python/mock-1.0.0/html/_sources/mocksignature.txt

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1 mocksignature
2 =============
3
4 .. currentmodule:: mock
5
6 .. note::
7
8 :ref:`auto-speccing`, added in mock 0.8, is a more advanced version of
9 `mocksignature` and can be used for many of the same use cases.
10
11 A problem with using mock objects to replace real objects in your tests is that
12 :class:`Mock` can be *too* flexible. Your code can treat the mock objects in
13 any way and you have to manually check that they were called correctly. If your
14 code calls functions or methods with the wrong number of arguments then mocks
15 don't complain.
16
17 The solution to this is `mocksignature`, which creates functions with the
18 same signature as the original, but delegating to a mock. You can interrogate
19 the mock in the usual way to check it has been called with the *right*
20 arguments, but if it is called with the wrong number of arguments it will
21 raise a `TypeError` in the same way your production code would.
22
23 Another advantage is that your mocked objects are real functions, which can
24 be useful when your code uses
25 `inspect <http://docs.python.org/library/inspect.html>`_ or depends on
26 functions being function objects.
27
28 .. function:: mocksignature(func, mock=None, skipfirst=False)
29
30 Create a new function with the same signature as `func` that delegates
31 to `mock`. If `skipfirst` is True the first argument is skipped, useful
32 for methods where `self` needs to be omitted from the new function.
33
34 If you don't pass in a `mock` then one will be created for you.
35
36 Functions returned by `mocksignature` have many of the same attributes
37 and assert methods as a mock object.
38
39 The mock is set as the `mock` attribute of the returned function for easy
40 access.
41
42 `mocksignature` can also be used with classes. It copies the signature of
43 the `__init__` method.
44
45 When used with callable objects (instances) it copies the signature of the
46 `__call__` method.
47
48 `mocksignature` will work out if it is mocking the signature of a method on
49 an instance or a method on a class and do the "right thing" with the `self`
50 argument in both cases.
51
52 Because of a limitation in the way that arguments are collected by functions
53 created by `mocksignature` they are *always* passed as positional arguments
54 (including defaults) and not keyword arguments.
55
56
57 mocksignature api
58 -----------------
59
60 Although the objects returned by `mocksignature` api are real function objects,
61 they have much of the same api as the :class:`Mock` class. This includes the
62 assert methods:
63
64 .. doctest::
65
66 >>> def func(a, b, c):
67 ... pass
68 ...
69 >>> func2 = mocksignature(func)
70 >>> func2.called
71 False
72 >>> func2.return_value = 3
73 >>> func2(1, 2, 3)
74 3
75 >>> func2.called
76 True
77 >>> func2.assert_called_once_with(1, 2, 3)
78 >>> func2.assert_called_with(1, 2, 4)
79 Traceback (most recent call last):
80 ...
81 AssertionError: Expected call: mock(1, 2, 4)
82 Actual call: mock(1, 2, 3)
83 >>> func2.call_count
84 1
85 >>> func2.side_effect = IndexError
86 >>> func2(4, 5, 6)
87 Traceback (most recent call last):
88 ...
89 IndexError
90
91 The mock object that is being delegated to is available as the `mock` attribute
92 of the function created by `mocksignature`.
93
94 .. doctest::
95
96 >>> func2.mock.mock_calls
97 [call(1, 2, 3), call(4, 5, 6)]
98
99 The methods and attributes available on functions returned by `mocksignature`
100 are:
101
102 :meth:`~Mock.assert_any_call`, :meth:`~Mock.assert_called_once_with`,
103 :meth:`~Mock.assert_called_with`, :meth:`~Mock.assert_has_calls`,
104 :attr:`~Mock.call_args`, :attr:`~Mock.call_args_list`,
105 :attr:`~Mock.call_count`, :attr:`~Mock.called`,
106 :attr:`~Mock.method_calls`, `mock`, :attr:`~Mock.mock_calls`,
107 :meth:`~Mock.reset_mock`, :attr:`~Mock.return_value`, and
108 :attr:`~Mock.side_effect`.
109
110
111 Example use
112 -----------
113
114 Basic use
115 ~~~~~~~~~
116
117 .. doctest::
118
119 >>> def function(a, b, c=None):
120 ... pass
121 ...
122 >>> mock = Mock()
123 >>> function = mocksignature(function, mock)
124 >>> function()
125 Traceback (most recent call last):
126 ...
127 TypeError: <lambda>() takes at least 2 arguments (0 given)
128 >>> function.return_value = 'some value'
129 >>> function(1, 2, 'foo')
130 'some value'
131 >>> function.assert_called_with(1, 2, 'foo')
132
133
134 Keyword arguments
135 ~~~~~~~~~~~~~~~~~
136
137 Note that arguments to functions created by `mocksignature` are always passed
138 in to the underlying mock by position even when called with keywords:
139
140 .. doctest::
141
142 >>> def function(a, b, c=None):
143 ... pass
144 ...
145 >>> function = mocksignature(function)
146 >>> function.return_value = None
147 >>> function(1, 2)
148 >>> function.assert_called_with(1, 2, None)
149
150
151 Mocking methods and self
152 ~~~~~~~~~~~~~~~~~~~~~~~~
153
154 When you use `mocksignature` to replace a method on a class then `self`
155 will be included in the method signature - and you will need to include
156 the instance when you do your asserts.
157
158 As a curious factor of the way Python (2) wraps methods fetched from a class,
159 we can *get* the `return_value` from a function set on a class, but we can't
160 set it. We have to do this through the exposed `mock` attribute instead:
161
162 .. doctest::
163
164 >>> class SomeClass(object):
165 ... def method(self, a, b, c=None):
166 ... pass
167 ...
168 >>> SomeClass.method = mocksignature(SomeClass.method)
169 >>> SomeClass.method.mock.return_value = None
170 >>> instance = SomeClass()
171 >>> instance.method()
172 Traceback (most recent call last):
173 ...
174 TypeError: <lambda>() takes at least 4 arguments (1 given)
175 >>> instance.method(1, 2, 3)
176 >>> instance.method.assert_called_with(instance, 1, 2, 3)
177
178 When you use `mocksignature` on instance methods `self` isn't included (and we
179 can set the `return_value` etc directly):
180
181 .. doctest::
182
183 >>> class SomeClass(object):
184 ... def method(self, a, b, c=None):
185 ... pass
186 ...
187 >>> instance = SomeClass()
188 >>> instance.method = mocksignature(instance.method)
189 >>> instance.method.return_value = None
190 >>> instance.method(1, 2, 3)
191 >>> instance.method.assert_called_with(1, 2, 3)
192
193
194 mocksignature with classes
195 ~~~~~~~~~~~~~~~~~~~~~~~~~~
196
197 When used with a class `mocksignature` copies the signature of the `__init__`
198 method.
199
200 .. doctest::
201
202 >>> class Something(object):
203 ... def __init__(self, foo, bar):
204 ... pass
205 ...
206 >>> MockSomething = mocksignature(Something)
207 >>> instance = MockSomething(10, 9)
208 >>> assert instance is MockSomething.return_value
209 >>> MockSomething.assert_called_with(10, 9)
210 >>> MockSomething()
211 Traceback (most recent call last):
212 ...
213 TypeError: <lambda>() takes at least 2 arguments (0 given)
214
215 Because the object returned by `mocksignature` is a function rather than a
216 `Mock` you lose the other capabilities of `Mock`, like dynamic attribute
217 creation.
218
219
220 mocksignature with callable objects
221 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
222
223 When used with a callable object `mocksignature` copies the signature of the
224 `__call__` method.
225
226 .. doctest::
227
228 >>> class Something(object):
229 ... def __call__(self, spam, eggs):
230 ... pass
231 ...
232 >>> something = Something()
233 >>> mock_something = mocksignature(something)
234 >>> result = mock_something(10, 9)
235 >>> mock_something.assert_called_with(10, 9)
236 >>> mock_something()
237 Traceback (most recent call last):
238 ...
239 TypeError: <lambda>() takes at least 2 arguments (0 given)
240
241
242 mocksignature argument to patch
243 -------------------------------
244
245 `mocksignature` is available as a keyword argument to :func:`patch` or
246 :func:`patch.object`. It can be used with functions / methods / classes and
247 callable objects.
248
249 .. doctest::
250
251 >>> class SomeClass(object):
252 ... def method(self, a, b, c=None):
253 ... pass
254 ...
255 >>> @patch.object(SomeClass, 'method', mocksignature=True)
256 ... def test(mock_method):
257 ... instance = SomeClass()
258 ... mock_method.return_value = None
259 ... instance.method(1, 2)
260 ... mock_method.assert_called_with(instance, 1, 2, None)
261 ...
262 >>> test()

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