pytest-mock patch context manager not restoring object on exit

Question

We've recently switched from unittest to pytest. I've encountered a strange problem when using mocker.patch as a context manager. Consider the following example.

module_a.py

class MyClass:
    def value(self):
        return 10

module_b.py

import module_a
class AnotherClass:
    def get_value(self):
        return module_a.MyClass().value()

test_module_b.py

from module_b import AnotherClass
def test_main_2(mocker):
    with mocker.patch('module_a.MyClass.value', return_value=20):
        value = AnotherClass().get_value()
        assert value == 20
    value = AnotherClass().get_value()
    assert value == 10

I would expect that once the context manager exits, MyClass's value method method would be restored (return value of 10), however the test fails on the second assert statement with an assertion error 20 != 10 If I use the exact same test, but replace mocker.patch with unittest.mock.patch, it passes. I thought that pytest-mock shared the same API as unittest.mock, so I'm confused as to why there is a difference.

wim · Accepted Answer · 2019-10-08T21:33:12.523

With pytest-mock, teardown is done when exiting the fixture context. The mocker.patch object is not just a simple alias for mock.patch.

You should not need context managers within the test functions when writing pytest-style tests, and in fact the purpose of the pytest-mock plugin is to make the use of context managers and function decorators for mocking unnecessary.

If for some reason you do need a teardown step from within the test itself, then you want the plain old mock API which also works fine within pytest.

from unittest.mock import patch

def test_main_2():
    with patch('module_a.MyClass.value', return_value=20):
        value = AnotherClass().get_value()
        assert value == 20
    value = AnotherClass().get_value()
    assert value == 10

Be aware that this nested structure is really what pytest intends to avoid, to make your tests more readable, so you're somewhat missing the point if you don't do setup and teardown entirely with fixtures.

Thank you for the clear explanation. I am running across this problem now by using hypothesis + pytest (and mocking). Hypothesis is great, but it does not play well with pytest-mock: `HypothesisDeprecationWarning: test_fileaccess.py::test_readfile uses the 'mocker' fixture, which is reset between function calls but not between test cases generated by `@given(...)`. You can change it to a module- or session-scoped fixture if it is safe to reuse; if not we recommend using a context manager inside your test function. See https://docs.pytest.org/en/latest/fixture.html#sharing-test-data` — Mike Williamson, Sep 17 '20 at 14:44

pytest-mock patch context manager not restoring object on exit

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