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.