Introducing sham

Some people, when their cats throw up in the middle of the night, go back to sleep. Some people write their own test double library. I am the latter person.

I just pushed my initial commit for sham, a test double library I started making because I have issues with the popular Mock.

Now, I love what Mock does. After Adam Tauno Williams' presentation on it at the Grand Rapids Python Users Group, I started using it for octothorpe, replacing some custom-built test doubles that recorded their behavior. Mock let me magically build those instead, making for a cleaner test suite.

But the API makes me chafe. My chief beef is this:

>>> m = mock.Mock()
>>> m(True)
<Mock name='mock()' id='4300958352'>
>>> m.assert_called_once_with(True)
>>> m.assert_called_once_with(False)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../mock.py", line 846, in assert_called_once_with
    return self.assert_called_with(*args, **kwargs)
  File ".../mock.py", line 835, in assert_called_with
    raise AssertionError(msg)
AssertionError: Expected call: mock(False)
Actual call: mock(True)

At first blush, nothing may seem particularly wrong here, right? But Mock has magic behavior, which, while useful, can cause some subtle problems to appear.

>>> m.assert_sky_is_pink()

This assertion should not have passed. Clearly, the sky isn't pink, but that's not our core problem—our core problem is actually that assert_sky_is_pink is not a valid assert method. But our Mock did what a good Mock should do: it spawned itself a brand new sub-Mock called assert_sky_is_pink and called that instead.

Now, obviously, I don't go around asserting the sky is pink. But I don't always keep Mock's API in my head, and as a result once thought it had a method called assert_called_once. It doesn't, but my tests did not fail when I attempted to make that assertion. I began to think that perhaps assertion methods should be kept off the Mock itself—moved to the module, perhaps—so that there was no ambiguity.

Of course, the logical response to this line of reasoning is that I'm not doing unit testing properly. I should have noticed my assertion did not fail, because I'm supposed to make tests that fail first, then code until they pass. I get that. But I'm not always testing first. In real life, I'm often testing after the fact. (And why am I calling a method that doesn't exist? Well, there's another problem here, and that is that Mock's API sometimes doesn't make a lot of sense. I'm looking at you, side_effect.)

sham is my attempt to make a compelling replacement for Mock. I'm starting clean, I'm experimenting a bit with some useful functionality like keeping a log. Right now, all it does is log calls and attribute gets, but I intend to add more functionality, such as returned values, side effects, and replays.

>>> s = sham.Sham()
>>> s(True)
>>> sham.assertCallCount(s, 1)
>>> sham.assertCallCount(s, 2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "sham/__init__.py", line 112, in assertCallCount
    assert count == log_count, '%d == %d' % (count, log_count)
AssertionError: 2 == 1
>>> sham.assertCalledWith(s, True)
>>> sham.assertCalledWith(s, False)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "sham/__init__.py", line 123, in assertCalledWith
    raise AssertionError(repr(match_entry))
AssertionError: <CallLogEntry False, >
>>> s.foo
<sham.Sham object at 0x1004aafd0>
>>> sham.getLog(s)
[(1385900294.091232, <CallLogEntry True, >),
 (1385900331.47395, <GetAttrLogEntry 'foo'>)]
>>> sham.filterLog(s, sham.CallLogEntry)
[(1385900294.091232, <CallLogEntry True, >)]

Astute observers will notice this is not very PEP 8. My primary reasoning for this is that unittest is also not very PEP 8. Shifting back and forth between styles in test assertions causes me mental friction I'd rather not have to deal with, especially when I've already found I have to unlearn a little bit of Mock's method call style to properly use sham. I think it will be worth it in the end, though.

sham is being developed at GitHub.