Python unittest mocking
The python "mock" library is the preferred mocking library for python unittests
in the ChromiumOS source. There still exist many tests that use "mox", but
"mock" is preferred for all new tests. python-mock is the standard mocking
framework starting in Python 3.3 (imported as
python-mock is already added to the chroot, but people who run unit tests
outside the chroot need to manually install it. Here are the commands to run:
sudo apt-get purge python-mock sudo apt-get install python-pip sudo pip install mock
Unlike pymox, python-mock is really easy to extend - no messing with
VerifyAll()s, etc. It's also more pythonic. Here's a quick
snippet of running a test with
somemodule.func() mocked out:
import somemodule with patch.object(somemodule, 'func') as mocked_f: mocked_f.returnvalue = 5 ret = somemodule.func('arg1', 'arg2') assertEquals(ret, 5) mocked_f.assert_called_once_with('arg1', 'arg2')
- Take a look at python-mock docs.
- There's also a talk given by the author on how to test with python-mock.
In the chromite source there is a partial mocking library, located at
lib/partial_mock.py. This extends python-mock to allow for writing
re-usable mocks. An example is the
class, which can be used to easily simulate command line behavior.
Here's a code snippet of a function we want to test:
def func(): try: result = cros_build_lib.run(['./some_command'], stdout=True, stderr=True) return 'passcode' in result.stdout except RunCommandError as e: if (e.result.returncode == 2 and 'monkey' in e.result.stderr): return False raise
Here's how we'd test it with the RunCommandMock partial mock:
from cros_build_lib_unittest import RunCommandMock, RunCommandError with RunCommandMock() as pmock: pmock.AddCmdResult(['./some_command'], returncode=2, stderr='12 monkeys') assertEquals(func(), False) pmock.AddCmdResult(['./some_command'], returncode=1) assertRaises(RunCommandError, func) pmock.AddCmdResult(['./some_command'], stdout='passcode') assertEquals(func(), True)
No mucking with setting up mock
CommandResults, or mock
testing for the right
error_code_ok=True arguments. The
partial mock handles all of that for you by using
run logic directly.
It's also easy to write your own partial mocks that can then be used by other unit tests. Having a corpus of re-usable mocks will make the barrier to testing lower, and will help us get into a more test-driven development workflow.
Thanks for reading, and happy mocking!