py.test patch on fixture

Question

I use the following to mock constant values for a test with py.test:

@patch('ConstantsModule.ConstantsClass.DELAY_TIME', 10)
def test_PowerUp():
    ...
    thing = Thing.Thing()
    assert thing.a == 1

This mocks DELAY_TIME as used in both the test, and in Thing, which is what I expected.

I wanted to do this for all the tests in this file, so I tried

@patch('ConstantsModule.ConstantsClass.DELAY_TIME', 10)
@pytest.fixture(autouse=True)
def NoDelay():
    pass

But that doesn't seem to have the same effect.

Here is a similar question: pytest-mock mocker in pytest fixture, but the mock seems done in a non-decorator way there.

score 63 · Answer 1 · answered Apr 26 '18 at 17:17

63

I'd say patching via decorator is not the optimal approach here. I'd use the context manager:

import pytest
from unittest.mock import patch


@pytest.fixture(autouse=True)
def no_delay():
    with patch('ConstantsModule.ConstantsClass.DELAY_TIME', 10):
        yield

This way, patching is cleanly reverted on test teardown.

answered Apr 26 '18 at 17:17

hoefling

59,418
12
147
194

how can I pass in the delay time to this fixture from within a test case? – GlaceCelery May 07 '19 at 10:11
Indirect parametrization allows you to give fixtures values to run over. https://docs.pytest.org/en/6.2.x/example/parametrize.html#indirect-parametrization – sinback Oct 14 '21 at 20:17
this solution is very clean – Francisco Albert Feb 24 '23 at 11:45

flub · Answer 2 · 2018-04-27T19:55:36.450

15

pytest provides builtin patching support via the monkeypatch fixture. So to patch the constant for all tests in the file you can create the following autouse fixture:

@pytest.fixture(autouse=True)
def no_delay(monkeypatch):
    monkeypatch.setattr(ConstantsModule.ConstantsClass, 'DELAY_TIME', 10)

If you don't want to have the ConstantsModule imported in your tests you can use a string, see the full API reference.

edited Apr 27 '18 at 19:55

answered Apr 26 '18 at 19:16

flub

5,953
27
24

3

Beware that `monkeypatch` is a function-scoped fixture, so your test will raise a `ScopeMismatch`. – hoefling Apr 26 '18 at 19:57
3

Whoops, that'll teach me for not testing the code I post. Thanks for the heads up, I've corrected it. – flub Apr 27 '18 at 19:56

py.test patch on fixture

2 Answers2

Linked