Understanding python mock framework

Question

I haven't been able to found a good explanation of this in the net, I'm guessing that i'm missing something trivial but I haven't been able to find it, so I came here to ask the experts :)

I have a test were I need to patch a constructor call, reading the docs as I understand, something like this should work, but:

import unittest.mock as mocker
import some_module

mock_class1 = mocker.patch('some_module.some_class')
print(mock_class1 is some_module.some_class)  # Returns False
print(mock_class1) # <unittest.mock._patch>
mock_instance1 = mock_class1.return_value # _patch object has no attr return_value

Instead I get a different output if I do this

with mocker.patch('some_module.some_class') as mock_class2:
    print(mock_class2 is some_module.some_class)  # Returns True
    print(mock_class2) # <MagicMock name=...>
    mock_instance2 = mock_class2.return_value  # No problem
    print(mock_instance2) # <NonCallableMagicMock name=...>

Now, for the test itself, i'm using pytest-mock module which gives a mocker fixture that behaves like the first code block.

I would like to know:

1. why the behavior differs depending on the way that one call the mock framework

2. is there a clean way to trigger the behavior of the second code block without the with clause?

Answer 1

1) pytest mocker plugin is being developer to avoid use of context managers; and probably not everybody fond of the way the standard mock plays with functions parameter 2) not really. It is intended to be used either as content manager or function decorator. I think it is possible use mocker package without pytest

References https://github.com/pytest-dev/pytest-mock https://www.packtpub.com/mapt/book/application_development/9781847198846/5/ch05lvl1sec45/integrating-with-python-mocker

What about installing pytest-mock and creating a test like this

import itertools

def test1(mocker):

    mock_class1 = mocker.patch('itertools.count')
    print(mock_class1 is itertools.count)
    print(mock_class1)
    mock_instance1 = mock_class1.return_value # Magic staff...

or may be using monkeypatching ? Just do not use the standard unittest.mock with pytest