Mock testing with nested function changes

I am adding testing to a pipeline project, code is already written and in production so it cannot be changed to accommodate the tests.

In simplest terms, if I have a function like so:

def other_foo():
    return 1

def foo():
    res = other_foo()
    return res

In practicality, the other_foo call will return a variety of responses, but for testing, I want to create a fixed response to test foo.

So in my test I want to create a fixed response to other_foo of 2. and my test evaluation to be something like:

def test_foo():
    # some mocking or nesting handle here for other_foo
    res = foo()
    assert res == 2

>Solution :

Use the patch decorator from unitest.mock and patch your module local variable.

from your.module import foo
from unitest.mock import patch

@patch('your.module.other_foo')
def test_foo(mock_other_foo):
    mock_other_foo.return_value = 3
    assert foo() == 3
    mock_other_foo.return_value = 42
    assert foo() == 42

You can find more information here and there.