:mod:`unittest.mock` --- mock object library ============================================ .. module:: unittest.mock :synopsis: Mock object library. .. moduleauthor:: Michael Foord .. currentmodule:: unittest.mock .. versionadded:: 3.3 :mod:`unittest.mock` is a library for testing in Python. It allows you to replace parts of your system under test with mock objects and make assertions about how they have been used. `unittest.mock` provides a core :class:`Mock` class removing the need to create a host of stubs throughout your test suite. After performing an action, you can make assertions about which methods / attributes were used and arguments they were called with. You can also specify return values and set needed attributes in the normal way. Additionally, mock provides a :func:`patch` decorator that handles patching module and class level attributes within the scope of a test, along with :const:`sentinel` for creating unique objects. See the `quick guide`_ for some examples of how to use :class:`Mock`, :class:`MagicMock` and :func:`patch`. Mock is very easy to use and is designed for use with :mod:`unittest`. Mock is based on the 'action -> assertion' pattern instead of `'record -> replay'` used by many mocking frameworks. There is a backport of `unittest.mock` for earlier versions of Python, available as `mock on PyPI `_. **Source code:** :source:`Lib/unittest/mock.py` Quick Guide ----------- :class:`Mock` and :class:`MagicMock` objects create all attributes and methods as you access them and store details of how they have been used. You can configure them, to specify return values or limit what attributes are available, and then make assertions about how they have been used: >>> from unittest.mock import MagicMock >>> thing = ProductionClass() >>> thing.method = MagicMock(return_value=3) >>> thing.method(3, 4, 5, key='value') 3 >>> thing.method.assert_called_with(3, 4, 5, key='value') :attr:`side_effect` allows you to perform side effects, including raising an exception when a mock is called: >>> mock = Mock(side_effect=KeyError('foo')) >>> mock() Traceback (most recent call last): ... KeyError: 'foo' >>> values = {'a': 1, 'b': 2, 'c': 3} >>> def side_effect(arg): ... return values[arg] ... >>> mock.side_effect = side_effect >>> mock('a'), mock('b'), mock('c') (1, 2, 3) >>> mock.side_effect = [5, 4, 3, 2, 1] >>> mock(), mock(), mock() (5, 4, 3) Mock has many other ways you can configure it and control its behaviour. For example the `spec` argument configures the mock to take its specification from another object. Attempting to access attributes or methods on the mock that don't exist on the spec will fail with an `AttributeError`. The :func:`patch` decorator / context manager makes it easy to mock classes or objects in a module under test. The object you specify will be replaced with a mock (or other object) during the test and restored when the test ends: >>> from unittest.mock import patch >>> @patch('module.ClassName2') ... @patch('module.ClassName1') ... def test(MockClass1, MockClass2): ... module.ClassName1() ... module.ClassName2() ... assert MockClass1 is module.ClassName1 ... assert MockClass2 is module.ClassName2 ... assert MockClass1.called ... assert MockClass2.called ... >>> test() .. note:: When you nest patch decorators the mocks are passed in to the decorated function in the same order they applied (the normal *python* order that decorators are applied). This means from the bottom up, so in the example above the mock for `module.ClassName1` is passed in first. With `patch` it matters that you patch objects in the namespace where they are looked up. This is normally straightforward, but for a quick guide read :ref:`where to patch `. As well as a decorator `patch` can be used as a context manager in a with statement: >>> with patch.object(ProductionClass, 'method', return_value=None) as mock_method: ... thing = ProductionClass() ... thing.method(1, 2, 3) ... >>> mock_method.assert_called_once_with(1, 2, 3) There is also :func:`patch.dict` for setting values in a dictionary just during a scope and restoring the dictionary to its original state when the test ends: >>> foo = {'key': 'value'} >>> original = foo.copy() >>> with patch.dict(foo, {'newkey': 'newvalue'}, clear=True): ... assert foo == {'newkey': 'newvalue'} ... >>> assert foo == original Mock supports the mocking of Python :ref:`magic methods `. The easiest way of using magic methods is with the :class:`MagicMock` class. It allows you to do things like: >>> mock = MagicMock() >>> mock.__str__.return_value = 'foobarbaz' >>> str(mock) 'foobarbaz' >>> mock.__str__.assert_called_with() Mock allows you to assign functions (or other Mock instances) to magic methods and they will be called appropriately. The `MagicMock` class is just a Mock variant that has all of the magic methods pre-created for you (well, all the useful ones anyway). The following is an example of using magic methods with the ordinary Mock class: >>> mock = Mock() >>> mock.__str__ = Mock(return_value='wheeeeee') >>> str(mock) 'wheeeeee' For ensuring that the mock objects in your tests have the same api as the objects they are replacing, you can use :ref:`auto-speccing `. Auto-speccing can be done through the `autospec` argument to patch, or the :func:`create_autospec` function. Auto-speccing creates mock objects that have the same attributes and methods as the objects they are replacing, and any functions and methods (including constructors) have the same call signature as the real object. This ensures that your mocks will fail in the same way as your production code if they are used incorrectly: >>> from unittest.mock import create_autospec >>> def function(a, b, c): ... pass ... >>> mock_function = create_autospec(function, return_value='fishy') >>> mock_function(1, 2, 3) 'fishy' >>> mock_function.assert_called_once_with(1, 2, 3) >>> mock_function('wrong arguments') Traceback (most recent call last): ... TypeError: () takes exactly 3 arguments (1 given) `create_autospec` can also be used on classes, where it copies the signature of the `__init__` method, and on callable objects where it copies the signature of the `__call__` method. The Mock Class -------------- `Mock` is a flexible mock object intended to replace the use of stubs and test doubles throughout your code. Mocks are callable and create attributes as new mocks when you access them [#]_. Accessing the same attribute will always return the same mock. Mocks record how you use them, allowing you to make assertions about what your code has done to them. :class:`MagicMock` is a subclass of `Mock` with all the magic methods pre-created and ready to use. There are also non-callable variants, useful when you are mocking out objects that aren't callable: :class:`NonCallableMock` and :class:`NonCallableMagicMock` The :func:`patch` decorators makes it easy to temporarily replace classes in a particular module with a `Mock` object. By default `patch` will create a `MagicMock` for you. You can specify an alternative class of `Mock` using the `new_callable` argument to `patch`. .. class:: Mock(spec=None, side_effect=None, return_value=DEFAULT, wraps=None, name=None, spec_set=None, **kwargs) Create a new `Mock` object. `Mock` takes several optional arguments that specify the behaviour of the Mock object: * `spec`: This can be either a list of strings or an existing object (a class or instance) that acts as the specification for the mock object. If you pass in an object then a list of strings is formed by calling dir on the object (excluding unsupported magic attributes and methods). Accessing any attribute not in this list will raise an `AttributeError`. If `spec` is an object (rather than a list of strings) then :attr:`__class__` returns the class of the spec object. This allows mocks to pass `isinstance` tests. * `spec_set`: A stricter variant of `spec`. If used, attempting to *set* or get an attribute on the mock that isn't on the object passed as `spec_set` will raise an `AttributeError`. * `side_effect`: A function to be called whenever the Mock is called. See the :attr:`~Mock.side_effect` attribute. Useful for raising exceptions or dynamically changing return values. The function is called with the same arguments as the mock, and unless it returns :data:`DEFAULT`, the return value of this function is used as the return value. Alternatively `side_effect` can be an exception class or instance. In this case the exception will be raised when the mock is called. If `side_effect` is an iterable then each call to the mock will return the next value from the iterable. A `side_effect` can be cleared by setting it to `None`. * `return_value`: The value returned when the mock is called. By default this is a new Mock (created on first access). See the :attr:`return_value` attribute. * `wraps`: Item for the mock object to wrap. If `wraps` is not None then calling the Mock will pass the call through to the wrapped object (returning the real result and ignoring `return_value`). Attribute access on the mock will return a Mock object that wraps the corresponding attribute of the wrapped object (so attempting to access an attribute that doesn't exist will raise an `AttributeError`). If the mock has an explicit `return_value` set then calls are not passed to the wrapped object and the `return_value` is returned instead. * `name`: If the mock has a name then it will be used in the repr of the mock. This can be useful for debugging. The name is propagated to child mocks. Mocks can also be called with arbitrary keyword arguments. These will be used to set attributes on the mock after it is created. See the :meth:`configure_mock` method for details. .. method:: assert_called_with(*args, **kwargs) This method is a convenient way of asserting that calls are made in a particular way: >>> mock = Mock() >>> mock.method(1, 2, 3, test='wow') >>> mock.method.assert_called_with(1, 2, 3, test='wow') .. method:: assert_called_once_with(*args, **kwargs) Assert that the mock was called exactly once and with the specified arguments. >>> mock = Mock(return_value=None) >>> mock('foo', bar='baz') >>> mock.assert_called_once_with('foo', bar='baz') >>> mock('foo', bar='baz') >>> mock.assert_called_once_with('foo', bar='baz') Traceback (most recent call last): ... AssertionError: Expected to be called once. Called 2 times. .. method:: assert_any_call(*args, **kwargs) assert the mock has been called with the specified arguments. The assert passes if the mock has *ever* been called, unlike :meth:`assert_called_with` and :meth:`assert_called_once_with` that only pass if the call is the most recent one. >>> mock = Mock(return_value=None) >>> mock(1, 2, arg='thing') >>> mock('some', 'thing', 'else') >>> mock.assert_any_call(1, 2, arg='thing') .. method:: assert_has_calls(calls, any_order=False) assert the mock has been called with the specified calls. The `mock_calls` list is checked for the calls. If `any_order` is False (the default) then the calls must be sequential. There can be extra calls before or after the specified calls. If `any_order` is True then the calls can be in any order, but they must all appear in :attr:`mock_calls`. >>> mock = Mock(return_value=None) >>> mock(1) >>> mock(2) >>> mock(3) >>> mock(4) >>> calls = [call(2), call(3)] >>> mock.assert_has_calls(calls) >>> calls = [call(4), call(2), call(3)] >>> mock.assert_has_calls(calls, any_order=True) .. method:: reset_mock() The reset_mock method resets all the call attributes on a mock object: >>> mock = Mock(return_value=None) >>> mock('hello') >>> mock.called True >>> mock.reset_mock() >>> mock.called False This can be useful where you want to make a series of assertions that reuse the same object. Note that `reset_mock` *doesn't* clear the return value, :attr:`side_effect` or any child attributes you have set using normal assignment. Child mocks and the return value mock (if any) are reset as well. .. method:: mock_add_spec(spec, spec_set=False) Add a spec to a mock. `spec` can either be an object or a list of strings. Only attributes on the `spec` can be fetched as attributes from the mock. If `spec_set` is `True` then only attributes on the spec can be set. .. method:: attach_mock(mock, attribute) Attach a mock as an attribute of this one, replacing its name and parent. Calls to the attached mock will be recorded in the :attr:`method_calls` and :attr:`mock_calls` attributes of this one. .. method:: configure_mock(**kwargs) Set attributes on the mock through keyword arguments. Attributes plus return values and side effects can be set on child mocks using standard dot notation and unpacking a dictionary in the method call: >>> mock = Mock() >>> attrs = {'method.return_value': 3, 'other.side_effect': KeyError} >>> mock.configure_mock(**attrs) >>> mock.method() 3 >>> mock.other() Traceback (most recent call last): ... KeyError The same thing can be achieved in the constructor call to mocks: >>> attrs = {'method.return_value': 3, 'other.side_effect': KeyError} >>> mock = Mock(some_attribute='eggs', **attrs) >>> mock.some_attribute 'eggs' >>> mock.method() 3 >>> mock.other() Traceback (most recent call last): ... KeyError `configure_mock` exists to make it easier to do configuration after the mock has been created. .. method:: __dir__() `Mock` objects limit the results of `dir(some_mock)` to useful results. For mocks with a `spec` this includes all the permitted attributes for the mock. See :data:`FILTER_DIR` for what this filtering does, and how to switch it off. .. method:: _get_child_mock(**kw) Create the child mocks for attributes and return value. By default child mocks will be the same type as the parent. Subclasses of Mock may want to override this to customize the way child mocks are made. For non-callable mocks the callable variant will be used (rather than any custom subclass). .. attribute:: called A boolean representing whether or not the mock object has been called: >>> mock = Mock(return_value=None) >>> mock.called False >>> mock() >>> mock.called True .. attribute:: call_count An integer telling you how many times the mock object has been called: >>> mock = Mock(return_value=None) >>> mock.call_count 0 >>> mock() >>> mock() >>> mock.call_count 2 .. attribute:: return_value Set this to configure the value returned by calling the mock: >>> mock = Mock() >>> mock.return_value = 'fish' >>> mock() 'fish' The default return value is a mock object and you can configure it in the normal way: >>> mock = Mock() >>> mock.return_value.attribute = sentinel.Attribute >>> mock.return_value() >>> mock.return_value.assert_called_with() `return_value` can also be set in the constructor: >>> mock = Mock(return_value=3) >>> mock.return_value 3 >>> mock() 3 .. attribute:: side_effect This can either be a function to be called when the mock is called, or an exception (class or instance) to be raised. If you pass in a function it will be called with same arguments as the mock and unless the function returns the :data:`DEFAULT` singleton the call to the mock will then return whatever the function returns. If the function returns :data:`DEFAULT` then the mock will return its normal value (from the :attr:`return_value`. An example of a mock that raises an exception (to test exception handling of an API): >>> mock = Mock() >>> mock.side_effect = Exception('Boom!') >>> mock() Traceback (most recent call last): ... Exception: Boom! Using `side_effect` to return a sequence of values: >>> mock = Mock() >>> mock.side_effect = [3, 2, 1] >>> mock(), mock(), mock() (3, 2, 1) The `side_effect` function is called with the same arguments as the mock (so it is wise for it to take arbitrary args and keyword arguments) and whatever it returns is used as the return value for the call. The exception is if `side_effect` returns :data:`DEFAULT`, in which case the normal :attr:`return_value` is used. >>> mock = Mock(return_value=3) >>> def side_effect(*args, **kwargs): ... return DEFAULT ... >>> mock.side_effect = side_effect >>> mock() 3 `side_effect` can be set in the constructor. Here's an example that adds one to the value the mock is called with and returns it: >>> side_effect = lambda value: value + 1 >>> mock = Mock(side_effect=side_effect) >>> mock(3) 4 >>> mock(-8) -7 Setting `side_effect` to `None` clears it: >>> m = Mock(side_effect=KeyError, return_value=3) >>> m() Traceback (most recent call last): ... KeyError >>> m.side_effect = None >>> m() 3 .. attribute:: call_args This is either `None` (if the mock hasn't been called), or the arguments that the mock was last called with. This will be in the form of a tuple: the first member is any ordered arguments the mock was called with (or an empty tuple) and the second member is any keyword arguments (or an empty dictionary). >>> mock = Mock(return_value=None) >>> print mock.call_args None >>> mock() >>> mock.call_args call() >>> mock.call_args == () True >>> mock(3, 4) >>> mock.call_args call(3, 4) >>> mock.call_args == ((3, 4),) True >>> mock(3, 4, 5, key='fish', next='w00t!') >>> mock.call_args call(3, 4, 5, key='fish', next='w00t!') `call_args`, along with members of the lists :attr:`call_args_list`, :attr:`method_calls` and :attr:`mock_calls` are :data:`call` objects. These are tuples, so they can be unpacked to get at the individual arguments and make more complex assertions. See :ref:`calls as tuples `. .. attribute:: call_args_list This is a list of all the calls made to the mock object in sequence (so the length of the list is the number of times it has been called). Before any calls have been made it is an empty list. The :data:`call` object can be used for conveniently constructing lists of calls to compare with `call_args_list`. >>> mock = Mock(return_value=None) >>> mock() >>> mock(3, 4) >>> mock(key='fish', next='w00t!') >>> mock.call_args_list [call(), call(3, 4), call(key='fish', next='w00t!')] >>> expected = [(), ((3, 4),), ({'key': 'fish', 'next': 'w00t!'},)] >>> mock.call_args_list == expected True Members of `call_args_list` are :data:`call` objects. These can be unpacked as tuples to get at the individual arguments. See :ref:`calls as tuples `. .. attribute:: method_calls As well as tracking calls to themselves, mocks also track calls to methods and attributes, and *their* methods and attributes: >>> mock = Mock() >>> mock.method() >>> mock.property.method.attribute() >>> mock.method_calls [call.method(), call.property.method.attribute()] Members of `method_calls` are :data:`call` objects. These can be unpacked as tuples to get at the individual arguments. See :ref:`calls as tuples `. .. attribute:: mock_calls `mock_calls` records *all* calls to the mock object, its methods, magic methods *and* return value mocks. >>> mock = MagicMock() >>> result = mock(1, 2, 3) >>> mock.first(a=3) >>> mock.second() >>> int(mock) 1 >>> result(1) >>> expected = [call(1, 2, 3), call.first(a=3), call.second(), ... call.__int__(), call()(1)] >>> mock.mock_calls == expected True Members of `mock_calls` are :data:`call` objects. These can be unpacked as tuples to get at the individual arguments. See :ref:`calls as tuples `. .. attribute:: __class__ Normally the `__class__` attribute of an object will return its type. For a mock object with a `spec` `__class__` returns the spec class instead. This allows mock objects to pass `isinstance` tests for the object they are replacing / masquerading as: >>> mock = Mock(spec=3) >>> isinstance(mock, int) True `__class__` is assignable to, this allows a mock to pass an `isinstance` check without forcing you to use a spec: >>> mock = Mock() >>> mock.__class__ = dict >>> isinstance(mock, dict) True .. class:: NonCallableMock(spec=None, wraps=None, name=None, spec_set=None, **kwargs) A non-callable version of `Mock`. The constructor parameters have the same meaning of `Mock`, with the exception of `return_value` and `side_effect` which have no meaning on a non-callable mock. Mock objects that use a class or an instance as a `spec` or `spec_set` are able to pass `isintance` tests: >>> mock = Mock(spec=SomeClass) >>> isinstance(mock, SomeClass) True >>> mock = Mock(spec_set=SomeClass()) >>> isinstance(mock, SomeClass) True The `Mock` classes have support for mocking magic methods. See :ref:`magic methods ` for the full details. The mock classes and the :func:`patch` decorators all take arbitrary keyword arguments for configuration. For the `patch` decorators the keywords are passed to the constructor of the mock being created. The keyword arguments are for configuring attributes of the mock: >>> m = MagicMock(attribute=3, other='fish') >>> m.attribute 3 >>> m.other 'fish' The return value and side effect of child mocks can be set in the same way, using dotted notation. As you can't use dotted names directly in a call you have to create a dictionary and unpack it using `**`: >>> attrs = {'method.return_value': 3, 'other.side_effect': KeyError} >>> mock = Mock(some_attribute='eggs', **attrs) >>> mock.some_attribute 'eggs' >>> mock.method() 3 >>> mock.other() Traceback (most recent call last): ... KeyError .. class:: PropertyMock(*args, **kwargs) A mock intended to be used as a property, or other descriptor, on a class. `PropertyMock` provides `__get__` and `__set__` methods so you can specify a return value when it is fetched. Fetching a `PropertyMock` instance from an object calls the mock, with no args. Setting it calls the mock with the value being set. >>> class Foo(object): ... @property ... def foo(self): ... return 'something' ... @foo.setter ... def foo(self, value): ... pass ... >>> with patch('__main__.Foo.foo', new_callable=PropertyMock) as mock_foo: ... mock_foo.return_value = 'mockity-mock' ... this_foo = Foo() ... print this_foo.foo ... this_foo.foo = 6 ... mockity-mock >>> mock_foo.mock_calls [call(), call(6)] Calling ~~~~~~~ Mock objects are callable. The call will return the value set as the :attr:`~Mock.return_value` attribute. The default return value is a new Mock object; it is created the first time the return value is accessed (either explicitly or by calling the Mock) - but it is stored and the same one returned each time. Calls made to the object will be recorded in the attributes like :attr:`~Mock.call_args` and :attr:`~Mock.call_args_list`. If :attr:`~Mock.side_effect` is set then it will be called after the call has been recorded, so if `side_effect` raises an exception the call is still recorded. The simplest way to make a mock raise an exception when called is to make :attr:`~Mock.side_effect` an exception class or instance: >>> m = MagicMock(side_effect=IndexError) >>> m(1, 2, 3) Traceback (most recent call last): ... IndexError >>> m.mock_calls [call(1, 2, 3)] >>> m.side_effect = KeyError('Bang!') >>> m('two', 'three', 'four') Traceback (most recent call last): ... KeyError: 'Bang!' >>> m.mock_calls [call(1, 2, 3), call('two', 'three', 'four')] If `side_effect` is a function then whatever that function returns is what calls to the mock return. The `side_effect` function is called with the same arguments as the mock. This allows you to vary the return value of the call dynamically, based on the input: >>> def side_effect(value): ... return value + 1 ... >>> m = MagicMock(side_effect=side_effect) >>> m(1) 2 >>> m(2) 3 >>> m.mock_calls [call(1), call(2)] If you want the mock to still return the default return value (a new mock), or any set return value, then there are two ways of doing this. Either return `mock.return_value` from inside `side_effect`, or return :data:`DEFAULT`: >>> m = MagicMock() >>> def side_effect(*args, **kwargs): ... return m.return_value ... >>> m.side_effect = side_effect >>> m.return_value = 3 >>> m() 3 >>> def side_effect(*args, **kwargs): ... return DEFAULT ... >>> m.side_effect = side_effect >>> m() 3 To remove a `side_effect`, and return to the default behaviour, set the `side_effect` to `None`: >>> m = MagicMock(return_value=6) >>> def side_effect(*args, **kwargs): ... return 3 ... >>> m.side_effect = side_effect >>> m() 3 >>> m.side_effect = None >>> m() 6 The `side_effect` can also be any iterable object. Repeated calls to the mock will return values from the iterable (until the iterable is exhausted and a `StopIteration` is raised): >>> m = MagicMock(side_effect=[1, 2, 3]) >>> m() 1 >>> m() 2 >>> m() 3 >>> m() Traceback (most recent call last): ... StopIteration .. _deleting-attributes: Deleting Attributes ~~~~~~~~~~~~~~~~~~~ Mock objects create attributes on demand. This allows them to pretend to be objects of any type. You may want a mock object to return `False` to a `hasattr` call, or raise an `AttributeError` when an attribute is fetched. You can do this by providing an object as a `spec` for a mock, but that isn't always convenient. You "block" attributes by deleting them. Once deleted, accessing an attribute will raise an `AttributeError`. >>> mock = MagicMock() >>> hasattr(mock, 'm') True >>> del mock.m >>> hasattr(mock, 'm') False >>> del mock.f >>> mock.f Traceback (most recent call last): ... AttributeError: f Attaching Mocks as Attributes ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When you attach a mock as an attribute of another mock (or as the return value) it becomes a "child" of that mock. Calls to the child are recorded in the :attr:`~Mock.method_calls` and :attr:`~Mock.mock_calls` attributes of the parent. This is useful for configuring child mocks and then attaching them to the parent, or for attaching mocks to a parent that records all calls to the children and allows you to make assertions about the order of calls between mocks: >>> parent = MagicMock() >>> child1 = MagicMock(return_value=None) >>> child2 = MagicMock(return_value=None) >>> parent.child1 = child1 >>> parent.child2 = child2 >>> child1(1) >>> child2(2) >>> parent.mock_calls [call.child1(1), call.child2(2)] The exception to this is if the mock has a name. This allows you to prevent the "parenting" if for some reason you don't want it to happen. >>> mock = MagicMock() >>> not_a_child = MagicMock(name='not-a-child') >>> mock.attribute = not_a_child >>> mock.attribute() >>> mock.mock_calls [] Mocks created for you by :func:`patch` are automatically given names. To attach mocks that have names to a parent you use the :meth:`~Mock.attach_mock` method: >>> thing1 = object() >>> thing2 = object() >>> parent = MagicMock() >>> with patch('__main__.thing1', return_value=None) as child1: ... with patch('__main__.thing2', return_value=None) as child2: ... parent.attach_mock(child1, 'child1') ... parent.attach_mock(child2, 'child2') ... child1('one') ... child2('two') ... >>> parent.mock_calls [call.child1('one'), call.child2('two')] .. [#] The only exceptions are magic methods and attributes (those that have leading and trailing double underscores). Mock doesn't create these but instead of raises an ``AttributeError``. This is because the interpreter will often implicitly request these methods, and gets *very* confused to get a new Mock object when it expects a magic method. If you need magic method support see :ref:`magic methods `.