Re: [boost] Boost::Python and Swig

From: John Hunter (jdhunter_at_[hidden])
Date: 2001-12-29 18:17:01

>>>>> "David" == David Abrahams <david.abrahams_at_[hidden]> writes:

    David> Hi John, What you want to do is certainly possible. If you
    David> know how to verify that a given Python object holds a
    David> wrapped wxDateTime, and extract a reference to that
    David> wxDateTime object, then you have everything you need. You
    David> just need to declare and define the appropriate
    David> from_python(PyObject, type<...>) functions 

Ok, thanks, that example was very helpful.  I learned enough about
SWIG to get everything working.  In a nutshell, if Foo is a SWIG
wrapped object, you need

BOOST_PYTHON_BEGIN_CONVERSION_NAMESPACE

extern "C" char * SWIG_GetPtrObj(PyObject *obj, void **ptr, char *type);
Foo from_python(PyObject* p, python::type<const Foo&>) {
  Foo *pf;
  if (SWIG_GetPtrObj(p, (void **) &pf, "_Foo_p")) {
    PyErr_SetString(PyExc_TypeError, "Type error in Bar. Expected a Foo.");
    throw python::error_already_set();
  }
  
  return *pf;
}

BOOST_PYTHON_END_CONVERSION_NAMESPACE

And then link the boost::python extension with the SWIG runtime
library -lswigpy.

As I was figuring this out, I put together some example files to
illustrate how to do this.  I will include them as a tar file with
this email for anyone else needing to do this.

Thanks for your help.

John Hunter

Here is the README included in the tar file:

MOTIVATION:

  The files here show how to use python extensions built with swig and
  boost together.  There are two simple classes, Foo and Bar.  Bar
  takes a Foo in it's constructor:

     Bar(const Foo& f)

  The goal is to take a Foo extension built with SWIG and pass it to a
  Bar constructor built with boost::python.  The motivation for this
  is that some extensive class libraries (such as wx windows) have
  python extensions built with SWIG.  I would like to use those python
  extensions with the extensions I build with boost::python.

  In other words, I want to be able to do this:

    from swigmod import Foo    #Foo was exposed with SWIG
    from boostmod import Bar   #Bar was exposed with Boost::Python

    f = Foo(3)
    b = Bar(f)
    print b.whats_foo_got()  #should print 3

FILES:

  Makefile          - The flags, required libs, etc...
  Foo.h             - The Foo class
  mixedmod.cpp      - The Bar boost::python extension with 
                      from_python for SWIG wrapped objects
  swigmod.i         - The Foo SWIG extension 
  test_swigmod.py   - Test the SWIG extension
  test_mixed.py     - Test the SWIG and boost extensions together

USE:

  Edit the Makefile to point to your SWIG Makefile.template and your
  boost and python distributions.  

    make swig   - Build the SWIG extension
    make mixed  - Build the boost::python extension
    make clean  - You know
    
  Each of the test files output '3' if all is well.