def helloworld():
    return "hello world"

def helloworld_helper():
    return helloworld()

import mylibrarywithlongname as ml
from mock import MagicMock

def test():
    ml.helloworld = MagicMock(return_value="oops")
    print(ml.helloworld_helper())

print(ml.helloworld_helper())
test()
print(ml.helloworld_helper())

hello world
oops
oops