casting PyCFunctionWithKeywords in PyMethodDef

Question

Recently I've been wrapping a lot of C++ code in python, and I find this block (taken directly from the python documentation) a bit troubling:

static PyMethodDef keywdarg_methods[] = {
/* The cast of the function is necessary since PyCFunction values
 * only take two PyObject* parameters, and keywdarg_parrot() takes
 * three.
 */
{"parrot", (PyCFunction)keywdarg_parrot, METH_VARARGS | METH_KEYWORDS,
 "Print a lovely skit to standard output."},
{NULL, NULL, 0, NULL}   /* sentinel */
};

The issue is the line which casts kwarg_parrot, of type PyCFunctionWithKeywords to a PyCFunction.

Coming from a C++ background (and given that I am wrapping C++ code), it seems wrong to use a C-style cast. I've tried static_cast, and dynamic_cast, both of which cause the compiler to complain (with good reason, this really is an unsafe cast in the general sense). The only viable C++ option seems to bereinterpret_cast, but so far as I can tell this is a more verbose version of a C-style cast.

Granted, the above is wrapped in an extern "C" block, so maybe the C way is the correct way. Does anyone have any better ideas? (What I'd really like to see would be a solution that could automatically generate the doc string based on the keywords.)

Unfortunately, solutions like Boost.Python and SWIG are off the table. (I'm working within an ugly framework)

Answer 1

Main Python implementation is written in C, not C++.

Thus, libpython code that evaluates keywdarg_methods[] is written in C and calls keywdarg_parrot via C calling conventions.

If you want C++ style integration with Python, find a way to use boost.python instead. Or perhaps cython.

Answer 2

PyMethodDef is part of the Python C API so I would just use a C cast. It works and it's what all Python extenaions are using. Stay consistent.

Use C++ casts when working with objects that aren't part of the Python API.