Mock autospec

updated: 2022-09-23 11:58

Mock autospec

In typical unit tests, almost all of the dependencies are mocked or patched (mock.patch), without any guarantee that the mocked methods actually exist, or if their signatures are respected. Because of this, actual issues can be easily overlooked and missed, as the unit tests are wrongfully passing.

The mock.Mock class accepts a spec as an argument, which only solves half the problem: it only checks if an attribute exists, based on the given spec. It does not guarantee that the given attribute is actually a method, or if its signature is respected. The Mock class does not accept an autospec argument [1].

mock.patch, mock.patch.object, mock.patch.multiple accept an autospec argument, but because of a bug [2], it cannot be used properly.

oslotest offers a solution for problems mentioned above.

  1. https://github.com/testing-cabal/mock/issues/393

  2. https://github.com/testing-cabal/mock/issues/396

Patching the mock module

The oslotest.mock_fixture module contains 2 components:

  • patch_mock_module

  • MockAutospecFixture

Both components need to be used in order to fix various issues within mock regarding autospec.

patch_mock_module

At the moment, mock.patch, mock.patch.object, mock.patch.multiple accepts the autospec argument, but it does not correctly consume the self / cls argument of methods or class methods.

patch_mock_module addresses this issue. In order to make sure that the original version of mock.patch is not used by the unit tests, this function has to be called as early as possible within the test module, or the base test module. E.g.:

nova/test.py
...
from oslotest import mock_fixture

mock_fixture.patch_mock_module()

Additionally, this function will set the autospec argument’s value to True, unless otherwise specified or these arguments are passed in: new_callable, create, spec.

MockAutospecFixture

mock.Mock and mock.MagicMock classes do not accept any autospec argument. This fixture will replace the mock.Mock and mock.MagicMock classes with subclasses which accepts the said argument.

The fixture can be used in the test setUp method. E.g.:

nova/test.py
...
from oslotest import mock_fixture

class TestCase(testtools.TestCase):
    def setUp(self):
        super(TestCase, self).setUp()
        self.useFixture(mock_fixture.MockAutospecFixture())

Mock autospec usage

Consider the following class as an example:

class Foo(object):
    def bar(self, a, b, c, d=None):
        pass

Using the setup described above, the following unit tests will now pass correctly:

class FooTestCase(TestCase):
    def test_mock_bar(self):
        mock_foo = mock.Mock(autospec=Foo)
        self.assertRaises(TypeError, mock_foo.bar, invalid='argument')

    @mock.patch.object(Foo, 'bar', autospec=True)
    def test_patch_bar(self, mock_bar):
        foo = Foo()
        self.assertRaises(TypeError, foo.bar, invalid='argument')
updated: 2022-09-23 11:58
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