I am a little confused about the usage of patch and of its arguments: new and side_effect (although this is not a proper argument of patch but is passed to the created MagicMock).

When should I use or the other? or both possibly?

In my case, I can achieve my intended mocking with either method.

In this case, I was mocking the get response to an external api, the classic of the classic. I exemplified this here in this barebones module:

# src/module.py

class API:

    def get():
        return {"actual request": True} # assume this were making an actual request

def function_with_api_get_request():

    api = API()

    resp = api.get()

    if resp['actual request']:
        return 0
    else:
        return 1

new

from unittest.mock import patch
from src.module import function_with_api_get_request

expected_if_correct_mocking = 1

def mock_get_request(*args) -> dict:

    print(f"I am a mocked API.get, called with args {args}.")
    return {"actual request": False}

@patch("src.module.API.get", new=mock_get_request)
def test_function_with_api_get_request():

    actual = function_with_api_get_request()
    assert actual == expected_if_correct_mocking      

When using new, I am telling patch which object I want to use to substitute the target object. Interestingly, I noted here that the Mocked object is passed as first argument to the mock_get_request function. Its type is that of the object being mocked (I was expecting this to be MagicMock instead! why is it so?).

output

I am a mocked API.get, called with args (<src.module.API object at 0x7f01046ddc70>,).

side_effect

from unittest.mock import patch
from src.module import function_with_api_get_request

expected_if_correct_mocking = 1

def mock_get_request(*args) -> dict:

    print(f"I am a mocked API.get, called with args {args}.")
    return {"actual request": False}

@patch("src.module.API.get", side_effect=mock_get_request)
def test_function_with_api_get_request(mocked):

    actual = function_with_api_get_request()
    assert actual == expected_if_correct_mocking     

When using side_effect, as per documentation, the Mock is passed to the decorated function. I achieve the same behaviour.

output

I am a mocked API.get, called with args ().

The question is: isn’t the point of side_effect of actually doing something else, as the name also suggests? Maybe it’s totally fine and I am nitpicking.

What are then the cases where one would choose new over side_effect? are there use cases for and can they be used together?