Re: [Swig-user] Passing a wrapped C++ object to a python callback

[Ben W. <be...@sa...>]

John Pye wrote:
> I'm having trouble using Py_BuildValue to pass a SWIG-wrapped *C++ 
> object* back to my python callback function. It says in the python docs 
> that I can pass a PyObject using the "o" string, but I'm not sure how I 
> can cast my C++ object to a PyObject from inside C++. What's the correct 
> way of doing that?

May or may not be of use to you, but we do something similar for our 
package (mixed Fortran 90 and C code - don't ask!). The C/Fortran 
objects simply keep a borrowed reference to the corresponding Python 
object (note: we don't use shadow classes or SWIG-generated 
constructors) which is used for callback functions.

For example, a typical constructor looks like:
struct foo *new_foo(const void *scriptobj)
{
   struct foo *newobj;
   newobj = .../* create new struct foo object */
   newobj->scriptobj = scriptobj;
   return newobj;
}

The SWIG interface then looks something like:
/* Pass scripting language objects to new_foo() routines as
    opaque void * */
%typemap(in) const void * scriptobj {
   $1 = $input;
}

and the Python class looks something like:
class foo(object):
     def __init__(self):
         self.this = _module.new_foo(self)
     def __del__(self):
         _module.free_foo(self.this)

(Note that we don't Py_INCREF the passed object, since the lifetime of 
the C/Fortran and Python objects should be the same, and we don't want 
circular references.)

	Ben
-- 
be...@sa...                      http://salilab.org/~ben/
"It is a capital mistake to theorize before one has data."
	- Sir Arthur Conan Doyle