Passing parameters to functions

[Jeff Epler]

If you want to have a varying number of positional arguments, and
require certain keyword arguments, I think you'll be forced to write it
in a clumsy way, similar to (untested):
    def extract_args(funcname, mapping, *names):
        for i in names:
            try:
                yield mapping[i]
                del mapping[i]
            except KeyError:
                raise TypeError, (
                    "%s requires a keyword argument %s" %
                    (funcname, repr(i))))

    def f(parg1, parg2, *pargs, **kargs):
        karg1, karg2 = extract_args("f()", kargs, "karg1", "karg2")
        print parg1, parg2, pargs, karg1, karg2, kargs

When you write code like
    def f(a, b=1)
Python will accept any of these calls to f:
    f(0)
    f(1, 2)
    f(3, b=4)
    f(a=5, b=6)
and even
    f(b=7, a=8)
.. the call that is relevant to your case is the second one.  The
value in b can come from a positional argument or a keyword argument.
In your case, when you cakk 'f(1, 2, 3, karg=3)' karg gets two
values---one from the third positional argument, and one from the
keyword argument.

Jeff