Basic python extension producing error: "dynamic module not initialized properly"

[Jason Veldicott]

Hi,

I am trying to create an extension to call into a c++ library, a library
which simply returns a string, given a string.  At first, a quick and
simple approach was tried (based on post
http://stackoverflow.com/questions/1615813/how-to-use-c-classes-with-ctypes)
in which the usual infrastructure for loading extensions is bypassed.  A
working simple example is shown beneath:

myextension.cpp

#include <python.h>

extern "C"
{
int test_func() {
return 10;
}
}

To execute in Python:

from ctypes import cdll
mlib=cdll.LoadLibrary("myextension.dll")
print( mlib.test_func() )

This works fine when function return type is a number, but not when a
string.  C++ code returns char *, which is received by Python as a pointer,
which prints as a number, eg1755066468.  Apparently, based on web posts
viewed, it seems data conversion is required, using a function such
as Py_BuildValue("s", str) from python.h.  But this gave a similar result,
in which a pointer is received by Python instead of a string. The modified
function used in this case:

PyObject * test_func() {
return Py_BuildValue("s", "aString");
}

Resort was then made to the conventional approach of defining a method
table and initializing method.  (Abandoned cdll.LoadLibrary(...) as
subsequently test_func was not recognised, and instead put the dll (.dll
changed to pyd) in /python26/DLLs.)  The revised code:

myextension.cpp

#include <python.h>

extern "C"
{
static PyObject * test_func(PyObject *self, PyObject *args)
{
return (PyObject *) 0;
}

static PyMethodDef TestMethods[] = {
{"test",  test_func, METH_VARARGS, "test"},
{NULL, NULL, 0, NULL}
};

PyMODINIT_FUNC initlibTestExtnModule(void) {
(void) Py_InitModule("TestExtnModule", TestMethods);
}
}

Using the import statement "import libTestExtnModule" however produced the
following error message:

"SystemError: dynamic module not initialized properly"

As far as I know there is nothing wrong with the initialisation of the
extension code above.   Perhaps the error is related to compilation.  I
have not used distutils, but rather compiled externally and dropped the dll
(as pyd) into the /DLLs directory as mentioned.  This should work, as far
as I know.  Related to compilation, both MinGW and Python2.6 are 32 bit
versions, and g++ statements used were:

mingw32-g++ "-IC:\\Python26\\include" -O0 -g3 -Wall -c -fmessage-length=0
-o myextension.o "..\\myextension.cpp"
mingw32-g++ -LC:/Python26/libs -shared -o
libTestExtnModule.pyd myextension.o -lpython26

Any suggestions as to why the dynamic module is not initialising properly,
or as to how otherwise a string can be returned, would be greatly
appreciated.

Thanks

Jason
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