mirror of https://github.com/python/cpython
2226 lines
78 KiB
ReStructuredText
2226 lines
78 KiB
ReStructuredText
:mod:`unittest.mock` --- mock object library
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============================================
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.. module:: unittest.mock
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:synopsis: Mock object library.
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.. moduleauthor:: Michael Foord <michael@python.org>
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.. currentmodule:: unittest.mock
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.. versionadded:: 3.3
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:mod:`unittest.mock` is a library for testing in Python. It allows you to
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replace parts of your system under test with mock objects and make assertions
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about how they have been used.
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`unittest.mock` provides a core :class:`Mock` class removing the need to
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create a host of stubs throughout your test suite. After performing an
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action, you can make assertions about which methods / attributes were used
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and arguments they were called with. You can also specify return values and
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set needed attributes in the normal way.
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Additionally, mock provides a :func:`patch` decorator that handles patching
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module and class level attributes within the scope of a test, along with
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:const:`sentinel` for creating unique objects. See the `quick guide`_ for
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some examples of how to use :class:`Mock`, :class:`MagicMock` and
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:func:`patch`.
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Mock is very easy to use and is designed for use with :mod:`unittest`. Mock
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is based on the 'action -> assertion' pattern instead of `'record -> replay'`
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used by many mocking frameworks.
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There is a backport of `unittest.mock` for earlier versions of Python,
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available as `mock on PyPI <http://pypi.python.org/pypi/mock>`_.
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**Source code:** :source:`Lib/unittest/mock.py`
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Quick Guide
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-----------
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:class:`Mock` and :class:`MagicMock` objects create all attributes and
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methods as you access them and store details of how they have been used. You
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can configure them, to specify return values or limit what attributes are
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available, and then make assertions about how they have been used:
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>>> from unittest.mock import MagicMock
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>>> thing = ProductionClass()
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>>> thing.method = MagicMock(return_value=3)
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>>> thing.method(3, 4, 5, key='value')
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3
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>>> thing.method.assert_called_with(3, 4, 5, key='value')
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:attr:`side_effect` allows you to perform side effects, including raising an
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exception when a mock is called:
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>>> mock = Mock(side_effect=KeyError('foo'))
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>>> mock()
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Traceback (most recent call last):
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...
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KeyError: 'foo'
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>>> values = {'a': 1, 'b': 2, 'c': 3}
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>>> def side_effect(arg):
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... return values[arg]
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...
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>>> mock.side_effect = side_effect
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>>> mock('a'), mock('b'), mock('c')
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(1, 2, 3)
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>>> mock.side_effect = [5, 4, 3, 2, 1]
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>>> mock(), mock(), mock()
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(5, 4, 3)
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Mock has many other ways you can configure it and control its behaviour. For
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example the `spec` argument configures the mock to take its specification
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from another object. Attempting to access attributes or methods on the mock
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that don't exist on the spec will fail with an `AttributeError`.
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The :func:`patch` decorator / context manager makes it easy to mock classes or
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objects in a module under test. The object you specify will be replaced with a
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mock (or other object) during the test and restored when the test ends:
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>>> from unittest.mock import patch
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>>> @patch('module.ClassName2')
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... @patch('module.ClassName1')
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... def test(MockClass1, MockClass2):
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... module.ClassName1()
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... module.ClassName2()
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... assert MockClass1 is module.ClassName1
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... assert MockClass2 is module.ClassName2
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... assert MockClass1.called
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... assert MockClass2.called
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...
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>>> test()
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.. note::
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When you nest patch decorators the mocks are passed in to the decorated
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function in the same order they applied (the normal *python* order that
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decorators are applied). This means from the bottom up, so in the example
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above the mock for `module.ClassName1` is passed in first.
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With `patch` it matters that you patch objects in the namespace where they
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are looked up. This is normally straightforward, but for a quick guide
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read :ref:`where to patch <where-to-patch>`.
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As well as a decorator `patch` can be used as a context manager in a with
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statement:
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>>> with patch.object(ProductionClass, 'method', return_value=None) as mock_method:
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... thing = ProductionClass()
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... thing.method(1, 2, 3)
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...
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>>> mock_method.assert_called_once_with(1, 2, 3)
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There is also :func:`patch.dict` for setting values in a dictionary just
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during a scope and restoring the dictionary to its original state when the test
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ends:
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>>> foo = {'key': 'value'}
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>>> original = foo.copy()
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>>> with patch.dict(foo, {'newkey': 'newvalue'}, clear=True):
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... assert foo == {'newkey': 'newvalue'}
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...
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>>> assert foo == original
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Mock supports the mocking of Python :ref:`magic methods <magic-methods>`. The
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easiest way of using magic methods is with the :class:`MagicMock` class. It
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allows you to do things like:
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>>> mock = MagicMock()
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>>> mock.__str__.return_value = 'foobarbaz'
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>>> str(mock)
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'foobarbaz'
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>>> mock.__str__.assert_called_with()
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Mock allows you to assign functions (or other Mock instances) to magic methods
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and they will be called appropriately. The `MagicMock` class is just a Mock
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variant that has all of the magic methods pre-created for you (well, all the
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useful ones anyway).
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The following is an example of using magic methods with the ordinary Mock
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class:
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>>> mock = Mock()
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>>> mock.__str__ = Mock(return_value='wheeeeee')
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>>> str(mock)
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'wheeeeee'
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For ensuring that the mock objects in your tests have the same api as the
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objects they are replacing, you can use :ref:`auto-speccing <auto-speccing>`.
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Auto-speccing can be done through the `autospec` argument to patch, or the
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:func:`create_autospec` function. Auto-speccing creates mock objects that
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have the same attributes and methods as the objects they are replacing, and
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any functions and methods (including constructors) have the same call
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signature as the real object.
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This ensures that your mocks will fail in the same way as your production
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code if they are used incorrectly:
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>>> from unittest.mock import create_autospec
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>>> def function(a, b, c):
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... pass
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...
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>>> mock_function = create_autospec(function, return_value='fishy')
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>>> mock_function(1, 2, 3)
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'fishy'
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>>> mock_function.assert_called_once_with(1, 2, 3)
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>>> mock_function('wrong arguments')
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Traceback (most recent call last):
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...
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TypeError: <lambda>() takes exactly 3 arguments (1 given)
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`create_autospec` can also be used on classes, where it copies the signature of
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the `__init__` method, and on callable objects where it copies the signature of
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the `__call__` method.
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The Mock Class
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--------------
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`Mock` is a flexible mock object intended to replace the use of stubs and
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test doubles throughout your code. Mocks are callable and create attributes as
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new mocks when you access them [#]_. Accessing the same attribute will always
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return the same mock. Mocks record how you use them, allowing you to make
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assertions about what your code has done to them.
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:class:`MagicMock` is a subclass of `Mock` with all the magic methods
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pre-created and ready to use. There are also non-callable variants, useful
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when you are mocking out objects that aren't callable:
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:class:`NonCallableMock` and :class:`NonCallableMagicMock`
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The :func:`patch` decorators makes it easy to temporarily replace classes
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in a particular module with a `Mock` object. By default `patch` will create
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a `MagicMock` for you. You can specify an alternative class of `Mock` using
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the `new_callable` argument to `patch`.
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.. class:: Mock(spec=None, side_effect=None, return_value=DEFAULT, wraps=None, name=None, spec_set=None, **kwargs)
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Create a new `Mock` object. `Mock` takes several optional arguments
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that specify the behaviour of the Mock object:
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* `spec`: This can be either a list of strings or an existing object (a
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class or instance) that acts as the specification for the mock object. If
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you pass in an object then a list of strings is formed by calling dir on
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the object (excluding unsupported magic attributes and methods).
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Accessing any attribute not in this list will raise an `AttributeError`.
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If `spec` is an object (rather than a list of strings) then
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:attr:`__class__` returns the class of the spec object. This allows mocks
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to pass `isinstance` tests.
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* `spec_set`: A stricter variant of `spec`. If used, attempting to *set*
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or get an attribute on the mock that isn't on the object passed as
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`spec_set` will raise an `AttributeError`.
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* `side_effect`: A function to be called whenever the Mock is called. See
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the :attr:`~Mock.side_effect` attribute. Useful for raising exceptions or
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dynamically changing return values. The function is called with the same
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arguments as the mock, and unless it returns :data:`DEFAULT`, the return
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value of this function is used as the return value.
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Alternatively `side_effect` can be an exception class or instance. In
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this case the exception will be raised when the mock is called.
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If `side_effect` is an iterable then each call to the mock will return
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the next value from the iterable.
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A `side_effect` can be cleared by setting it to `None`.
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* `return_value`: The value returned when the mock is called. By default
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this is a new Mock (created on first access). See the
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:attr:`return_value` attribute.
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* `wraps`: Item for the mock object to wrap. If `wraps` is not None then
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calling the Mock will pass the call through to the wrapped object
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(returning the real result). Attribute access on the mock will return a
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Mock object that wraps the corresponding attribute of the wrapped
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object (so attempting to access an attribute that doesn't exist will
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raise an `AttributeError`).
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If the mock has an explicit `return_value` set then calls are not passed
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to the wrapped object and the `return_value` is returned instead.
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* `name`: If the mock has a name then it will be used in the repr of the
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mock. This can be useful for debugging. The name is propagated to child
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mocks.
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Mocks can also be called with arbitrary keyword arguments. These will be
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used to set attributes on the mock after it is created. See the
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:meth:`configure_mock` method for details.
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.. method:: assert_called_with(*args, **kwargs)
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This method is a convenient way of asserting that calls are made in a
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particular way:
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>>> mock = Mock()
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>>> mock.method(1, 2, 3, test='wow')
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<Mock name='mock.method()' id='...'>
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>>> mock.method.assert_called_with(1, 2, 3, test='wow')
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.. method:: assert_called_once_with(*args, **kwargs)
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Assert that the mock was called exactly once and with the specified
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arguments.
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>>> mock = Mock(return_value=None)
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>>> mock('foo', bar='baz')
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>>> mock.assert_called_once_with('foo', bar='baz')
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>>> mock('foo', bar='baz')
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>>> mock.assert_called_once_with('foo', bar='baz')
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Traceback (most recent call last):
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...
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AssertionError: Expected to be called once. Called 2 times.
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.. method:: assert_any_call(*args, **kwargs)
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assert the mock has been called with the specified arguments.
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The assert passes if the mock has *ever* been called, unlike
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:meth:`assert_called_with` and :meth:`assert_called_once_with` that
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only pass if the call is the most recent one.
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>>> mock = Mock(return_value=None)
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>>> mock(1, 2, arg='thing')
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>>> mock('some', 'thing', 'else')
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>>> mock.assert_any_call(1, 2, arg='thing')
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.. method:: assert_has_calls(calls, any_order=False)
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assert the mock has been called with the specified calls.
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The `mock_calls` list is checked for the calls.
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If `any_order` is False (the default) then the calls must be
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sequential. There can be extra calls before or after the
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specified calls.
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If `any_order` is True then the calls can be in any order, but
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they must all appear in :attr:`mock_calls`.
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>>> mock = Mock(return_value=None)
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>>> mock(1)
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>>> mock(2)
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>>> mock(3)
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>>> mock(4)
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>>> calls = [call(2), call(3)]
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>>> mock.assert_has_calls(calls)
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>>> calls = [call(4), call(2), call(3)]
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>>> mock.assert_has_calls(calls, any_order=True)
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.. method:: reset_mock()
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The reset_mock method resets all the call attributes on a mock object:
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>>> mock = Mock(return_value=None)
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>>> mock('hello')
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>>> mock.called
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True
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>>> mock.reset_mock()
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>>> mock.called
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False
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This can be useful where you want to make a series of assertions that
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reuse the same object. Note that `reset_mock` *doesn't* clear the
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return value, :attr:`side_effect` or any child attributes you have
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set using normal assignment. Child mocks and the return value mock
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(if any) are reset as well.
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.. method:: mock_add_spec(spec, spec_set=False)
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Add a spec to a mock. `spec` can either be an object or a
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list of strings. Only attributes on the `spec` can be fetched as
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attributes from the mock.
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If `spec_set` is `True` then only attributes on the spec can be set.
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.. method:: attach_mock(mock, attribute)
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Attach a mock as an attribute of this one, replacing its name and
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parent. Calls to the attached mock will be recorded in the
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:attr:`method_calls` and :attr:`mock_calls` attributes of this one.
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.. method:: configure_mock(**kwargs)
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Set attributes on the mock through keyword arguments.
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Attributes plus return values and side effects can be set on child
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mocks using standard dot notation and unpacking a dictionary in the
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method call:
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>>> mock = Mock()
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>>> attrs = {'method.return_value': 3, 'other.side_effect': KeyError}
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>>> mock.configure_mock(**attrs)
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>>> mock.method()
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3
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>>> mock.other()
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Traceback (most recent call last):
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...
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KeyError
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The same thing can be achieved in the constructor call to mocks:
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>>> attrs = {'method.return_value': 3, 'other.side_effect': KeyError}
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>>> mock = Mock(some_attribute='eggs', **attrs)
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>>> mock.some_attribute
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'eggs'
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>>> mock.method()
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3
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>>> mock.other()
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Traceback (most recent call last):
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...
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KeyError
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`configure_mock` exists to make it easier to do configuration
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after the mock has been created.
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.. method:: __dir__()
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`Mock` objects limit the results of `dir(some_mock)` to useful results.
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For mocks with a `spec` this includes all the permitted attributes
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for the mock.
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See :data:`FILTER_DIR` for what this filtering does, and how to
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switch it off.
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.. method:: _get_child_mock(**kw)
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Create the child mocks for attributes and return value.
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By default child mocks will be the same type as the parent.
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Subclasses of Mock may want to override this to customize the way
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child mocks are made.
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For non-callable mocks the callable variant will be used (rather than
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any custom subclass).
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.. attribute:: called
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A boolean representing whether or not the mock object has been called:
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>>> mock = Mock(return_value=None)
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>>> mock.called
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False
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>>> mock()
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>>> mock.called
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True
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.. attribute:: call_count
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An integer telling you how many times the mock object has been called:
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>>> mock = Mock(return_value=None)
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>>> mock.call_count
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0
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>>> mock()
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>>> mock()
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>>> mock.call_count
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2
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.. attribute:: return_value
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Set this to configure the value returned by calling the mock:
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>>> mock = Mock()
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>>> mock.return_value = 'fish'
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>>> mock()
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'fish'
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The default return value is a mock object and you can configure it in
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the normal way:
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>>> mock = Mock()
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>>> mock.return_value.attribute = sentinel.Attribute
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>>> mock.return_value()
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<Mock name='mock()()' id='...'>
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>>> mock.return_value.assert_called_with()
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`return_value` can also be set in the constructor:
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>>> mock = Mock(return_value=3)
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>>> mock.return_value
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3
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>>> mock()
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3
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.. attribute:: side_effect
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This can either be a function to be called when the mock is called,
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or an exception (class or instance) to be raised.
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If you pass in a function it will be called with same arguments as the
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mock and unless the function returns the :data:`DEFAULT` singleton the
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call to the mock will then return whatever the function returns. If the
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function returns :data:`DEFAULT` then the mock will return its normal
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value (from the :attr:`return_value`.
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An example of a mock that raises an exception (to test exception
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handling of an API):
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>>> mock = Mock()
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>>> mock.side_effect = Exception('Boom!')
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>>> mock()
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Traceback (most recent call last):
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...
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Exception: Boom!
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Using `side_effect` to return a sequence of values:
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>>> mock = Mock()
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>>> mock.side_effect = [3, 2, 1]
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>>> mock(), mock(), mock()
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(3, 2, 1)
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The `side_effect` function is called with the same arguments as the
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mock (so it is wise for it to take arbitrary args and keyword
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arguments) and whatever it returns is used as the return value for
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the call. The exception is if `side_effect` returns :data:`DEFAULT`,
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in which case the normal :attr:`return_value` is used.
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>>> mock = Mock(return_value=3)
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>>> def side_effect(*args, **kwargs):
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... return DEFAULT
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...
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>>> mock.side_effect = side_effect
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>>> mock()
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3
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`side_effect` can be set in the constructor. Here's an example that
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adds one to the value the mock is called with and returns it:
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>>> side_effect = lambda value: value + 1
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>>> mock = Mock(side_effect=side_effect)
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>>> mock(3)
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4
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>>> mock(-8)
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-7
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Setting `side_effect` to `None` clears it:
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>>> m = Mock(side_effect=KeyError, return_value=3)
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>>> m()
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Traceback (most recent call last):
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...
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KeyError
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>>> m.side_effect = None
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>>> m()
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3
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.. attribute:: call_args
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|
|
|
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 <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 <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 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 :data:`call` objects. These can be
|
|
unpacked as tuples to get at the individual arguments. See
|
|
:ref:`calls as tuples <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)
|
|
<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 :data:`call` objects. These can be
|
|
unpacked as tuples to get at the individual arguments. See
|
|
:ref:`calls as tuples <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 <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)]
|
|
|
|
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
|
|
: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
|
|
|
|
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:
|
|
|
|
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()
|
|
<MagicMock name='not-a-child()' id='...'>
|
|
>>> 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 <magic-methods>`.
|
|
|
|
|
|
The patchers
|
|
============
|
|
|
|
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:
|
|
|
|
>>> @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:
|
|
|
|
>>> 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.
|
|
|
|
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
|
|
: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>`_.
|
|
|
|
|
|
MagicMock and magic method support
|
|
==================================
|
|
|
|
.. _magic-methods:
|
|
|
|
Mocking Magic Methods
|
|
---------------------
|
|
|
|
:class:`Mock` supports mocking the Python protocol methods, also known as
|
|
"magic methods". This allows mock objects to replace containers or other
|
|
objects that implement Python protocols.
|
|
|
|
Because magic methods are looked up differently from normal methods [#]_, this
|
|
support has been specially implemented. This means that only specific magic
|
|
methods are supported. The supported list includes *almost* all of them. If
|
|
there are any missing that you need please let us know.
|
|
|
|
You mock magic methods by setting the method you are interested in to a function
|
|
or a mock instance. If you are using a function then it *must* take ``self`` as
|
|
the first argument [#]_.
|
|
|
|
>>> def __str__(self):
|
|
... return 'fooble'
|
|
...
|
|
>>> mock = Mock()
|
|
>>> mock.__str__ = __str__
|
|
>>> str(mock)
|
|
'fooble'
|
|
|
|
>>> mock = Mock()
|
|
>>> mock.__str__ = Mock()
|
|
>>> mock.__str__.return_value = 'fooble'
|
|
>>> str(mock)
|
|
'fooble'
|
|
|
|
>>> mock = Mock()
|
|
>>> mock.__iter__ = Mock(return_value=iter([]))
|
|
>>> list(mock)
|
|
[]
|
|
|
|
One use case for this is for mocking objects used as context managers in a
|
|
`with` statement:
|
|
|
|
>>> mock = Mock()
|
|
>>> mock.__enter__ = Mock(return_value='foo')
|
|
>>> mock.__exit__ = Mock(return_value=False)
|
|
>>> with mock as m:
|
|
... assert m == 'foo'
|
|
...
|
|
>>> mock.__enter__.assert_called_with()
|
|
>>> mock.__exit__.assert_called_with(None, None, None)
|
|
|
|
Calls to magic methods do not appear in :attr:`~Mock.method_calls`, but they
|
|
are recorded in :attr:`~Mock.mock_calls`.
|
|
|
|
.. note::
|
|
|
|
If you use the `spec` keyword argument to create a mock then attempting to
|
|
set a magic method that isn't in the spec will raise an `AttributeError`.
|
|
|
|
The full list of supported magic methods is:
|
|
|
|
* ``__hash__``, ``__sizeof__``, ``__repr__`` and ``__str__``
|
|
* ``__dir__``, ``__format__`` and ``__subclasses__``
|
|
* ``__floor__``, ``__trunc__`` and ``__ceil__``
|
|
* Comparisons: ``__cmp__``, ``__lt__``, ``__gt__``, ``__le__``, ``__ge__``,
|
|
``__eq__`` and ``__ne__``
|
|
* Container methods: ``__getitem__``, ``__setitem__``, ``__delitem__``,
|
|
``__contains__``, ``__len__``, ``__iter__``, ``__getslice__``,
|
|
``__setslice__``, ``__reversed__`` and ``__missing__``
|
|
* Context manager: ``__enter__`` and ``__exit__``
|
|
* Unary numeric methods: ``__neg__``, ``__pos__`` and ``__invert__``
|
|
* The numeric methods (including right hand and in-place variants):
|
|
``__add__``, ``__sub__``, ``__mul__``, ``__div__``,
|
|
``__floordiv__``, ``__mod__``, ``__divmod__``, ``__lshift__``,
|
|
``__rshift__``, ``__and__``, ``__xor__``, ``__or__``, and ``__pow__``
|
|
* Numeric conversion methods: ``__complex__``, ``__int__``, ``__float__``,
|
|
``__index__`` and ``__coerce__``
|
|
* Descriptor methods: ``__get__``, ``__set__`` and ``__delete__``
|
|
* Pickling: ``__reduce__``, ``__reduce_ex__``, ``__getinitargs__``,
|
|
``__getnewargs__``, ``__getstate__`` and ``__setstate__``
|
|
|
|
|
|
The following methods exist but are *not* supported as they are either in use
|
|
by mock, can't be set dynamically, or can cause problems:
|
|
|
|
* ``__getattr__``, ``__setattr__``, ``__init__`` and ``__new__``
|
|
* ``__prepare__``, ``__instancecheck__``, ``__subclasscheck__``, ``__del__``
|
|
|
|
|
|
|
|
Magic Mock
|
|
----------
|
|
|
|
There are two `MagicMock` variants: `MagicMock` and `NonCallableMagicMock`.
|
|
|
|
|
|
.. class:: MagicMock(*args, **kw)
|
|
|
|
``MagicMock`` is a subclass of :class:`Mock` with default implementations
|
|
of most of the magic methods. You can use ``MagicMock`` without having to
|
|
configure the magic methods yourself.
|
|
|
|
The constructor parameters have the same meaning as for :class:`Mock`.
|
|
|
|
If you use the `spec` or `spec_set` arguments then *only* magic methods
|
|
that exist in the spec will be created.
|
|
|
|
|
|
.. class:: NonCallableMagicMock(*args, **kw)
|
|
|
|
A non-callable version of `MagicMock`.
|
|
|
|
The constructor parameters have the same meaning as for
|
|
:class:`MagicMock`, with the exception of `return_value` and
|
|
`side_effect` which have no meaning on a non-callable mock.
|
|
|
|
The magic methods are setup with `MagicMock` objects, so you can configure them
|
|
and use them in the usual way:
|
|
|
|
>>> mock = MagicMock()
|
|
>>> mock[3] = 'fish'
|
|
>>> mock.__setitem__.assert_called_with(3, 'fish')
|
|
>>> mock.__getitem__.return_value = 'result'
|
|
>>> mock[2]
|
|
'result'
|
|
|
|
By default many of the protocol methods are required to return objects of a
|
|
specific type. These methods are preconfigured with a default return value, so
|
|
that they can be used without you having to do anything if you aren't interested
|
|
in the return value. You can still *set* the return value manually if you want
|
|
to change the default.
|
|
|
|
Methods and their defaults:
|
|
|
|
* ``__lt__``: NotImplemented
|
|
* ``__gt__``: NotImplemented
|
|
* ``__le__``: NotImplemented
|
|
* ``__ge__``: NotImplemented
|
|
* ``__int__`` : 1
|
|
* ``__contains__`` : False
|
|
* ``__len__`` : 1
|
|
* ``__iter__`` : iter([])
|
|
* ``__exit__`` : False
|
|
* ``__complex__`` : 1j
|
|
* ``__float__`` : 1.0
|
|
* ``__bool__`` : True
|
|
* ``__index__`` : 1
|
|
* ``__hash__`` : default hash for the mock
|
|
* ``__str__`` : default str for the mock
|
|
* ``__sizeof__``: default sizeof for the mock
|
|
|
|
For example:
|
|
|
|
>>> mock = MagicMock()
|
|
>>> int(mock)
|
|
1
|
|
>>> len(mock)
|
|
0
|
|
>>> list(mock)
|
|
[]
|
|
>>> object() in mock
|
|
False
|
|
|
|
The two equality method, `__eq__` and `__ne__`, are special.
|
|
They do the default equality comparison on identity, using a side
|
|
effect, unless you change their return value to return something else:
|
|
|
|
>>> MagicMock() == 3
|
|
False
|
|
>>> MagicMock() != 3
|
|
True
|
|
>>> mock = MagicMock()
|
|
>>> mock.__eq__.return_value = True
|
|
>>> mock == 3
|
|
True
|
|
|
|
The return value of `MagicMock.__iter__` can be any iterable object and isn't
|
|
required to be an iterator:
|
|
|
|
>>> mock = MagicMock()
|
|
>>> mock.__iter__.return_value = ['a', 'b', 'c']
|
|
>>> list(mock)
|
|
['a', 'b', 'c']
|
|
>>> list(mock)
|
|
['a', 'b', 'c']
|
|
|
|
If the return value *is* an iterator, then iterating over it once will consume
|
|
it and subsequent iterations will result in an empty list:
|
|
|
|
>>> mock.__iter__.return_value = iter(['a', 'b', 'c'])
|
|
>>> list(mock)
|
|
['a', 'b', 'c']
|
|
>>> list(mock)
|
|
[]
|
|
|
|
``MagicMock`` has all of the supported magic methods configured except for some
|
|
of the obscure and obsolete ones. You can still set these up if you want.
|
|
|
|
Magic methods that are supported but not setup by default in ``MagicMock`` are:
|
|
|
|
* ``__subclasses__``
|
|
* ``__dir__``
|
|
* ``__format__``
|
|
* ``__get__``, ``__set__`` and ``__delete__``
|
|
* ``__reversed__`` and ``__missing__``
|
|
* ``__reduce__``, ``__reduce_ex__``, ``__getinitargs__``, ``__getnewargs__``,
|
|
``__getstate__`` and ``__setstate__``
|
|
* ``__getformat__`` and ``__setformat__``
|
|
|
|
|
|
|
|
.. [#] Magic methods *should* be looked up on the class rather than the
|
|
instance. Different versions of Python are inconsistent about applying this
|
|
rule. The supported protocol methods should work with all supported versions
|
|
of Python.
|
|
.. [#] The function is basically hooked up to the class, but each ``Mock``
|
|
instance is kept isolated from the others.
|
|
|
|
|
|
Helpers
|
|
=======
|
|
|
|
sentinel
|
|
--------
|
|
|
|
.. data:: sentinel
|
|
|
|
The ``sentinel`` object provides a convenient way of providing unique
|
|
objects for your tests.
|
|
|
|
Attributes are created on demand when you access them by name. Accessing
|
|
the same attribute will always return the same object. The objects
|
|
returned have a sensible repr so that test failure messages are readable.
|
|
|
|
Sometimes when testing you need to test that a specific object is passed as an
|
|
argument to another method, or returned. It can be common to create named
|
|
sentinel objects to test this. `sentinel` provides a convenient way of
|
|
creating and testing the identity of objects like this.
|
|
|
|
In this example we monkey patch `method` to return `sentinel.some_object`:
|
|
|
|
>>> real = ProductionClass()
|
|
>>> real.method = Mock(name="method")
|
|
>>> real.method.return_value = sentinel.some_object
|
|
>>> result = real.method()
|
|
>>> assert result is sentinel.some_object
|
|
>>> sentinel.some_object
|
|
sentinel.some_object
|
|
|
|
|
|
DEFAULT
|
|
-------
|
|
|
|
|
|
.. data:: DEFAULT
|
|
|
|
The `DEFAULT` object is a pre-created sentinel (actually
|
|
`sentinel.DEFAULT`). It can be used by :attr:`~Mock.side_effect`
|
|
functions to indicate that the normal return value should be used.
|
|
|
|
|
|
|
|
call
|
|
----
|
|
|
|
.. function:: call(*args, **kwargs)
|
|
|
|
`call` is a helper object for making simpler assertions, for comparing with
|
|
:attr:`~Mock.call_args`, :attr:`~Mock.call_args_list`,
|
|
:attr:`~Mock.mock_calls` and :attr:`~Mock.method_calls`. `call` can also be
|
|
used with :meth:`~Mock.assert_has_calls`.
|
|
|
|
>>> m = MagicMock(return_value=None)
|
|
>>> m(1, 2, a='foo', b='bar')
|
|
>>> m()
|
|
>>> m.call_args_list == [call(1, 2, a='foo', b='bar'), call()]
|
|
True
|
|
|
|
.. method:: call.call_list()
|
|
|
|
For a call object that represents multiple calls, `call_list`
|
|
returns a list of all the intermediate calls as well as the
|
|
final call.
|
|
|
|
`call_list` is particularly useful for making assertions on "chained calls". A
|
|
chained call is multiple calls on a single line of code. This results in
|
|
multiple entries in :attr:`~Mock.mock_calls` on a mock. Manually constructing
|
|
the sequence of calls can be tedious.
|
|
|
|
:meth:`~call.call_list` can construct the sequence of calls from the same
|
|
chained call:
|
|
|
|
>>> m = MagicMock()
|
|
>>> m(1).method(arg='foo').other('bar')(2.0)
|
|
<MagicMock name='mock().method().other()()' id='...'>
|
|
>>> kall = call(1).method(arg='foo').other('bar')(2.0)
|
|
>>> kall.call_list()
|
|
[call(1),
|
|
call().method(arg='foo'),
|
|
call().method().other('bar'),
|
|
call().method().other()(2.0)]
|
|
>>> m.mock_calls == kall.call_list()
|
|
True
|
|
|
|
.. _calls-as-tuples:
|
|
|
|
A `call` object is either a tuple of (positional args, keyword args) or
|
|
(name, positional args, keyword args) depending on how it was constructed. When
|
|
you construct them yourself this isn't particularly interesting, but the `call`
|
|
objects that are in the :attr:`Mock.call_args`, :attr:`Mock.call_args_list` and
|
|
:attr:`Mock.mock_calls` attributes can be introspected to get at the individual
|
|
arguments they contain.
|
|
|
|
The `call` objects in :attr:`Mock.call_args` and :attr:`Mock.call_args_list`
|
|
are two-tuples of (positional args, keyword args) whereas the `call` objects
|
|
in :attr:`Mock.mock_calls`, along with ones you construct yourself, are
|
|
three-tuples of (name, positional args, keyword args).
|
|
|
|
You can use their "tupleness" to pull out the individual arguments for more
|
|
complex introspection and assertions. The positional arguments are a tuple
|
|
(an empty tuple if there are no positional arguments) and the keyword
|
|
arguments are a dictionary:
|
|
|
|
>>> m = MagicMock(return_value=None)
|
|
>>> m(1, 2, 3, arg='one', arg2='two')
|
|
>>> kall = m.call_args
|
|
>>> args, kwargs = kall
|
|
>>> args
|
|
(1, 2, 3)
|
|
>>> kwargs
|
|
{'arg2': 'two', 'arg': 'one'}
|
|
>>> args is kall[0]
|
|
True
|
|
>>> kwargs is kall[1]
|
|
True
|
|
|
|
>>> m = MagicMock()
|
|
>>> m.foo(4, 5, 6, arg='two', arg2='three')
|
|
<MagicMock name='mock.foo()' id='...'>
|
|
>>> kall = m.mock_calls[0]
|
|
>>> name, args, kwargs = kall
|
|
>>> name
|
|
'foo'
|
|
>>> args
|
|
(4, 5, 6)
|
|
>>> kwargs
|
|
{'arg2': 'three', 'arg': 'two'}
|
|
>>> name is m.mock_calls[0][0]
|
|
True
|
|
|
|
|
|
create_autospec
|
|
---------------
|
|
|
|
.. function:: create_autospec(spec, spec_set=False, instance=False, **kwargs)
|
|
|
|
Create a mock object using another object as a spec. Attributes on the
|
|
mock will use the corresponding attribute on the `spec` object as their
|
|
spec.
|
|
|
|
Functions or methods being mocked will have their arguments checked to
|
|
ensure that they are called with the correct signature.
|
|
|
|
If `spec_set` is `True` then attempting to set attributes that don't exist
|
|
on the spec object will raise an `AttributeError`.
|
|
|
|
If a class is used as a spec then the return value of the mock (the
|
|
instance of the class) will have the same spec. You can use a class as the
|
|
spec for an instance object by passing `instance=True`. The returned mock
|
|
will only be callable if instances of the mock are callable.
|
|
|
|
`create_autospec` also takes arbitrary keyword arguments that are passed to
|
|
the constructor of the created mock.
|
|
|
|
See :ref:`auto-speccing` for examples of how to use auto-speccing with
|
|
`create_autospec` and the `autospec` argument to :func:`patch`.
|
|
|
|
|
|
ANY
|
|
---
|
|
|
|
.. data:: ANY
|
|
|
|
Sometimes you may need to make assertions about *some* of the arguments in a
|
|
call to mock, but either not care about some of the arguments or want to pull
|
|
them individually out of :attr:`~Mock.call_args` and make more complex
|
|
assertions on them.
|
|
|
|
To ignore certain arguments you can pass in objects that compare equal to
|
|
*everything*. Calls to :meth:`~Mock.assert_called_with` and
|
|
:meth:`~Mock.assert_called_once_with` will then succeed no matter what was
|
|
passed in.
|
|
|
|
>>> mock = Mock(return_value=None)
|
|
>>> mock('foo', bar=object())
|
|
>>> mock.assert_called_once_with('foo', bar=ANY)
|
|
|
|
`ANY` can also be used in comparisons with call lists like
|
|
:attr:`~Mock.mock_calls`:
|
|
|
|
>>> m = MagicMock(return_value=None)
|
|
>>> m(1)
|
|
>>> m(1, 2)
|
|
>>> m(object())
|
|
>>> m.mock_calls == [call(1), call(1, 2), ANY]
|
|
True
|
|
|
|
|
|
|
|
FILTER_DIR
|
|
----------
|
|
|
|
.. data:: FILTER_DIR
|
|
|
|
`FILTER_DIR` is a module level variable that controls the way mock objects
|
|
respond to `dir` (only for Python 2.6 or more recent). The default is `True`,
|
|
which uses the filtering described below, to only show useful members. If you
|
|
dislike this filtering, or need to switch it off for diagnostic purposes, then
|
|
set `mock.FILTER_DIR = False`.
|
|
|
|
With filtering on, `dir(some_mock)` shows only useful attributes and will
|
|
include any dynamically created attributes that wouldn't normally be shown.
|
|
If the mock was created with a `spec` (or `autospec` of course) then all the
|
|
attributes from the original are shown, even if they haven't been accessed
|
|
yet:
|
|
|
|
>>> dir(Mock())
|
|
['assert_any_call',
|
|
'assert_called_once_with',
|
|
'assert_called_with',
|
|
'assert_has_calls',
|
|
'attach_mock',
|
|
...
|
|
>>> from urllib import request
|
|
>>> dir(Mock(spec=request))
|
|
['AbstractBasicAuthHandler',
|
|
'AbstractDigestAuthHandler',
|
|
'AbstractHTTPHandler',
|
|
'BaseHandler',
|
|
...
|
|
|
|
Many of the not-very-useful (private to `Mock` rather than the thing being
|
|
mocked) underscore and double underscore prefixed attributes have been
|
|
filtered from the result of calling `dir` on a `Mock`. If you dislike this
|
|
behaviour you can switch it off by setting the module level switch
|
|
`FILTER_DIR`:
|
|
|
|
>>> from unittest import mock
|
|
>>> mock.FILTER_DIR = False
|
|
>>> dir(mock.Mock())
|
|
['_NonCallableMock__get_return_value',
|
|
'_NonCallableMock__get_side_effect',
|
|
'_NonCallableMock__return_value_doc',
|
|
'_NonCallableMock__set_return_value',
|
|
'_NonCallableMock__set_side_effect',
|
|
'__call__',
|
|
'__class__',
|
|
...
|
|
|
|
Alternatively you can just use `vars(my_mock)` (instance members) and
|
|
`dir(type(my_mock))` (type members) to bypass the filtering irrespective of
|
|
`mock.FILTER_DIR`.
|
|
|
|
|
|
mock_open
|
|
---------
|
|
|
|
.. function:: mock_open(mock=None, read_data=None)
|
|
|
|
A helper function to create a mock to replace the use of `open`. It works
|
|
for `open` called directly or used as a context manager.
|
|
|
|
The `mock` argument is the mock object to configure. If `None` (the
|
|
default) then a `MagicMock` will be created for you, with the API limited
|
|
to methods or attributes available on standard file handles.
|
|
|
|
`read_data` is a string for the `read` method of the file handle to return.
|
|
This is an empty string by default.
|
|
|
|
Using `open` as a context manager is a great way to ensure your file handles
|
|
are closed properly and is becoming common::
|
|
|
|
with open('/some/path', 'w') as f:
|
|
f.write('something')
|
|
|
|
The issue is that even if you mock out the call to `open` it is the
|
|
*returned object* that is used as a context manager (and has `__enter__` and
|
|
`__exit__` called).
|
|
|
|
Mocking context managers with a :class:`MagicMock` is common enough and fiddly
|
|
enough that a helper function is useful.
|
|
|
|
>>> m = mock_open()
|
|
>>> with patch('__main__.open', m, create=True):
|
|
... with open('foo', 'w') as h:
|
|
... h.write('some stuff')
|
|
...
|
|
>>> m.mock_calls
|
|
[call('foo', 'w'),
|
|
call().__enter__(),
|
|
call().write('some stuff'),
|
|
call().__exit__(None, None, None)]
|
|
>>> m.assert_called_once_with('foo', 'w')
|
|
>>> handle = m()
|
|
>>> handle.write.assert_called_once_with('some stuff')
|
|
|
|
And for reading files:
|
|
|
|
>>> with patch('__main__.open', mock_open(read_data='bibble'), create=True) as m:
|
|
... with open('foo') as h:
|
|
... result = h.read()
|
|
...
|
|
>>> m.assert_called_once_with('foo')
|
|
>>> assert result == 'bibble'
|
|
|
|
|
|
.. _auto-speccing:
|
|
|
|
Autospeccing
|
|
------------
|
|
|
|
Autospeccing is based on the existing `spec` feature of mock. It limits the
|
|
api of mocks to the api of an original object (the spec), but it is recursive
|
|
(implemented lazily) so that attributes of mocks only have the same api as
|
|
the attributes of the spec. In addition mocked functions / methods have the
|
|
same call signature as the original so they raise a `TypeError` if they are
|
|
called incorrectly.
|
|
|
|
Before I explain how auto-speccing works, here's why it is needed.
|
|
|
|
`Mock` is a very powerful and flexible object, but it suffers from two flaws
|
|
when used to mock out objects from a system under test. One of these flaws is
|
|
specific to the `Mock` api and the other is a more general problem with using
|
|
mock objects.
|
|
|
|
First the problem specific to `Mock`. `Mock` has two assert methods that are
|
|
extremely handy: :meth:`~Mock.assert_called_with` and
|
|
:meth:`~Mock.assert_called_once_with`.
|
|
|
|
>>> mock = Mock(name='Thing', return_value=None)
|
|
>>> mock(1, 2, 3)
|
|
>>> mock.assert_called_once_with(1, 2, 3)
|
|
>>> mock(1, 2, 3)
|
|
>>> mock.assert_called_once_with(1, 2, 3)
|
|
Traceback (most recent call last):
|
|
...
|
|
AssertionError: Expected to be called once. Called 2 times.
|
|
|
|
Because mocks auto-create attributes on demand, and allow you to call them
|
|
with arbitrary arguments, if you misspell one of these assert methods then
|
|
your assertion is gone:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> mock = Mock(name='Thing', return_value=None)
|
|
>>> mock(1, 2, 3)
|
|
>>> mock.assret_called_once_with(4, 5, 6)
|
|
|
|
Your tests can pass silently and incorrectly because of the typo.
|
|
|
|
The second issue is more general to mocking. If you refactor some of your
|
|
code, rename members and so on, any tests for code that is still using the
|
|
*old api* but uses mocks instead of the real objects will still pass. This
|
|
means your tests can all pass even though your code is broken.
|
|
|
|
Note that this is another reason why you need integration tests as well as
|
|
unit tests. Testing everything in isolation is all fine and dandy, but if you
|
|
don't test how your units are "wired together" there is still lots of room
|
|
for bugs that tests might have caught.
|
|
|
|
`mock` already provides a feature to help with this, called speccing. If you
|
|
use a class or instance as the `spec` for a mock then you can only access
|
|
attributes on the mock that exist on the real class:
|
|
|
|
>>> from urllib import request
|
|
>>> mock = Mock(spec=request.Request)
|
|
>>> mock.assret_called_with
|
|
Traceback (most recent call last):
|
|
...
|
|
AttributeError: Mock object has no attribute 'assret_called_with'
|
|
|
|
The spec only applies to the mock itself, so we still have the same issue
|
|
with any methods on the mock:
|
|
|
|
.. code-block:: pycon
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>>> mock.has_data()
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<mock.Mock object at 0x...>
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>>> mock.has_data.assret_called_with()
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Auto-speccing solves this problem. You can either pass `autospec=True` to
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`patch` / `patch.object` or use the `create_autospec` function to create a
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mock with a spec. If you use the `autospec=True` argument to `patch` then the
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object that is being replaced will be used as the spec object. Because the
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speccing is done "lazily" (the spec is created as attributes on the mock are
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accessed) you can use it with very complex or deeply nested objects (like
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modules that import modules that import modules) without a big performance
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hit.
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Here's an example of it in use:
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>>> from urllib import request
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>>> patcher = patch('__main__.request', autospec=True)
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>>> mock_request = patcher.start()
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>>> request is mock_request
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True
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>>> mock_request.Request
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<MagicMock name='request.Request' spec='Request' id='...'>
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You can see that `request.Request` has a spec. `request.Request` takes two
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arguments in the constructor (one of which is `self`). Here's what happens if
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we try to call it incorrectly:
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>>> req = request.Request()
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Traceback (most recent call last):
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...
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TypeError: <lambda>() takes at least 2 arguments (1 given)
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The spec also applies to instantiated classes (i.e. the return value of
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specced mocks):
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>>> req = request.Request('foo')
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>>> req
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<NonCallableMagicMock name='request.Request()' spec='Request' id='...'>
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`Request` objects are not callable, so the return value of instantiating our
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mocked out `request.Request` is a non-callable mock. With the spec in place
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any typos in our asserts will raise the correct error:
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>>> req.add_header('spam', 'eggs')
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<MagicMock name='request.Request().add_header()' id='...'>
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>>> req.add_header.assret_called_with
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Traceback (most recent call last):
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...
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AttributeError: Mock object has no attribute 'assret_called_with'
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>>> req.add_header.assert_called_with('spam', 'eggs')
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In many cases you will just be able to add `autospec=True` to your existing
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`patch` calls and then be protected against bugs due to typos and api
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changes.
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As well as using `autospec` through `patch` there is a
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:func:`create_autospec` for creating autospecced mocks directly:
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>>> from urllib import request
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>>> mock_request = create_autospec(request)
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>>> mock_request.Request('foo', 'bar')
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<NonCallableMagicMock name='mock.Request()' spec='Request' id='...'>
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This isn't without caveats and limitations however, which is why it is not
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the default behaviour. In order to know what attributes are available on the
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spec object, autospec has to introspect (access attributes) the spec. As you
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traverse attributes on the mock a corresponding traversal of the original
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object is happening under the hood. If any of your specced objects have
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properties or descriptors that can trigger code execution then you may not be
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able to use autospec. On the other hand it is much better to design your
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objects so that introspection is safe [#]_.
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A more serious problem is that it is common for instance attributes to be
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created in the `__init__` method and not to exist on the class at all.
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`autospec` can't know about any dynamically created attributes and restricts
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the api to visible attributes.
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>>> class Something(object):
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... def __init__(self):
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... self.a = 33
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...
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>>> with patch('__main__.Something', autospec=True):
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... thing = Something()
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... thing.a
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...
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Traceback (most recent call last):
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...
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AttributeError: Mock object has no attribute 'a'
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There are a few different ways of resolving this problem. The easiest, but
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not necessarily the least annoying, way is to simply set the required
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attributes on the mock after creation. Just because `autospec` doesn't allow
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you to fetch attributes that don't exist on the spec it doesn't prevent you
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setting them:
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>>> with patch('__main__.Something', autospec=True):
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... thing = Something()
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... thing.a = 33
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...
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There is a more aggressive version of both `spec` and `autospec` that *does*
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prevent you setting non-existent attributes. This is useful if you want to
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ensure your code only *sets* valid attributes too, but obviously it prevents
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this particular scenario:
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>>> with patch('__main__.Something', autospec=True, spec_set=True):
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... thing = Something()
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... thing.a = 33
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...
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Traceback (most recent call last):
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...
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AttributeError: Mock object has no attribute 'a'
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Probably the best way of solving the problem is to add class attributes as
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default values for instance members initialised in `__init__`. Note that if
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you are only setting default attributes in `__init__` then providing them via
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class attributes (shared between instances of course) is faster too. e.g.
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.. code-block:: python
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class Something(object):
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a = 33
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This brings up another issue. It is relatively common to provide a default
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value of `None` for members that will later be an object of a different type.
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`None` would be useless as a spec because it wouldn't let you access *any*
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attributes or methods on it. As `None` is *never* going to be useful as a
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spec, and probably indicates a member that will normally of some other type,
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`autospec` doesn't use a spec for members that are set to `None`. These will
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just be ordinary mocks (well - `MagicMocks`):
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>>> class Something(object):
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... member = None
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...
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>>> mock = create_autospec(Something)
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>>> mock.member.foo.bar.baz()
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<MagicMock name='mock.member.foo.bar.baz()' id='...'>
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If modifying your production classes to add defaults isn't to your liking
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then there are more options. One of these is simply to use an instance as the
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spec rather than the class. The other is to create a subclass of the
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production class and add the defaults to the subclass without affecting the
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production class. Both of these require you to use an alternative object as
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the spec. Thankfully `patch` supports this - you can simply pass the
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alternative object as the `autospec` argument:
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>>> class Something(object):
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... def __init__(self):
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... self.a = 33
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...
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>>> class SomethingForTest(Something):
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... a = 33
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...
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>>> p = patch('__main__.Something', autospec=SomethingForTest)
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>>> mock = p.start()
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>>> mock.a
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<NonCallableMagicMock name='Something.a' spec='int' id='...'>
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.. [#] This only applies to classes or already instantiated objects. Calling
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a mocked class to create a mock instance *does not* create a real instance.
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It is only attribute lookups - along with calls to `dir` - that are done.
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