gistfile1.py

#!/usr/bin/env python

class A(object):
    def __iter__(self):
        yield 'i'
        yield 'am'
        yield 'A'

    def fiddleIter(self):
        def newIter(_):
            return iter(['i', 'am', 'new'])
        self.__iter__ = newIter


class B(A):
    def __iter__(self):
        yield 'i'
        yield 'am'
        yield 'B'


b = B()
b.fiddleIter()

print b.__iter__

print list(b)

In Python 2.7.2 this prints

<function newIter at 0x108663140>
['i', 'am', 'B']