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diff --git a/Doc/library/unittest.mock-patch.rst b/Doc/library/unittest.mock-patch.rst new file mode 100644 index 0000000000..e3e3525beb --- /dev/null +++ b/Doc/library/unittest.mock-patch.rst @@ -0,0 +1,538 @@ +:mod:`unittest.mock` --- the patchers +===================================== + +.. module:: unittest.mock + :synopsis: Mock object library. +.. moduleauthor:: Michael Foord <michael@python.org> +.. currentmodule:: unittest.mock + +.. versionadded:: 3.3 + +The patch decorators are used for patching objects only within the scope of +the function they decorate. They automatically handle the unpatching for you, +even if exceptions are raised. All of these functions can also be used in with +statements or as class decorators. + + +patch +----- + +.. note:: + + `patch` is straightforward to use. The key is to do the patching in the + right namespace. See the section `where to patch`_. + +.. function:: patch(target, new=DEFAULT, spec=None, create=False, spec_set=None, autospec=None, new_callable=None, **kwargs) + + `patch` acts as a function decorator, class decorator or a context + manager. Inside the body of the function or with statement, the `target` + (specified in the form `'package.module.ClassName'`) is patched + with a `new` object. When the function/with statement exits the patch is + undone. + + The `target` is imported and the specified attribute patched with the new + object, so it must be importable from the environment you are calling the + decorator from. The target is imported when the decorated function is + executed, not at decoration time. + + If `new` is omitted, then a new `MagicMock` is created and passed in as an + extra argument to the decorated function. + + The `spec` and `spec_set` keyword arguments are passed to the `MagicMock` + if patch is creating one for you. + + In addition you can pass `spec=True` or `spec_set=True`, which causes + patch to pass in the object being mocked as the spec/spec_set object. + + `new_callable` allows you to specify a different class, or callable object, + that will be called to create the `new` object. By default `MagicMock` is + used. + + A more powerful form of `spec` is `autospec`. If you set `autospec=True` + then the mock with be created with a spec from the object being replaced. + All attributes of the mock will also have the spec of the corresponding + attribute of the object being replaced. Methods and functions being mocked + will have their arguments checked and will raise a `TypeError` if they are + called with the wrong signature. For mocks + replacing a class, their return value (the 'instance') will have the same + spec as the class. See the :func:`create_autospec` function and + :ref:`auto-speccing`. + + Instead of `autospec=True` you can pass `autospec=some_object` to use an + arbitrary object as the spec instead of the one being replaced. + + By default `patch` will fail to replace attributes that don't exist. If + you pass in `create=True`, and the attribute doesn't exist, patch will + create the attribute for you when the patched function is called, and + delete it again afterwards. This is useful for writing tests against + attributes that your production code creates at runtime. It is off by by + default because it can be dangerous. With it switched on you can write + passing tests against APIs that don't actually exist! + + Patch can be used as a `TestCase` class decorator. It works by + decorating each test method in the class. This reduces the boilerplate + code when your test methods share a common patchings set. `patch` finds + tests by looking for method names that start with `patch.TEST_PREFIX`. + By default this is `test`, which matches the way `unittest` finds tests. + You can specify an alternative prefix by setting `patch.TEST_PREFIX`. + + Patch can be used as a context manager, with the with statement. Here the + patching applies to the indented block after the with statement. If you + use "as" then the patched object will be bound to the name after the + "as"; very useful if `patch` is creating a mock object for you. + + `patch` takes arbitrary keyword arguments. These will be passed to + the `Mock` (or `new_callable`) on construction. + + `patch.dict(...)`, `patch.multiple(...)` and `patch.object(...)` are + available for alternate use-cases. + + +Patching a class replaces the class with a `MagicMock` *instance*. If the +class is instantiated in the code under test then it will be the +:attr:`~Mock.return_value` of the mock that will be used. + +If the class is instantiated multiple times you could use +:attr:`~Mock.side_effect` to return a new mock each time. Alternatively you +can set the `return_value` to be anything you want. + +To configure return values on methods of *instances* on the patched class +you must do this on the `return_value`. For example: + + >>> class Class(object): + ... def method(self): + ... pass + ... + >>> with patch('__main__.Class') as MockClass: + ... instance = MockClass.return_value + ... instance.method.return_value = 'foo' + ... assert Class() is instance + ... assert Class().method() == 'foo' + ... + +If you use `spec` or `spec_set` and `patch` is replacing a *class*, then the +return value of the created mock will have the same spec. + + >>> Original = Class + >>> patcher = patch('__main__.Class', spec=True) + >>> MockClass = patcher.start() + >>> instance = MockClass() + >>> assert isinstance(instance, Original) + >>> patcher.stop() + +The `new_callable` argument is useful where you want to use an alternative +class to the default :class:`MagicMock` for the created mock. For example, if +you wanted a :class:`NonCallableMock` to be used: + + >>> thing = object() + >>> with patch('__main__.thing', new_callable=NonCallableMock) as mock_thing: + ... assert thing is mock_thing + ... thing() + ... + Traceback (most recent call last): + ... + TypeError: 'NonCallableMock' object is not callable + +Another use case might be to replace an object with a `StringIO` instance: + + >>> from StringIO import StringIO + >>> def foo(): + ... print 'Something' + ... + >>> @patch('sys.stdout', new_callable=StringIO) + ... def test(mock_stdout): + ... foo() + ... assert mock_stdout.getvalue() == 'Something\n' + ... + >>> test() + +When `patch` is creating a mock for you, it is common that the first thing +you need to do is to configure the mock. Some of that configuration can be done +in the call to patch. Any arbitrary keywords you pass into the call will be +used to set attributes on the created mock: + + >>> patcher = patch('__main__.thing', first='one', second='two') + >>> mock_thing = patcher.start() + >>> mock_thing.first + 'one' + >>> mock_thing.second + 'two' + +As well as attributes on the created mock attributes, like the +:attr:`~Mock.return_value` and :attr:`~Mock.side_effect`, of child mocks can +also be configured. These aren't syntactically valid to pass in directly as +keyword arguments, but a dictionary with these as keys can still be expanded +into a `patch` call using `**`: + + >>> config = {'method.return_value': 3, 'other.side_effect': KeyError} + >>> patcher = patch('__main__.thing', **config) + >>> mock_thing = patcher.start() + >>> mock_thing.method() + 3 + >>> mock_thing.other() + Traceback (most recent call last): + ... + KeyError + + +patch.object +------------ + +.. function:: patch.object(target, attribute, new=DEFAULT, spec=None, create=False, spec_set=None, autospec=None, new_callable=None, **kwargs) + + patch the named member (`attribute`) on an object (`target`) with a mock + object. + + `patch.object` can be used as a decorator, class decorator or a context + manager. Arguments `new`, `spec`, `create`, `spec_set`, `autospec` and + `new_callable` have the same meaning as for `patch`. Like `patch`, + `patch.object` takes arbitrary keyword arguments for configuring the mock + object it creates. + + When used as a class decorator `patch.object` honours `patch.TEST_PREFIX` + for choosing which methods to wrap. + +You can either call `patch.object` with three arguments or two arguments. The +three argument form takes the object to be patched, the attribute name and the +object to replace the attribute with. + +When calling with the two argument form you omit the replacement object, and a +mock is created for you and passed in as an extra argument to the decorated +function: + + >>> @patch.object(SomeClass, 'class_method') + ... def test(mock_method): + ... SomeClass.class_method(3) + ... mock_method.assert_called_with(3) + ... + >>> test() + +`spec`, `create` and the other arguments to `patch.object` have the same +meaning as they do for `patch`. + + +patch.dict +---------- + +.. function:: patch.dict(in_dict, values=(), clear=False, **kwargs) + + Patch a dictionary, or dictionary like object, and restore the dictionary + to its original state after the test. + + `in_dict` can be a dictionary or a mapping like container. If it is a + mapping then it must at least support getting, setting and deleting items + plus iterating over keys. + + `in_dict` can also be a string specifying the name of the dictionary, which + will then be fetched by importing it. + + `values` can be a dictionary of values to set in the dictionary. `values` + can also be an iterable of `(key, value)` pairs. + + If `clear` is True then the dictionary will be cleared before the new + values are set. + + `patch.dict` can also be called with arbitrary keyword arguments to set + values in the dictionary. + + `patch.dict` can be used as a context manager, decorator or class + decorator. When used as a class decorator `patch.dict` honours + `patch.TEST_PREFIX` for choosing which methods to wrap. + +`patch.dict` can be used to add members to a dictionary, or simply let a test +change a dictionary, and ensure the dictionary is restored when the test +ends. + + >>> foo = {} + >>> with patch.dict(foo, {'newkey': 'newvalue'}): + ... assert foo == {'newkey': 'newvalue'} + ... + >>> assert foo == {} + + >>> import os + >>> with patch.dict('os.environ', {'newkey': 'newvalue'}): + ... print os.environ['newkey'] + ... + newvalue + >>> assert 'newkey' not in os.environ + +Keywords can be used in the `patch.dict` call to set values in the dictionary: + + >>> mymodule = MagicMock() + >>> mymodule.function.return_value = 'fish' + >>> with patch.dict('sys.modules', mymodule=mymodule): + ... import mymodule + ... mymodule.function('some', 'args') + ... + 'fish' + +`patch.dict` can be used with dictionary like objects that aren't actually +dictionaries. At the very minimum they must support item getting, setting, +deleting and either iteration or membership test. This corresponds to the +magic methods `__getitem__`, `__setitem__`, `__delitem__` and either +`__iter__` or `__contains__`. + + >>> class Container(object): + ... def __init__(self): + ... self.values = {} + ... def __getitem__(self, name): + ... return self.values[name] + ... def __setitem__(self, name, value): + ... self.values[name] = value + ... def __delitem__(self, name): + ... del self.values[name] + ... def __iter__(self): + ... return iter(self.values) + ... + >>> thing = Container() + >>> thing['one'] = 1 + >>> with patch.dict(thing, one=2, two=3): + ... assert thing['one'] == 2 + ... assert thing['two'] == 3 + ... + >>> assert thing['one'] == 1 + >>> assert list(thing) == ['one'] + + +patch.multiple +-------------- + +.. function:: patch.multiple(target, spec=None, create=False, spec_set=None, autospec=None, new_callable=None, **kwargs) + + Perform multiple patches in a single call. It takes the object to be + patched (either as an object or a string to fetch the object by importing) + and keyword arguments for the patches:: + + with patch.multiple(settings, FIRST_PATCH='one', SECOND_PATCH='two'): + ... + + Use :data:`DEFAULT` as the value if you want `patch.multiple` to create + mocks for you. In this case the created mocks are passed into a decorated + function by keyword, and a dictionary is returned when `patch.multiple` is + used as a context manager. + + `patch.multiple` can be used as a decorator, class decorator or a context + manager. The arguments `spec`, `spec_set`, `create`, `autospec` and + `new_callable` have the same meaning as for `patch`. These arguments will + be applied to *all* patches done by `patch.multiple`. + + When used as a class decorator `patch.multiple` honours `patch.TEST_PREFIX` + for choosing which methods to wrap. + +If you want `patch.multiple` to create mocks for you, then you can use +:data:`DEFAULT` as the value. If you use `patch.multiple` as a decorator +then the created mocks are passed into the decorated function by keyword. + + >>> thing = object() + >>> other = object() + + >>> @patch.multiple('__main__', thing=DEFAULT, other=DEFAULT) + ... def test_function(thing, other): + ... assert isinstance(thing, MagicMock) + ... assert isinstance(other, MagicMock) + ... + >>> test_function() + +`patch.multiple` can be nested with other `patch` decorators, but put arguments +passed by keyword *after* any of the standard arguments created by `patch`: + + >>> @patch('sys.exit') + ... @patch.multiple('__main__', thing=DEFAULT, other=DEFAULT) + ... def test_function(mock_exit, other, thing): + ... assert 'other' in repr(other) + ... assert 'thing' in repr(thing) + ... assert 'exit' in repr(mock_exit) + ... + >>> test_function() + +If `patch.multiple` is used as a context manager, the value returned by the +context manger is a dictionary where created mocks are keyed by name: + + >>> with patch.multiple('__main__', thing=DEFAULT, other=DEFAULT) as values: + ... assert 'other' in repr(values['other']) + ... assert 'thing' in repr(values['thing']) + ... assert values['thing'] is thing + ... assert values['other'] is other + ... + + +.. _start-and-stop: + +patch methods: start and stop +----------------------------- + +All the patchers have `start` and `stop` methods. These make it simpler to do +patching in `setUp` methods or where you want to do multiple patches without +nesting decorators or with statements. + +To use them call `patch`, `patch.object` or `patch.dict` as normal and keep a +reference to the returned `patcher` object. You can then call `start` to put +the patch in place and `stop` to undo it. + +If you are using `patch` to create a mock for you then it will be returned by +the call to `patcher.start`. + + >>> patcher = patch('package.module.ClassName') + >>> from package import module + >>> original = module.ClassName + >>> new_mock = patcher.start() + >>> assert module.ClassName is not original + >>> assert module.ClassName is new_mock + >>> patcher.stop() + >>> assert module.ClassName is original + >>> assert module.ClassName is not new_mock + + +A typical use case for this might be for doing multiple patches in the `setUp` +method of a `TestCase`: + + >>> class MyTest(TestCase): + ... def setUp(self): + ... self.patcher1 = patch('package.module.Class1') + ... self.patcher2 = patch('package.module.Class2') + ... self.MockClass1 = self.patcher1.start() + ... self.MockClass2 = self.patcher2.start() + ... + ... def tearDown(self): + ... self.patcher1.stop() + ... self.patcher2.stop() + ... + ... def test_something(self): + ... assert package.module.Class1 is self.MockClass1 + ... assert package.module.Class2 is self.MockClass2 + ... + >>> MyTest('test_something').run() + +.. caution:: + + If you use this technique you must ensure that the patching is "undone" by + calling `stop`. This can be fiddlier than you might think, because if an + exception is raised in the ``setUp`` then ``tearDown`` is not called. + :meth:`unittest.TestCase.addCleanup` makes this easier: + + >>> class MyTest(TestCase): + ... def setUp(self): + ... patcher = patch('package.module.Class') + ... self.MockClass = patcher.start() + ... self.addCleanup(patcher.stop) + ... + ... def test_something(self): + ... assert package.module.Class is self.MockClass + ... + + As an added bonus you no longer need to keep a reference to the `patcher` + object. + +In fact `start` and `stop` are just aliases for the context manager +`__enter__` and `__exit__` methods. + + +TEST_PREFIX +----------- + +All of the patchers can be used as class decorators. When used in this way +they wrap every test method on the class. The patchers recognise methods that +start with `test` as being test methods. This is the same way that the +:class:`unittest.TestLoader` finds test methods by default. + +It is possible that you want to use a different prefix for your tests. You can +inform the patchers of the different prefix by setting `patch.TEST_PREFIX`: + + >>> patch.TEST_PREFIX = 'foo' + >>> value = 3 + >>> + >>> @patch('__main__.value', 'not three') + ... class Thing(object): + ... def foo_one(self): + ... print value + ... def foo_two(self): + ... print value + ... + >>> + >>> Thing().foo_one() + not three + >>> Thing().foo_two() + not three + >>> value + 3 + + +Nesting Patch Decorators +------------------------ + +If you want to perform multiple patches then you can simply stack up the +decorators. + +You can stack up multiple patch decorators using this pattern: + + >>> @patch.object(SomeClass, 'class_method') + ... @patch.object(SomeClass, 'static_method') + ... def test(mock1, mock2): + ... assert SomeClass.static_method is mock1 + ... assert SomeClass.class_method is mock2 + ... SomeClass.static_method('foo') + ... SomeClass.class_method('bar') + ... return mock1, mock2 + ... + >>> mock1, mock2 = test() + >>> mock1.assert_called_once_with('foo') + >>> mock2.assert_called_once_with('bar') + + +Note that the decorators are applied from the bottom upwards. This is the +standard way that Python applies decorators. The order of the created mocks +passed into your test function matches this order. + + +.. _where-to-patch: + +Where to patch +-------------- + +`patch` works by (temporarily) changing the object that a *name* points to with +another one. There can be many names pointing to any individual object, so +for patching to work you must ensure that you patch the name used by the system +under test. + +The basic principle is that you patch where an object is *looked up*, which +is not necessarily the same place as where it is defined. A couple of +examples will help to clarify this. + +Imagine we have a project that we want to test with the following structure:: + + a.py + -> Defines SomeClass + + b.py + -> from a import SomeClass + -> some_function instantiates SomeClass + +Now we want to test `some_function` but we want to mock out `SomeClass` using +`patch`. The problem is that when we import module b, which we will have to +do then it imports `SomeClass` from module a. If we use `patch` to mock out +`a.SomeClass` then it will have no effect on our test; module b already has a +reference to the *real* `SomeClass` and it looks like our patching had no +effect. + +The key is to patch out `SomeClass` where it is used (or where it is looked up +). In this case `some_function` will actually look up `SomeClass` in module b, +where we have imported it. The patching should look like:: + + @patch('b.SomeClass') + +However, consider the alternative scenario where instead of `from a import +SomeClass` module b does `import a` and `some_function` uses `a.SomeClass`. Both +of these import forms are common. In this case the class we want to patch is +being looked up on the a module and so we have to patch `a.SomeClass` instead:: + + @patch('a.SomeClass') + + +Patching Descriptors and Proxy Objects +-------------------------------------- + +Both patch_ and patch.object_ correctly patch and restore descriptors: class +methods, static methods and properties. You should patch these on the *class* +rather than an instance. They also work with *some* objects +that proxy attribute access, like the `django setttings object +<http://www.voidspace.org.uk/python/weblog/arch_d7_2010_12_04.shtml#e1198>`_. |