How to load my fake module every time when `import a real module` in python?

Question

I need to test on a function.py, and in this function.py there is a import statement:

from my.functions import cleaner
from my.functions import worker

I don't want the cleaner get imported because it's very complicated. So I try to set the sys.path :

sys.path.insert(0, './fakes')

in fakes module, a my.functions.cleaner also exists there but with no function, that works but it will also impact worker, I really want the worker works for me.

So my question is there any way to "mock" this cleaner to my fake cleaner, I need my fake cleaner replace it every time import cleaner.

I tried sys.modules but failed.

Answer 1

If you're trying to dynamically replace a function, you can do so with an assignment statement, I.E.:

To see this in action check out this example:

import my.functions

/my

functions.py

def cleaner():
    print("Cleaner from functions!")

def worker():
    print("Worker from functions!")

base.py

import my.functions

def cleaner():
    print("Replacement fake cleaner!")

my.functions.cleaner = cleaner


def method_to_test():
    from my.functions import cleaner
    from my.functions import worker
    cleaner()
    worker()


if __name__ == "__main__":
    method_to_test()

Python loads the my.functions module into sys.modules, then on subsequent from imports gets objects from the already loaded module, so we can set a mock function before your function does its import.