What are the semantics of the following:

    b = ComplexObject (...)
    # do things with b

    def foo (self, arg=b):
        # use b

    return foo

Should it create a copy of b every time the function is invoked? If you want that right now, you can just call b.copy (), when you always create that copy, then you can not implement the current choice.

Should the semantic of this be any different? :

    def foo (self, arg=ComplexObject (...)):

Now imagine a:

    ComplexObject = list