Python monkeypatch.setattr() with pytest fixture at module scope

Question

First of all, the relevant portion of my project directory looks like:

└── my_package
    ├── my_subpackage
    │   ├── my_module.py
    |   └── other_module.py
    └── tests
        └── my_subpackage
            └── unit_test.py

I am writing some tests in unit_test.py that require mocking of an external resource at the module level. I would like to use a pytest fixture with module level scope and pytest monkeypatch to acomplish this. Here is a snippet of what I have tried in unit_test.py:

import unittest.mock as mock
import pytest
from my_package.my_subpackage.my_module import MyClass


@pytest.fixture(scope='function')
def external_access(monkeypatch):
    external_access = mock.MagicMock()
    external_access.get_something = mock.MagicMock(
        return_value='Mock was used.')
    monkeypatch.setattr(
        'my_package.my_subpackage.my_module.ExternalAccess.get_something',
        external_access.get_something)


def test_get_something(external_access):
    instance = MyClass()
    instance.get_something()
    assert instance.data == 'Mock was used.'

Everything works just fine. But when I try to change line 8 from @pytest.fixture(scope='function') to @pytest.fixture(scope='module'), I get the following error.

ScopeMismatch: You tried to access the 'function' scoped fixture 'monkeypatch' with a 'module' scoped request object, involved factories
my_packagetestsunit_test.py:7:  def external_access(monkeypatch)
....Anaconda3envspy37libsite-packages_pytestmonkeypatch.py:20:  def monkeypatch()

Does anyone know how to monkeypatch with module level scope?

In case anyone wants to know, this is what the two modules look like as well.

my_module.py

from my_package.my_subpackage.other_module import ExternalAccess


class MyClass(object):
    def __init__(self):
        self.external_access = ExternalAccess()
        self.data = None

    def get_something(self):
        self.data = self.external_access.get_something()

other_module.py

class ExternalAccess(object):
    def get_something(self):
        return 'Call to external resource.'

Asked By: Rich Inman

||

Source

Answer 1

I found this issue which guided the way. I needed to make a few changes to the solution for module level scope. unit_test.py now looks like this:

import unittest.mock as mock

import pytest

from my_package.my_subpackage.my_module import MyClass


@pytest.fixture(scope='module')
def monkeymodule():
    from _pytest.monkeypatch import MonkeyPatch
    mpatch = MonkeyPatch()
    yield mpatch
    mpatch.undo()

@pytest.fixture(scope='module')
def external_access(monkeymodule):
    external_access = mock.MagicMock()
    external_access.get_something = mock.MagicMock(
        return_value='Mock was used.')
    monkeymodule.setattr(
        'my_package.my_subpackage.my_module.ExternalAccess.get_something',
        external_access.get_something)


def test_get_something(external_access):
    instance = MyClass()
    instance.get_something()
    assert instance.data == 'Mock was used.'

Answered By: Rich Inman

Answer 2

This has gotten simpler as of pytest 6.2, thanks to the pytest.MonkeyPatch class and context-manager (https://docs.pytest.org/en/6.2.x/reference.html#pytest.MonkeyPatch). Building off Rich’s answer, the monkeymodule fixture can now be written as follows:

@pytest.fixture(scope='module')
def monkeymodule():
    with pytest.MonkeyPatch.context() as mp:
        yield mp

@pytest.fixture(scope='function')
def external_access(monkeymodule):
    external_access = mock.MagicMock()
    external_access.get_something = mock.MagicMock(
        return_value='Mock was used.')
    monkeymodule.setattr(
        'my_package.my_subpackage.my_module.ExternalAccess.get_something',
        external_access.get_something)

Answered By: Steven D.

Python monkeypatch.setattr() with pytest fixture at module scope

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