C ++ Python module import error: "undefined: Py_InitModule3" (Py_InitModule ())
I'm just getting started trying to write my first Python extension module in C and using the instructions provided at https://www.tutorialspoint.com/python/python_further_extensions.htm
I am on Linux Mint 18.1 using Python 3.6.1 in my virtualenv version.
As a first step, I've put together a very minimalistic version of the Python module I'm planning to write.
Here is my C code:
include <Python.h>
static PyObject* uniqueCombinations(PyObject* self)
{
return Py_BuildValue("s", "uniqueCombinations() return value (is of type 'string')");
}
static char uniqueCombinations_docs[] =
"usage: uniqueCombinations(lstSortableItems, comboSize)\n";
static PyMethodDef uniqueCombinations_funcs[] = {
{"uniqueCombinations", (PyCFunction)uniqueCombinations,
METH_NOARGS, uniqueCombinations_docs},
{NULL}
};
void inituniqueCombinations(void)
{
Py_InitModule3("uniqueCombinations", uniqueCombinations_funcs,
"Extension module uniqueCombinations v. 0.01");
}
and here is the setup.py file code I used to compile the C code to uniqueCombinations.cpython-36m-x86_64-linux-gnu.so
:
from distutils.core import setup, Extension
setup(name='uniqueCombinations', version='0.01', \
ext_modules=[Extension('uniqueCombinations', ['uniqueCombinations_pythonCmodule_v-0.01_Cg.c'])])
To my surprise, the module doesn't load as expected on import:
>>> import uniqueCombinations
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ImportError: uniqueCombinations.cpython-36m-x86_64-linux-gnu.so: undefined symbol: Py_InitModule3
Why am I getting this error?
And what should I do to be able to load the compiled module correctly?
Here's the build log with a warning about the implicit declaration of Py_InitModule3:
$ python uniqueCombinations_pythonCmodule_v-0.01_setup.py_Cg.py build
running build
running build_ext
building 'uniqueCombinations' extension
gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fPIC -I/usr/local/include/python3.6m -c uniqueCombinations_pythonCmodule_v-0.01_Cg.c -o build/temp.linux-x86_64-3.6/uniqueCombinations_pythonCmodule_v-0.01_Cg.o
uniqueCombinations_pythonCmodule_v-0.01_Cg.c: In function ‘inituniqueCombinations’:
uniqueCombinations_pythonCmodule_v-0.01_Cg.c:19:5: warning: implicit declaration of function ‘Py_InitModule3’ [-Wimplicit-function-declaration]
Py_InitModule3("uniqueCombinations", uniqueCombinations_funcs,
^
creating build/lib.linux-x86_64-3.6
gcc -pthread -shared build/temp.linux-x86_64-3.6/uniqueCombinations_pythonCmodule_v-0.01_Cg.o -o build/lib.linux-x86_64-3.6/uniqueCombinations.cpython-36m-x86_64-linux-gnu.so
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Py_InitModule (Py_InitModule3) is no longer used. You need to create a PyModuleDef structure and then pass a reference to it to PyModule_Create.
Below is the C code from which the generated module does not throw an error on import:
#include <Python.h>
static PyObject* uniqueCombinations(PyObject* self)
{
return Py_BuildValue("s", "uniqueCombinations() return value (is of type 'string')");
}
static char uniqueCombinations_docs[] =
"usage: uniqueCombinations(lstSortableItems, comboSize)\n";
/* deprecated:
static PyMethodDef uniqueCombinations_funcs[] = {
{"uniqueCombinations", (PyCFunction)uniqueCombinations,
METH_NOARGS, uniqueCombinations_docs},
{NULL}
};
use instead of the above: */
static PyMethodDef module_methods[] = {
{"uniqueCombinations", (PyCFunction) uniqueCombinations,
METH_NOARGS, uniqueCombinations_docs},
{NULL}
};
/* deprecated :
PyMODINIT_FUNC init_uniqueCombinations(void)
{
Py_InitModule3("uniqueCombinations", uniqueCombinations_funcs,
"Extension module uniqueCombinations v. 0.01");
}
*/
static struct PyModuleDef Combinations =
{
PyModuleDef_HEAD_INIT,
"Combinations", /* name of module */
"usage: Combinations.uniqueCombinations(lstSortableItems, comboSize)\n", /* module documentation, may be NULL */
-1, /* size of per-interpreter state of the module, or -1 if the module keeps state in global variables. */
module_methods
};
PyMODINIT_FUNC PyInit_Combinations(void)
{
return PyModule_Create(&Combinations);
}
Here's the build log:
$ python uniqueCombinations_pythonCmodule_v-0.01_setup.py_Cg.py build
running build
running build_ext
building 'Combinations' extension
gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fPIC -I/usr/local/include/python3.6m -c uniqueCombinations_pythonCmodule_v-0.01_Cg.c -o build/temp.linux-x86_64-3.6/uniqueCombinations_pythonCmodule_v-0.01_Cg.o
gcc -pthread -shared build/temp.linux-x86_64-3.6/uniqueCombinations_pythonCmodule_v-0.01_Cg.o -o build/lib.linux-x86_64-3.6/Combinations.cpython-36m-x86_64-linux-gnu.so
and an interactive Python console session with successful import:
>>> import Combinations
>>> dir(Combinations)
['__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__', 'uniqueCombinations']
>>> Combinations.__doc__
'usage: Combinations.uniqueCombinations(lstSortableItems, comboSize)\n'
>>> Combinations.uniqueCombinations()
"uniqueCombinations() return value (is of type 'string')"
>>>
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