Use types.MethodType for that:

    >>> import types
    >>> 
    >>> def hello(self):
    ...     print self.name
    ... 
    >>> class Foo(object):
    ...     name = 'blah'
    ... 
    >>> goo = Foo()
    >>> goo.hi = types.MethodType(hello, Foo)
    >>> 
    >>> goo.hi()
    blah

Edit: pre-2.6 you'd use "new.instancemethod()"