Patch Decorators

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.

patch(target, new=DEFAULT, spec=None, create=False, mocksignature=False, spec_set=None, autospec=False, 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 (similar to mocksignature). For mocks replacing a class, their return value (the ‘instance’) will have the same spec as the class. See the create_autospec() function and Autospeccing.

Instead of autospec=True you can pass autospec=some_object to use an arbitrary object as the spec instead of the one being replaced.

If mocksignature is True then the patch will be done with a function created by mocking the one being replaced. If the object being replaced is a class then the signature of __init__ will be copied. If the object being replaced is a callable object then the signature of __call__ will be copied.

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 return_value of the mock that will be used.

If the class is instantiated multiple times you could use 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 MagicMock for the created mock. For example, if you wanted a 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 return_value and 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

patch.object(target, attribute, new=DEFAULT, spec=None, create=False, mocksignature=False, spec_set=None, autospec=False, 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, mocksignature, 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

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.

>>> 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:

>>> 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

patch.multiple(target, spec=None, create=False, mocksignature=False, spec_set=None, autospec=False, 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 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, mocksignature, 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 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
...

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. unittest2 cleanup functions make 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
...
>>> MyTest('test_something').run()

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 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.

Like all context-managers patches can be nested using contextlib’s nested function; every patching will appear in the tuple after “as”:

>>> 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

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.

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...