Why isn't my mock.assert_called_once() passing?

Question

I have function 1 here:

def f1(x):
    f2(x+3)
    return x * x

And function 2 here:

def f2(y):
    print("Whatever you do, don't print this.")

I wrote my test here:

def test_f1(mocker):
    mock = mocker.patch("functions.f2")
    assert f1(1) == 1
    assert f2(1) is None
    mock.f2.assert_called_once()

When I run pytest, it throws the error:

>       mock.f2.assert_called_once()
E       AssertionError: Expected 'f2' to have been called once. Called 0 times.

I literally just called "f2" but it's not picking up on it? Not sure why.

Answer 1

For a number of reasons:

1. mocker.patch returns the mock, so the mocked f2 is mock , not mock.f2 .

2. From the code you posted, it appears you have a line like

   from functions import f1, f2

   or

   from functions import *

   This means that if you monkey-patch functions.f2 , you are not monkey-patching your local variable f2 . mock is the same as the mocked functions.f2 , but f2 is no longer the same as functions.f2 (because it's the original functions.f2 , not the mocked one). You can fix this (along with problem #1) with code like:

    f2 = mocker.patch("functions.f2") #... f2.assert_called_once()

3. Except now assert f2(1) is None will fail. This is because you mocked f2 . That means the real f2 isn't called. Did you mean to do a spy instead of a mock? With a spy, the real function is called. pytest-mock can handle spies, though you might need to change your imports slightly:

    import functions from functions import f1, f2 def test_f2(mocker): f2 = mocker.spy(functions, 'f2') assert f1(1) == 1 assert f2(1) is None f2.assert_called_once()

4. And now we finally...get a failed assert_called_once() again, this time because f2 has been called twice. That's because it's called once when you call f1(1) , and then a second time when you call f2(1) .

Answer 2

You're asserting that mock.f2() was done, not that f2() was done.

I'm not exactly clear on what kind of object your fixture mocker is, nor am I sure I understand what you actually want to do, but perhaps you want mock.assert_called_once() ? You can also check mock.mock_calls() .