Julia callback upon Python object finalization

example output

julia> require("test.jl")
Julia code
  lfile PyPtr address: Ptr{Void} @0x000000000361d390
Julia code
  weakref address: Ptr{Void} @0x0000000003622f70
  Julia code: calling decref
C code: cb_wrapper
  self: 0
  arg:  3622f70
Julia code:
  got callback from Python with arg address: Ptr{Void} @0x0000000003622f70

test.c

#include "Python.h"

// Compiles on ubuntu 12.04 with system python 2.7 using:
//   gcc -I /usr/include/python2.7 -shared -fPIC -g -lpython2.7 test.c -o libtest

void(*jlcallback)(void*,void*);

void set_jl_callback(void* cb)
{
  jlcallback = cb;
}

PyObject* cb_wrapper(PyObject* self, PyObject* arg)
{
  printf("C code: cb_wrapper\n");
  printf("  self: %lx\n", (unsigned long)self);
  printf("  arg:  %lx\n", (unsigned long)arg);

  jlcallback(self,arg);
  Py_RETURN_NONE;
}

PyMethodDef methods;
PyObject* pycb;

PyObject* gen_py_callback()
{
  methods.ml_name = "callback";
  methods.ml_meth = cb_wrapper;
  methods.ml_flags = METH_O;
  methods.ml_doc = NULL;
  pycb = PyCFunction_New(&methods, NULL);
  
  return pycb;
}

test.jl

module F

using PyCall
pyinitialize()

lfile = PyObject( ccall( pyfunc(:PyFile_FromString), PyPtr, (Ptr{Uint8}, Ptr{Uint8}), bytestring("test.txt"), bytestring("w") ), None )
println("Julia code")
println("  lfile PyPtr address: ", lfile.o)

function jlcb(self::Ptr{Void}, arg::Ptr{Void})
  println("Julia code:")
  println("  got callback from Python with arg address: ", arg)
  return convert(Int32, 0)
end

cb_jl = cfunction(jlcb, Int32, (Ptr{Void},Ptr{Void}))
cb_py = ccall( (:gen_py_callback, "./libtest"), Ptr{Void}, () )

ccall( (:set_jl_callback, "./libtest"), Void, (Ptr{Void},), cb_jl)

function do_test()
  weakref = ccall( pyfunc(:PyWeakref_NewRef), PyPtr, (Ptr{Void}, Ptr{Void}), lfile, cb_py)
  println("Julia code")
  println("  weakref address: ", weakref)
  println("  Julia code: calling decref")
  pydecref(lfile)
end

end # Module

F.do_test()