How to access to Maya python objects from a plug-in written in C++.

This is a modified copy of my old document (you can still see the original somewhere on the net).
When I wrote this, Maya8.5 was latest, and it is still my latest unfortunately, so in this doc Maya8.5 is used. If you use Maya2008, you need to use Python2.5 (e.g. Python25.dll). And I was using windows at that time. I will add to this post how to do it on Linux and Mac soon. (Encouraging me to do so by posting a comment will become a good motivation :)

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Maya8.5 comes with Python. It is really not a must to know how to access to it from a plug-in written in C++ since you can make a plug-in in Python, but for some reason e.g. performance, you may want to know the way to access to it directly from your Maya .mll/.so/.bundle plug-in.

Python itself is a shared library which you can find at Maya8.5/bin/python24.dll (on windows) so if Maya can access to Python, our plug-ins can get access to it.

First you need to include Python.h before every includes like any other python programs. The version of Python must be 2.4 since Python inside Maya is version 2.4.3. Other version of Python e.g. 2.5 uses different shared library e.g. python25.dll and you cannot use the same Python as the one Maya uses.

Maya8.5 has already initialized Python interpreter so you don't need to (and you shouldn't) call Py_Initialize(), but in the doIt() (in case of a command plugin) of your plug-in, you need to call
PyGILState_Ensure()
Python interpreter is not thread safe and this function call locks the interpreter properly so that only the thread that is running your plug-in can access to Python. In the test plug-in code show below, I used PyRun_SimpleString() to access to Python. See this post to know how to use Python C/API. When you leave doIt(), you need to call
PyGILState_Release()
It is needed even if you don't use Python thread modules. Forgetting this freezes or crashes Maya easily.

#include <Python.h>
#include <maya/MGlobal.h>
#include <maya/MSimple.h>
DeclareSimpleCommand( maya85py, "", "8.5");

MStatus maya85py::doIt( const MArgList& args )
{
PyGILState_STATE state = PyGILState_Ensure();
PyRun_SimpleString("tamtam = 1");
PyGILState_Release(state);
return MS::kSuccess;
}

Here's the result of above test plug-in.

tamtam
# Error: name 'tamtam' is not defined
# Traceback (most recent call last):
#   File "<maya console>", line 1, in ?
# NameError: name 'tamtam' is not defined #

import maya
maya.mel.eval("maya85py")

tamtam
# Result: 1 #

Debug build:
On windows, the compiler tries to link python24.lib or python24_d.lib automatically (on Linux and Mac you need to link the appropriate one manually). python24_d.lib is a debug version and you need to build python itself to get it. I surfed the web but couldn't find it, and tried to build python but full build was a little bit bothering (install tcl/tk, brabrabra...), so I just modified pyconfig.h in my Python24/include directory. It's all what you need to debug build your plug-in (You don't need this if you don't use the debugger). Here I just used Python header files and import library that comes with standard Python distribution, since Maya doesn't come with them. Recently I heard one of my friend made an import library using dumpbin command. It'll be better if you have time to do it (still not safe though).

//#                pragma comment(lib,"python24_d.lib")
#                pragma comment(lib,"python24.lib")
#            else
#                pragma comment(lib,"python24.lib")

Finally, in an actual plug-in PyRun_SimpleString("tamtam = 1"); can be as complicated as you want, missing PyGILState_Release() execution by some exception is quite dangerous. I would use try-catch clause.