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.
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')
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