How to access a Python global variable from C?

Question

I have some global variables in a Python script. Some functions in that script call into C - is it possible to set one of those variables while in C and if so, how?

I appreciate that this isn't a very nice design in the first place, but I need to make a small change to existing code, I don't want to embark on major refactoring of existing scripts.

Answer 1

I'm not a python guru, but I found this question interesting so I googled around. This was the first hit on "python embedding API" - does it help?

If the attributes belong to the global scope of a module, then you can use "PyImport_AddModule" to get a handle to the module object. For example, if you wanted to get the value of an integer in the main module named "foobar", you would do the following:

PyObject *m = PyImport_AddModule("__main__");
PyObject *v = PyObject_GetAttrString(m,"foobar");

int foobar = PyInt_AsLong(v);

Py_DECREF(v);

Answer 2

I recommend using pyrex to make an extension module you can store the values in in python, and cdef a bunch of functions which can be called from C to return the values there.

Otherwise, much depends on the type of values you're trying to transmit.

Answer 3

Are you willing to modify the API a little bit?

• You can make the C function return the new value for the global, and then call it like this:

  my_global = my_c_func(...)

• If you're using Robin or the Python C API directly, you can pass the globals dictionary as an additional parameter and modify it

• If your global is always in the same module, Sherm's solution looks great