Programs: Extending Embedded Python with SWIG

2002

I wrote a C++ application that embeds the Python programming language. My Python code needs to access some C++ functionality. Thus, I extend embedded Python. This involves a lot of wrapper code.

The SWIG tool generates such wrapper code. Its documentation discusses how you can use this wrapped code

• as an import-able Python module; or
• in a re-compiled python executable.

...but not how to smush it into some other executable. Here's what I came up with when I scrounged in the source:

SWIG's generated _wrap.c defines a function init_module(), where module is the module name. E.g., if your .i file has a line "%module swig", this function is named init_swig. Call this function to create the module.

extern "C" {  // I'm writing a C++ app, but initswig() is C.
    void init_swig(void); // we get this from the swig-generated file.
}
...
    Py_Initialize();
    init_swig(); 

That's it! If the .i file defines, e.g., a function HitWallP() in module swig, my Python code can now use swig.HitWallP().

My test code shows:

• code in scripty.cc loads and uses
  
• SWIG-generated wrappers based on swig.i which use
• code in mappy.cc.

Notes & Disclaimers:

• I'm not sure that SWIG is the best tool for easing extending embedded Python. I'm trying to find out. (Update, a few weeks later: I like SWIG. Using SWIG is much nicer than writing my own boilerplate code.)
• I'm using stable Debian Linux. Oh, except that I had to use an older version of their python2.2 module to stay compatible with the python2.2-dev module. (And I switched to using the then-bleeding-edge SWIG version 1.3.17 after starting out with the stable-Debian 1.3.11.)
• If you're using an older version of SWIG (e.g., version 1.3.11), that init_module() function will be called initmodule() instead.
• If you're using the C++ shadow class feature with an older version of SWIG, you'll also need to import the generated .py file:

      Py_Initialize();
      initswigc(); // shadow class feature added 'c' suffix to my C-module name
      PyRun_SimpleString("import swigc \n");
      PyRun_SimpleString("import sys \n");
      PyRun_SimpleString("sys.path.append('./') \n");
      PyRun_SimpleString("import swig \n"); // import the generated .py file
  

  There are many ways to import such a thing. Use your favorite.

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