Yep! It's slightly worse than you would think. Here's a (slightly edited) version of how the argument passing works static PyObject * unicode_new(PyTypeO

Yep! It's slightly worse than you would think. Here's a (slightly edited) version of how the argument passing works static PyObject * unicode_new(PyTypeObject *type, PyObject *args, PyObject *kwds) { PyObject *x = NULL; static char *kwlist[] = {"object", "encoding", "errors", 0}; char *encoding = NULL; char *errors = NULL; PyArg_ParseTupleAndKeywords(args, kwds, "|Oss:str", kwlist, &x, &encoding, &errors)) } Notice the call to PyArg_ParseTupleAndKeywords -- it takes an arguments tuple and a format string and executes a mini interpreter to parse the arguments from the tuple. It has to be ready to receive arguments as any combination of keywords and positional, but for a given callsite the matching will generally be static.