some numbers for the profiletest.i test, which is used to check the speed of the wrapped code:

Fully threaded code decreases the wrapping performance by around 60%. If that is important to your application, you can tune each method using the different 'nothread', 'nothreadblock' or 'nothreadallow' features as needed. Note that for some methods deactivating the 'thread block' or 'thread allow' code is not an option, so, be careful.