Mox: stub out a python builtin method like 'exit' or 'open'

Occasionally you need to stub out a part of core python for a test to work. In my case it was 'exit', another example is 'open'. This is what your test code will look like:

import __builtin__

m = mox.Mox()
m.StubOutWithMock(__builtin__, 'exit')
exit(my_module.EXIT_ERROR)

m.ReplayAll()
my_module.myfunction()
m.VerifyAll()