How do I set a callback in Python?

[catsclaw]

I can't for the life of me figure out how to set a callback in
Python.  I have a class, which wraps another class.  The second class
needs a callback assigned.  I don't want to use globals for it.
Here's what I'd like to do:

class MyWrapper:
    def get_login(self, username):
        return self.user, self.pass

    def __init__(self, user, pass):
        self.user = user
        self.pass = pass

        self.client = Client("connection string")
        self.client.callback_login = get_login

... but obviously, the Client class, when it calls the callback,
doesn't pass a reference to the "self" object.  How do I do this?

-- Chris