Mock

Links

• unittest.mock Python doc
  • patch
  • patch.object
  • PropertyMock
  • mock_calls

Asserts

Assert Methods

• assert_called()
• assert_called_once()
• assert_called_with(*args, **kwargs) - only passes if the call is the most recent one
• assert_called_once_with(*args, **kwargs) - passes if the mock was called exactly once with the specified arguments
• assert_any_call(*args, **kwargs) - passes if the mock has ever been called with the specified arguments
• assert_has_calls(calls, any_order=False)
• assert_not_called()

Assert only some Arguments

Assert not all exact arguments but just some of them.

Example:

from unittest.mock import Mock, ANY
mock = Mock(return_value=None)
mock("foo", bar="something_I_do_not_care_about")
mock.assert_called_once_with("foo", bar=ANY)

Spy

If you do not want to replace a function with a mock, but want to observe the parameters passed to the function, a so-called spy can be used.

Spy on a Function

To do this we use a combination of patch and the wraps argument. Note that the first argument of patch is a string.

Example:

def my_function(x):
    return x + 1

with patch("__main__.my_function", wraps=my_function) as wrapped_function:
    result = my_function(5)
    assert result == 6
    wrapped_function.assert_called_once_with(5)

Spy on a Method

To do this we use a combination of patch.object and the wraps argument. Note that the first argument of patch.object is a object (instance).

Example:

class MyClass:
    def my_function(self, x):
        return x + 1

my_instance = MyClass()
with patch.object(my_instance, 'my_function', wraps=my_instance.my_function) as wrapped_method:
    result = my_instance.my_function(5)
    assert result == 6
    wrapped_method.assert_called_once_with(5)

Seal a Mock

Seal disables the automatic creation of mocks when accessing an attribute of the mock. Also see: https://docs.python.org/3/library/unittest.mock.html#unittest.mock.seal

One problem with normal mocks is that they auto-create attributes on demand. If you misspell one of these assert methods then your assertion is simply swallowed and ignored. If you seal the mock after creation this avoids this problem.

Example:

from unittest.mock import Mock, seal
mock = Mock()
# more configuration here
seal(mock)