Use assert functions instead of methods to avoid insidious test problems

mocked.py

>>> import mock
>>> m = mock.Mock()
>>> m.assert_called_once()  # Old version of Mock? This may not be what you expect.
<Mock name='mock.assert_called_once()' id='4558624144'>

>>> assert_called_once = mock.Mock.assert_called_once  # Doesn't exist. Now you know!
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'Mock' has no attribute 'assert_called_once'

>>> assert_called_once_with = mock.Mock.assert_called_once_with  # There it is.

>>> assert_called_once_with(m)  # This does the right thing.
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../mock.py", line 845, in assert_called_once_with
    raise AssertionError(msg)
AssertionError: Expected to be called once. Called 0 times.

mock is a beautiful and incredibly useful module, but it's easy to misspell its .assert_* methods. Starting in Python 3.5, misspelling a method starting with .assert or .assret will raise an AttributeError. But even with that help, my_mock.ssert_called_once_with() (missing the "a" in "assert") will cheerfully succeed. If your mock assertions are critical - and why would you be testing them if they're not? - consider writing them as function calls instead of method invocation.

Remember that given obj = Cls(), that obj.method() is exactly the same as Cls.method(obj). You can define a list of assert functions that you'd like to use and call only those functions, not the methods they represent. Then your tests will fail with a NameError if you ever misspell a function and good editors will alert you to the problem as you code.