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+==================
+ Patch Decorators
+==================
+
+
+.. currentmodule:: mock
+
+.. testsetup::
+
+ class SomeClass(object):
+ static_method = None
+ class_method = None
+ attribute = None
+
+ sys.modules['package'] = package = Mock(name='package')
+ sys.modules['package.module'] = package.module
+
+ class TestCase(unittest2.TestCase):
+ def run(self):
+ result = unittest2.TestResult()
+ super(unittest2.TestCase, self).run(result)
+ assert result.wasSuccessful()
+
+.. testcleanup::
+
+ patch.TEST_PREFIX = 'test'
+
+
+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`
+ is patched with a `new` object. When the function/with statement exits
+ the patch is undone.
+
+ If `new` is omitted, then the target is replaced with a
+ :class:`MagicMock`. If `patch` is used as a decorator and `new` is
+ omitted, the created mock is passed in as an extra argument to the
+ decorated function. If `patch` is used as a context manager the created
+ mock is returned by the context manager.
+
+ `target` should be a string in the form `'package.module.ClassName'`. The
+ `target` is imported and the specified object replaced with the `new`
+ object, so the `target` must be importable from the environment you are
+ calling `patch` from. The target is imported when the decorated function
+ is executed, not at decoration time.
+
+ 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.
+
+`patch` as function decorator, creating the mock for you and passing it into
+the decorated function:
+
+.. doctest::
+
+ >>> @patch('__main__.SomeClass')
+ ... def function(normal_argument, mock_class):
+ ... print mock_class is SomeClass
+ ...
+ >>> function(None)
+ True
+
+
+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:
+
+.. doctest::
+
+ >>> 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.
+
+.. doctest::
+
+ >>> 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:
+
+.. doctest::
+
+ >>> 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:
+
+.. doctest::
+
+ >>> 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:
+
+.. doctest::
+
+ >>> 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 `**`:
+
+.. doctest::
+
+ >>> 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:
+
+.. doctest::
+
+ >>> @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.
+
+.. doctest::
+
+ >>> from mock import patch
+ >>> 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:
+
+.. doctest::
+
+ >>> 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__`.
+
+.. doctest::
+
+ >>> 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.
+
+.. doctest::
+
+ >>> 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`:
+
+.. doctest::
+
+ >>> @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:
+
+.. doctest::
+
+ >>> 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`.
+
+.. doctest::
+
+ >>> 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`:
+
+.. doctest::
+
+ >>> 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. `unittest2
+ <http://pypi.python.org/pypi/unittest2>`_ cleanup functions make this
+ easier.
+
+ .. doctest::
+
+ >>> 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
+ ...
+ >>> MyTest('test_something').run()
+
+ As an added bonus you no longer need to keep a reference to the `patcher`
+ object.
+
+It is also possible to stop all patches which have been started by using
+`patch.stopall`.
+
+.. function:: patch.stopall
+
+ Stop all active patches. Only stops patches started with `start`.
+
+
+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
+`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`:
+
+.. doctest::
+
+ >>> 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:
+
+.. doctest::
+
+ >>> @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.
+
+Like all context-managers patches can be nested using contextlib's nested
+function; *every* patching will appear in the tuple after "as":
+
+.. doctest::
+
+ >>> from contextlib import nested
+ >>> with nested(
+ ... patch('package.module.ClassName1'),
+ ... patch('package.module.ClassName2')
+ ... ) as (MockClass1, MockClass2):
+ ... assert package.module.ClassName1 is MockClass1
+ ... assert package.module.ClassName2 is MockClass2
+ ...
+
+
+.. _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
+======================================
+
+Since version 0.6.0 both patch_ and patch.object_ have been able to correctly
+patch and restore descriptors: class methods, static methods and properties.
+You should patch these on the *class* rather than an instance.
+
+Since version 0.7.0 patch_ and patch.object_ work correctly 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>`_.
+
+.. note::
+
+ In django `import settings` and `from django.conf import settings`
+ return different objects. If you are using libraries / apps that do both you
+ may have to patch both. Grrr...