That's tame. Here's a scary example [1] (redefining a (built in) function in the middle of a running coroutine):

  def foo(x):
    yield len(x)
    yield len(x)
  >>> g = foo(range(5))
  >>> g.next()
  5
  >>> len = lambda y: 8
  >>> g.next()
  8

This causes problems when trying to implement an efficient python. You have to re-lookup your function bindings every time.

[1]: Example taken from Colin's Unladen Swallow presentation http://llvm.org/devmtg/2009-10/Winter_UnladenSwallowLLVM.pdf