The Mock Class — Mock 1.0.0 documentation

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# 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 [1]. 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.

`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: `NonCallableMock` and `NonCallableMagicMock`

The `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 `__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 `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 `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. If any of the members of the iterable are exceptions they will be raised instead of returned.

    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 `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 `configure_mock()` method for details.

  `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 name='mock.method()' id='...'>
        >>> mock.method.assert_called_with(1, 2, 3, test='wow')

  `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.

  `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 `assert_called_with()` and `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')

  `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 `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)

  `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, `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.

  `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.

  `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 `method_calls` and `mock_calls` attributes of this one.

  `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.

  `__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 `FILTER_DIR` for what this filtering does, and how to switch it off.

  `_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).

  `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

  `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

  `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 name='mock()()' id='...'>
        >>> 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

  `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 `DEFAULT` singleton the call to the mock will then return whatever the function returns. If the function returns `DEFAULT` then the mock will return its normal value (from the `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 `DEFAULT`, in which case the normal `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:

        >>> from mock import Mock
        >>> m = Mock(side_effect=KeyError, return_value=3)
        >>> m()
        Traceback (most recent call last):
         ...
        KeyError
        >>> m.side_effect = None
        >>> m()
        3

  `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 `call_args_list`, `method_calls` and `mock_calls` are `call` objects. These are tuples, so they can be unpacked to get at the individual arguments and make more complex assertions. See calls as tuples.

  `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 `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 `call` objects. These can be unpacked as tuples to get at the individual arguments. See calls as tuples.

  `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 name='mock.method()' id='...'>
        >>> mock.property.method.attribute()
        <Mock name='mock.property.method.attribute()' id='...'>
        >>> mock.method_calls
        [call.method(), call.property.method.attribute()]

    Members of method_calls are `call` objects. These can be unpacked as tuples to get at the individual arguments. See calls as tuples.

  `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)
        <MagicMock name='mock.first()' id='...'>
        >>> mock.second()
        <MagicMock name='mock.second()' id='...'>
        >>> int(mock)
        1
        >>> result(1)
        <MagicMock name='mock()()' id='...'>
        >>> 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 `call` objects. These can be unpacked as tuples to get at the individual arguments. See calls as tuples.

  `__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 magic methods for the full details.

The mock classes and the `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)]

Because of the way mock attributes are stored you can’t directly attach a PropertyMock to a mock object. Instead you can attach it to the mock type object:

    >>> m = MagicMock()
    >>> p = PropertyMock(return_value=3)
    >>> type(m).foo = p
    >>> m.foo
    3
    >>> p.assert_called_once_with()

# Calling

Mock objects are callable. The call will return the value set as the `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 `call_args` and `call_args_list`.

If `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 `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 `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

If any members of the iterable are exceptions they will be raised instead of returned:

    >>> iterable = (33, ValueError, 66)
    >>> m = MagicMock(side_effect=iterable)
    >>> m()
    33
    >>> m()
    Traceback (most recent call last):
     ...
    ValueError
    >>> m()
    66

# 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 `method_calls` and `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()
    <MagicMock name='not-a-child()' id='...'>
    >>> mock.mock_calls
    []

Mocks created for you by `patch()` are automatically given names. To attach mocks that have names to a parent you use the `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')]

[1] | 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 magic methods.

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