.so module doesnt import in python: dynamic module does not define init function

Question

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?

@Ignacio: I am just trying to follow the examples. Is there a simpler example you could point me to? — highBandWidth
– highBandWidth, Commented Nov 6, 2010 at 7:55
Does the code in the top box really reflect what you have in your source file? — Ignacio Vazquez-Abrams
– Ignacio Vazquez-Abrams, Commented Nov 6, 2010 at 7:56
@Ignacio. Sorry, you're right. It messed up when I copied it from the file. I recopied the code. It now reflects what I have in my source file. — highBandWidth
– highBandWidth, Commented Nov 6, 2010 at 8:00

Frédéric Hamidi · Accepted Answer · 2010-11-10 21:59:56Z

5

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

edited Nov 10, 2010 at 21:59

answered Nov 6, 2010 at 12:21

Frédéric Hamidi

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2 Comments

Ignacio Vazquez-Abrams Over a year ago

The PyMODINIT_FUNC macro as given in the documentation will handle this for you.

RDK Over a year ago

@IgnacioVazquez-Abrams: Could you please elaborate how to carry out your solution? Thanks.

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