Saved from https://stackoverflow.com/questions/53215786/when-writing-a-python-extension-in-c-how-does-one-pass-a-python-function-in-to

callback.c

/* this code uses only C features */
static PyObject *
foo(PyObject *self, PyObject *args)
{
    PyObject *cb;    

    // Receive a single argument which can be any Python object
    // note: the object's reference count is NOT increased (but it's pinned by
    // the argument tuple).
    if (!PyArg_ParseTuple(args, "O", &cb)) {
        return 0;
    }
    // determine whether the object is in fact callable
    if (!PyCallable_Check(cb)) {
        PyErr_SetString(PyExc_TypeError, "foo: a callable is required");
        return 0;
    }
    // call it (no arguments supplied)
    // there are a whole bunch of other PyObject_Call* functions for when you want
    // to supply arguments
    PyObject *rv = PyObject_CallObject(cb, 0);
    // if calling it returned 0, must return 0 to propagate the exception
    if (!rv) return 0;
    // otherwise, discard the object returned and return None
    Py_CLEAR(rv);
    Py_RETURN_NONE;
}