def myfunc(a, b, *c = lambda: expression):

It's currently a syntax error, although the fact that "*arg" and
"*arg=default" would mean something completely different is
problematic. Still the same idea can be applied for some other
operator (either valid already or not).

Regardless of the actual operator, I came up with the following
additional subproposals.

Subproposal (1): Get rid of the explicit lambda for dynamic arguments. That is,

    def myfunc(a, b, *x=[]):

would be equivalent to what previous proposals would write as

    def myfunc(a, b, *x=lambda: []):

Subproposal (2): If subproposal (1) is accepted, we could get for free
(in terms of syntax at least) dynamic args depending on previous ones.
That is,

    def myfunc(a, b, *m=(a+b)/2):

would mean

    def myfunc(a, b, *m = lambda a,b: (a+b)/2):

with the lambda being passed the values of a and b at runtime.

Thoughts ?