No module named ctypes using Python/C API

Question

I have PythonCode.py:

import os, ctypes
print "ctypes are imported"

And CCode.c:

...
PySys_SetPath(path_to_PythonCode);
PyObject *pModule = PyImport_ImportModule("PythonCode");
if (!pModule) {
  PyErr_Print();
  return;
}
...

PyErr_Print() prints:

Traceback (most recent call last):   File ".../PythonCode.py", line 1,
  in <module> import os, ctypes
ImportError: No module named ctypes

Executing Python code in terminal shows that ctypes do exist:

$ python --version
Python 2.7.1
$ python PythonCode.py
ctypes are imported

Why Python/C API cannot import ctypes?

This looks suspiciously like a classic circular import problem in Python, only taking a trip through C to get there. You try to import the Python module in C, but the Python module is trying to import the C library as part of ctypes, so you end up trying to get ctypes from sys.modules before the interpreter has had a chance to completely construct it, since the C code is still in the middle of executing. It's not that it doesn't exist, it's that at the point you try to access it in your program flow, the state of the module is incomplete, and thus both unloaded and unloadable. — Silas Ray, Commented Dec 20, 2012 at 12:54
You replaced the default sys.path list with a single path. Instead you can append or insert the new path: PyObject *sysPath = PySys_GetObject("path"); PyObject *path = PyString_FromString(path_to_PythonCode); PyList_Append(sysPath, path); // or PyList_Insert(sysPath, 0, path); (needs error handling). — Eryk Sun, Commented Dec 20, 2012 at 13:42
@eryksun: Thank you! That did the trick! Why are you answering in comments? — AlexP, Commented Dec 20, 2012 at 14:05
@AlexP: I was just waiting to see if someone else was going to write a more complete answer. I'll add it as answer. — Eryk Sun, Commented Dec 20, 2012 at 14:50

Eryk Sun · Accepted Answer · 2012-12-20 14:55:11Z

4

You replaced the default sys.path list with a single path. Instead you can insert the new path:

PyObject *sys_path, *path;

sys_path = PySys_GetObject("path");
if (sys_path == NULL || !PyList_Check(sys_path)) {   
    /* ZOMG! That's so wrong... */
    return;
}
path = PyString_FromString(path_to_PythonCode);
PyList_Insert(sys_path, 0, path);
Py_DECREF(path);

answered Dec 20, 2012 at 14:55

Eryk Sun

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