Mock v0.7.2 documentation

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mocksignature

A problem with using mock objects to replace real objects in your tests is that Mock can be too flexible. Your code can treat the mock objects in any way and you have to manually check that they were called correctly. If your code calls functions or methods with the wrong number of arguments then mocks don’t complain.

The solution to this is mocksignature, which creates functions with the same signature as the original, but delegating to a mock. You can interrogate the mock in the usual way to check it has been called with the right arguments, but if it is called with the wrong number of arguments it will raise a TypeError in the same way your production code would.

Another advantage is that your mocked objects are real functions, which can be useful when your code uses inspect or depends on functions being functions.

mocksignature(func, mock=None, skipfirst=False)

Create a new function with the same signature as func that delegates to mock. If skipfirst is True the first argument is skipped, useful for methods where self needs to be omitted from the new function.

If you don’t pass in a mock then one will be created for you.

The mock is set as the mock attribute of the returned function for easy access.

mocksignature can also be used with classes. It copies the signature of the __init__ method.

When used with callable objects (instances) it copies the signature of the __call__ method.

mocksignature will work out if it is mocking the signature of a method on an instance or a method on a class and do the “right thing” with the self argument in both cases.

Because of a limitation in the way that arguments are collected by functions created by mocksignature they are always passed as positional arguments (including defaults) and not keyword arguments.

Example use

Basic use

>>> from mock import mocksignature, Mock
>>> def function(a, b, c=None):
...     pass
...
>>> mock = Mock()
>>> function = mocksignature(function, mock)
>>> function()
Traceback (most recent call last):
  ...
TypeError: <lambda>() takes at least 2 arguments (0 given)
>>> mock.return_value = 'some value'
>>> function(1, 2, 'foo')
'some value'
>>> function.mock.assert_called_with(1, 2, 'foo')

Keyword arguments

Note that arguments to functions created by mocksignature are always passed in to the underlying mock by position even when called with keywords:

>>> from mock import mocksignature
>>> def function(a, b, c=None):
...     pass
...
>>> function = mocksignature(function)
>>> function.mock.return_value = None
>>> function(1, 2)
>>> function.mock.assert_called_with(1, 2, None)

Mocking methods and self

When you use mocksignature to replace a method on a class then self will be included in the method signature - and you will need to include the instance when you do your asserts:

>>> from mock import mocksignature
>>> class SomeClass(object):
...     def method(self, a, b, c=None):
...         pass
...
>>> SomeClass.method = mocksignature(SomeClass.method)
>>> SomeClass.method.mock.return_value = None
>>> instance = SomeClass()
>>> instance.method()
Traceback (most recent call last):
  ...
TypeError: <lambda>() takes at least 4 arguments (1 given)
>>> instance.method(1, 2, 3)
>>> instance.method.mock.assert_called_with(instance, 1, 2, 3)

When you use mocksignature on instance methods self isn’t included:

>>> from mock import mocksignature
>>> class SomeClass(object):
...     def method(self, a, b, c=None):
...         pass
...
>>> instance = SomeClass()
>>> instance.method = mocksignature(instance.method)
>>> instance.method.mock.return_value = None
>>> instance.method(1, 2, 3)
>>> instance.method.mock.assert_called_with(1, 2, 3)

mocksignature with classes

When used with a class mocksignature copies the signature of the __init__ method.

>>> from mock import mocksignature
>>> class Something(object):
...     def __init__(self, foo, bar):
...         pass
...
>>> MockSomething = mocksignature(Something)
>>> instance = MockSomething(10, 9)
>>> assert instance is MockSomething.mock.return_value
>>> MockSomething.mock.assert_called_with(10, 9)
>>> MockSomething()
Traceback (most recent call last):
  ...
TypeError: <lambda>() takes at least 2 arguments (0 given)

Because the object returned by mocksignature is a function rather than a Mock you lose the other capabilities of Mock, like dynamic attribute creation.

mocksignature with callable objects

When used with a callable object mocksignature copies the signature of the __call__ method.

>>> from mock import mocksignature
>>> class Something(object):
...     def __call__(self, spam, eggs):
...         pass
...
>>> something = Something()
>>> mock_something = mocksignature(something)
>>> result = mock_something(10, 9)
>>> mock_something.mock.assert_called_with(10, 9)
>>> mock_something()
Traceback (most recent call last):
  ...
TypeError: <lambda>() takes at least 2 arguments (0 given)

Because the object returned by mocksignature is a function rather than a Mock you lose the other capabilities of Mock, like dynamic attribute creation.

mocksignature argument to patch

mocksignature is available as a keyword argument to patch() or patch.object(). It can be used with functions / methods / classes and callable objects.

>>> from mock import patch
>>> class SomeClass(object):
...     def method(self, a, b, c=None):
...         pass
...
>>> @patch.object(SomeClass, 'method', mocksignature=True)
... def test(mock_method):
...     instance = SomeClass()
...     mock_method.return_value = None
...     instance.method(1, 2)
...     mock_method.assert_called_with(instance, 1, 2, None)
...
>>> test()