As a final note, notice that when keyword-only arguments were implemented, the parameter was not added at the end of the constructor:

commit 4f72a78684bbfcdc43ceeabb240ceee54706c4b0
Author: Guido van Rossum <guido@python.org>
Date:   Fri Oct 27 23:31:49 2006 +0000

    Jiwon Seo's PEP 3102 implementation.
    See SF#1549670.
    The compiler package has not yet been updated.


--- a/Objects/codeobject.c
+++ b/Objects/codeobject.c
@@ -41,7 +41,8 @@ intern_strings(PyObject *tuple)


 PyCodeObject *
-PyCode_New(int argcount, int nlocals, int stacksize, int flags,
+PyCode_New(int argcount, int kwonlyargcount,
+          int nlocals, int stacksize, int flags,
           PyObject *code, PyObject *consts, PyObject *names,
           PyObject *varnames, PyObject *freevars, PyObject *cellvars,
           PyObject *filename, PyObject *name, int firstlineno,