Python automatic testing: mocking an imported module?

[Silfheed]

Heyas

So we have the following situation: we have a testee.py that we want
to automatically test out and verifiy that it is worthy of being
deployed.  We want our tester.py to test the code for testee.py
without changing the code for testee.py.  testee.py has a module in it
that we want to mock in some tests and in others use the real module.

/foo.py:  (real module)
class bar:
   def __init__(self):
        "I am real"

/foo_fake/foo.py: (fake module)
class bar:
   def ___init__(self):
        "I am a banana"

/testee.py:
import foo
foo.bar()

/tester.py:
from foo_fake import foo
   foo.bar()    # prints I am a banana
   testee.py  # also prints I am a banana
import foo
   foo.bar()   # prints I am real
   testee.py # also prints I am real


This isnt working as we would like, does anyone have any tips  on how
to get something like this working?