Mocking a script with no `if __name__=="__main__":` block

Suppose I have files main.py and common.py in my root directory. These represent third party modules which I would not like to touch for the time being. Suppose I also have a test.py file which I’d like to use to test main.py.

common.py contains the following:

def hello():
  print("hello")

main.py contains the following:

from common import hello

hello()

Is there a way to monkey patch the hello function in test.py such that importing main.py will use the mocked function? For instance, I’d like to monkey patch hello to print out goodbye.

>Solution :

Pretty straightforward:

import common

common.hello = lambda: print("goodbye")

import main