implementation of "complex" type

[Russ]

I tried the following:

>>> x = complex(4)
>>> y = x
>>> y *= 2
>>> print x, y
(4+0j) (8+0j)

But when I tried the same thing with my own class in place of
"complex" above, I found that both x and y were doubled. I'd like to
make my class behave like the "complex" class. Can someone tell me the
trick? Also, where can I find the code for for the "complex" class? I
hope it's written in Python! Thanks.