I am trying to write a python wrapper for a C function. After writing all the code, and getting it to compile, Python can't import the module. I am following the example given here. I reproduce it here, after fixing some typos. There is a file myModule.c:
#include <Python.h>
/*
* Function to be called from Python
*/
static PyObject* py_myFunction(PyObject* self, PyObject* args)
{
char *s = "Hello from C!";
return Py_BuildValue("s", s);
}
/*
* Bind Python function names to our C functions
*/
static PyMethodDef myModule_methods[] = {
{"myFunction", py_myFunction, METH_VARARGS},
{NULL, NULL}
};
/*
* Python calls this to let us initialize our module
*/
void initmyModule()
{
(void) Py_InitModule("myModule", myModule_methods);
}
Since I am on a Mac with Macports python, I compile it as
$ g++ -dynamiclib -I/opt/local/Library/Frameworks/Python.framework/Headers -lpython2.6 -o myModule.dylib myModule.c
$ mv myModule.dylib myModule.so
However, I get an error when I try to import it.
$ ipython
In[1]: import myModule
---------------------------------------------------------------------------
ImportError Traceback (most recent call last)
/Users/.../blahblah/.../<ipython console> in <module>()
ImportError: dynamic module does not define init function (initmyModule)
Why can't I import it?
Since you're using a C++ compiler, function names will be mangled (for instance, my g++
mangles void initmyModule()
into _Z12initmyModulev
). Therefore, the python interpreter won't find your module's init function.
You need to either use a plain C compiler, or force C linkage throughout your module with an extern "C" directive:
#ifdef __cplusplus
extern "C" {
#endif
#include <Python.h>
/*
* Function to be called from Python
*/
static PyObject* py_myFunction(PyObject* self, PyObject* args)
{
char *s = "Hello from C!";
return Py_BuildValue("s", s);
}
/*
* Bind Python function names to our C functions
*/
static PyMethodDef myModule_methods[] = {
{"myFunction", py_myFunction, METH_VARARGS},
{NULL, NULL}
};
/*
* Python calls this to let us initialize our module
*/
void initmyModule()
{
(void) Py_InitModule("myModule", myModule_methods);
}
#ifdef __cplusplus
} // extern "C"
#endif