Import functions from dll (programmed in C++) into Python script when the dll has an embedded Python interpreter

Question

I wrote a library (dll) in c++ that uses an embedded Python interpreter. Executing Python scripts in the embedded interpreter works fine when the library is used by a c++ program.

If I call the functions of the dll from a Python program I run into trouble. When the dll would usually start the embedded interpreter by calling Py_Initialize() there is an interpreter running already (I can test this by calling Py_IsInitialized() ). When I try to use this interpreter the program crashes.

Minimal example that reproduces the behavior:

Code for dll:

#include "stdafx.h"
#include <Python.h>

extern "C" __declspec(dllexport)
int testIsInitialized()
{
  return Py_IsInitialized();
}

extern "C" __declspec(dllexport)
void testRunSimpleString()
{
  PyRun_SimpleString("print('test')");
}

Python script:

import ctypes

dll=ctypes.windll.LoadLibrary('pytest.dll')

# test if Python interpreter is initialized
print(dll.testIsInitialized())

# test if we can run a simple Python script
dll.testRunSimpleString()

Output:

1
Traceback (most recent call last):
  File "test_dll.py", line 9, in <module>
    dll.testRunSimpleString()
OSError: exception: access violation reading 0x0000000000000010 

My main question is: How can I execute Python code from a c++ library that is imported by a different Python program?

UPDATE

The example works when I aquire the global interpreter lock (GIL) in my testRunSImpleString() function. I do not understand why this is necessary here.

Answer 1

The ctypes library has several classes to load different types of DLLs. Two relevant classes are:

• ctypes.CDLL (ie C DLL) - used to load a DLL that uses a standard C calling convention. This includes C++ DLLs that define extern "C" .
• ctypes.PyDLL - similar to C DLL, except that the Python GIL is not released during the function call. Useful to call Python C API functions directly.

When loading a C++ DLL that uses the Python C API (ie Python.h ), use ctypes.PyDLL :

import ctypes

dll = ctypes.PyDLL('pytest.dll')

# then do whatever
dll.testRunSimpleString.argtypes = []
dll.testRunSimpleString.restype = None
dll.testRunSimpleString()

One thing to keep in mind with PyDLL is to that you don't need to call Py_Initialize() or Py_Finalize() / Py_FinalizeEx() from the C++ API, since the interpreter is already running. You can first check (and store) Py_IsInitialized() to see if a new interpreter is need to be started/stopped.