Just mentioning it here again, but "type(a).__index__(a)" is still not perfectly correct.  Attached is a case where it differs.

I think you get always the correct answer by evaluating "range(a).stop".  It's admittedly obscure...  For example:

    class A:
        def __index__(self):
            return -42**100

    a = A()
    print(range(a).stop)