Calling Python from C++ and back

Imagine the following situation:

You have a existing C++ application. You want to migrate parts of it to python. That is, you want to be able to call python code from C++ code.
You also want to be able to call C++ code from python.
Your C++ code is multithreaded and there is no guarantee that any one thread calling python has initialized the interpreter.
You need to distribute the resulting app with minimum overhead.

Enter pyfromc. It is a sample Win32 C++ application using an embedded python interpreter, to be used as a potential starting point.

Download

You can download pyfromc here. Note that the installer is only 692.031 bytes large, but includes the c++ sample app, the embedded python interpreter and the full sourcecode.

License

There is none. Use at own risk.

Calling Python from C++

pyfromc.py is a sample python code that exposes two global functions, test and dumpmods. The C++ class Python inside pyfromc.cpp/.h defines wrapper methods for these calls. E.g. if you want to add a new method to pyfromc.py, you must also define a wrapper method in pyfromc.h and implement it in pyfromc.cpp.

The C++ wrapper method uses the helper class PythonCall, which in turn uses techniques described in the Python manual, section Embedding Python in Another Application.

Calling C++ from Python

Basically, cfrompy.cpp is a standard python module, as described in the python manual, section Extending Python with C or C++ . The only difference is that cfrompy.cpp is not a DLL, it is part of the executable code you write, so you have easier access to your existing C++ code.

Thread safety

The code is (or rather: should be) threadsafe. That is, you should be able to call python code from any native Win32 thread, without any additional precautions (provided your C++ code is already threadsafe). The code uses the techniques described in PEP 311 - Simplified Global Interpreter Lock Acquisition for Extensions.

Distribution your app

Starting with python 2.3, you can distribute modules in a zipfile, called "python23.zip", which must be in the same directory as python23.dll. So, what you need is

Your C++ app
python23.dll
all modules you need encoded in python23.zip
all .pyd extensions you need (cannot be stored in python23.zip because loaded as a DLL)

The only problem is: how to find out what modles you need. Well, enter dumpmods.py, a small python script that enumerates all loaded modules and dumps them into a target directory. Usage: When your program ends, add these two lines:

import dumpmods
dumpmods.analyze_modules(path to store files in)

where "path to store files in" has obvious connotations.

Historical Note