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 <div class="section" id="unittest-mock-getting-started">
<h1>26.6. <a class="reference internal" href="unittest.mock.html#module-unittest.mock" title="unittest.mock: Mock object library."><code class="xref py py-mod docutils literal notranslate">unittest.mock</code></a> — getting started<a class="headerlink" href="#unittest-mock-getting-started" title="Permalink to this headline">¶</a></h1>
<div class="versionadded">
New in version 3.3.
</div>
<div class="section" id="using-mock">
<h2>26.6.1. Using Mock<a class="headerlink" href="#using-mock" title="Permalink to this headline">¶</a></h2>
<div class="section" id="mock-patching-methods">
<h3>26.6.1.1. Mock Patching Methods<a class="headerlink" href="#mock-patching-methods" title="Permalink to this headline">¶</a></h3>
Common uses for <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock" title="unittest.mock.Mock"><code class="xref py py-class docutils literal notranslate">Mock</code></a> objects include:
<ul class="simple">
<li>Patching methods</li>
<li>Recording method calls on objects</li>
</ul>
You might want to replace a method on an object to check that
it is called with the correct arguments by another part of the system:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; real = SomeClass()
&gt;&gt;&gt; real.method = MagicMock(name=&#39;method&#39;)
&gt;&gt;&gt; real.method(3, 4, 5, key=&#39;value&#39;)
&lt;MagicMock name=&#39;method()&#39; id=&#39;...&#39;&gt;
</pre></div>
</div>
Once our mock has been used (<code class="docutils literal notranslate">real.method</code> in this example) it has methods
and attributes that allow you to make assertions about how it has been used.
<div class="admonition note">
Note
In most of these examples the <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock" title="unittest.mock.Mock"><code class="xref py py-class docutils literal notranslate">Mock</code></a> and <a class="reference internal" href="unittest.mock.html#unittest.mock.MagicMock" title="unittest.mock.MagicMock"><code class="xref py py-class docutils literal notranslate">MagicMock</code></a> classes
are interchangeable. As the <code class="docutils literal notranslate">MagicMock</code> is the more capable class it makes
a sensible one to use by default.
</div>
Once the mock has been called its <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.called" title="unittest.mock.Mock.called"><code class="xref py py-attr docutils literal notranslate">called</code></a> attribute is set to
<code class="docutils literal notranslate">True</code>. More importantly we can use the <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.assert_called_with" title="unittest.mock.Mock.assert_called_with"><code class="xref py py-meth docutils literal notranslate">assert_called_with()</code></a> or
<a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.assert_called_once_with" title="unittest.mock.Mock.assert_called_once_with"><code class="xref py py-meth docutils literal notranslate">assert_called_once_with()</code></a> method to check that it was called with
the correct arguments.
This example tests that calling <code class="docutils literal notranslate">ProductionClass().method</code> results in a call to
the <code class="docutils literal notranslate">something</code> method:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class ProductionClass:
... def method(self):
... self.something(1, 2, 3)
... def something(self, a, b, c):
... pass
...
&gt;&gt;&gt; real = ProductionClass()
&gt;&gt;&gt; real.something = MagicMock()
&gt;&gt;&gt; real.method()
&gt;&gt;&gt; real.something.assert_called_once_with(1, 2, 3)
</pre></div>
</div>
</div>
<div class="section" id="mock-for-method-calls-on-an-object">
<h3>26.6.1.2. Mock for Method Calls on an Object<a class="headerlink" href="#mock-for-method-calls-on-an-object" title="Permalink to this headline">¶</a></h3>
In the last example we patched a method directly on an object to check that it
was called correctly. Another common use case is to pass an object into a
method (or some part of the system under test) and then check that it is used
in the correct way.
The simple <code class="docutils literal notranslate">ProductionClass</code> below has a <code class="docutils literal notranslate">closer</code> method. If it is called with
an object then it calls <code class="docutils literal notranslate">close</code> on it.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class ProductionClass:
... def closer(self, something):
... something.close()
...
</pre></div>
</div>
So to test it we need to pass in an object with a <code class="docutils literal notranslate">close</code> method and check
that it was called correctly.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; real = ProductionClass()
&gt;&gt;&gt; mock = Mock()
&gt;&gt;&gt; real.closer(mock)
&gt;&gt;&gt; mock.close.assert_called_with()
</pre></div>
</div>
We don’t have to do any work to provide the ‘close’ method on our mock.
Accessing close creates it. So, if ‘close’ hasn’t already been called then
accessing it in the test will create it, but <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.assert_called_with" title="unittest.mock.Mock.assert_called_with"><code class="xref py py-meth docutils literal notranslate">assert_called_with()</code></a>
will raise a failure exception.
</div>
<div class="section" id="mocking-classes">
<h3>26.6.1.3. Mocking Classes<a class="headerlink" href="#mocking-classes" title="Permalink to this headline">¶</a></h3>
A common use case is to mock out classes instantiated by your code under test.
When you patch a class, then that class is replaced with a mock. Instances
are created by calling the class. This means you access the “mock instance”
by looking at the return value of the mocked class.
In the example below we have a function <code class="docutils literal notranslate">some_function</code> that instantiates <code class="docutils literal notranslate">Foo</code>
and calls a method on it. The call to <a class="reference internal" href="unittest.mock.html#unittest.mock.patch" title="unittest.mock.patch"><code class="xref py py-func docutils literal notranslate">patch()</code></a> replaces the class <code class="docutils literal notranslate">Foo</code> with a
mock. The <code class="docutils literal notranslate">Foo</code> instance is the result of calling the mock, so it is configured
by modifying the mock <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.return_value" title="unittest.mock.Mock.return_value"><code class="xref py py-attr docutils literal notranslate">return_value</code></a>.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def some_function():
... instance = module.Foo()
... return instance.method()
...
&gt;&gt;&gt; with patch(&#39;module.Foo&#39;) as mock:
... instance = mock.return_value
... instance.method.return_value = &#39;the result&#39;
... result = some_function()
... assert result == &#39;the result&#39;
</pre></div>
</div>
</div>
<div class="section" id="naming-your-mocks">
<h3>26.6.1.4. Naming your mocks<a class="headerlink" href="#naming-your-mocks" title="Permalink to this headline">¶</a></h3>
It can be useful to give your mocks a name. The name is shown in the repr of
the mock and can be helpful when the mock appears in test failure messages. The
name is also propagated to attributes or methods of the mock:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = MagicMock(name=&#39;foo&#39;)
&gt;&gt;&gt; mock
&lt;MagicMock name=&#39;foo&#39; id=&#39;...&#39;&gt;
&gt;&gt;&gt; mock.method
&lt;MagicMock name=&#39;foo.method&#39; id=&#39;...&#39;&gt;
</pre></div>
</div>
</div>
<div class="section" id="tracking-all-calls">
<h3>26.6.1.5. Tracking all Calls<a class="headerlink" href="#tracking-all-calls" title="Permalink to this headline">¶</a></h3>
Often you want to track more than a single call to a method. The
<a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.mock_calls" title="unittest.mock.Mock.mock_calls"><code class="xref py py-attr docutils literal notranslate">mock_calls</code></a> attribute records all calls
to child attributes of the mock - and also to their children.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = MagicMock()
&gt;&gt;&gt; mock.method()
&lt;MagicMock name=&#39;mock.method()&#39; id=&#39;...&#39;&gt;
&gt;&gt;&gt; mock.attribute.method(10, x=53)
&lt;MagicMock name=&#39;mock.attribute.method()&#39; id=&#39;...&#39;&gt;
&gt;&gt;&gt; mock.mock_calls
[call.method(), call.attribute.method(10, x=53)]
</pre></div>
</div>
If you make an assertion about <code class="docutils literal notranslate">mock_calls</code> and any unexpected methods
have been called, then the assertion will fail. This is useful because as well
as asserting that the calls you expected have been made, you are also checking
that they were made in the right order and with no additional calls:
You use the <a class="reference internal" href="unittest.mock.html#unittest.mock.call" title="unittest.mock.call"><code class="xref py py-data docutils literal notranslate">call</code></a> object to construct lists for comparing with
<code class="docutils literal notranslate">mock_calls</code>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; expected = [call.method(), call.attribute.method(10, x=53)]
&gt;&gt;&gt; mock.mock_calls == expected
True
</pre></div>
</div>
</div>
<div class="section" id="setting-return-values-and-attributes">
<h3>26.6.1.6. Setting Return Values and Attributes<a class="headerlink" href="#setting-return-values-and-attributes" title="Permalink to this headline">¶</a></h3>
Setting the return values on a mock object is trivially easy:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = Mock()
&gt;&gt;&gt; mock.return_value = 3
&gt;&gt;&gt; mock()
3
</pre></div>
</div>
Of course you can do the same for methods on the mock:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = Mock()
&gt;&gt;&gt; mock.method.return_value = 3
&gt;&gt;&gt; mock.method()
3
</pre></div>
</div>
The return value can also be set in the constructor:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = Mock(return_value=3)
&gt;&gt;&gt; mock()
3
</pre></div>
</div>
If you need an attribute setting on your mock, just do it:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = Mock()
&gt;&gt;&gt; mock.x = 3
&gt;&gt;&gt; mock.x
3
</pre></div>
</div>
Sometimes you want to mock up a more complex situation, like for example
<code class="docutils literal notranslate">mock.connection.cursor().execute(&quot;SELECT 1&quot;)</code>. If we wanted this call to
return a list, then we have to configure the result of the nested call.
We can use <a class="reference internal" href="unittest.mock.html#unittest.mock.call" title="unittest.mock.call"><code class="xref py py-data docutils literal notranslate">call</code></a> to construct the set of calls in a “chained call” like
this for easy assertion afterwards:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = Mock()
&gt;&gt;&gt; cursor = mock.connection.cursor.return_value
&gt;&gt;&gt; cursor.execute.return_value = [&#39;foo&#39;]
&gt;&gt;&gt; mock.connection.cursor().execute(&quot;SELECT 1&quot;)
[&#39;foo&#39;]
&gt;&gt;&gt; expected = call.connection.cursor().execute(&quot;SELECT 1&quot;).call_list()
&gt;&gt;&gt; mock.mock_calls
[call.connection.cursor(), call.connection.cursor().execute(&#39;SELECT 1&#39;)]
&gt;&gt;&gt; mock.mock_calls == expected
True
</pre></div>
</div>
It is the call to <code class="docutils literal notranslate">.call_list()</code> that turns our call object into a list of
calls representing the chained calls.
</div>
<div class="section" id="raising-exceptions-with-mocks">
<h3>26.6.1.7. Raising exceptions with mocks<a class="headerlink" href="#raising-exceptions-with-mocks" title="Permalink to this headline">¶</a></h3>
A useful attribute is <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.side_effect" title="unittest.mock.Mock.side_effect"><code class="xref py py-attr docutils literal notranslate">side_effect</code></a>. If you set this to an
exception class or instance then the exception will be raised when the mock
is called.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = Mock(side_effect=Exception(&#39;Boom!&#39;))
&gt;&gt;&gt; mock()
Traceback (most recent call last):
 ...
Exception: Boom!
</pre></div>
</div>
</div>
<div class="section" id="side-effect-functions-and-iterables">
<h3>26.6.1.8. Side effect functions and iterables<a class="headerlink" href="#side-effect-functions-and-iterables" title="Permalink to this headline">¶</a></h3>
<code class="docutils literal notranslate">side_effect</code> can also be set to a function or an iterable. The use case for
<code class="docutils literal notranslate">side_effect</code> as an iterable is where your mock is going to be called several
times, and you want each call to return a different value. When you set
<code class="docutils literal notranslate">side_effect</code> to an iterable every call to the mock returns the next value
from the iterable:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = MagicMock(side_effect=[4, 5, 6])
&gt;&gt;&gt; mock()
4
&gt;&gt;&gt; mock()
5
&gt;&gt;&gt; mock()
6
</pre></div>
</div>
For more advanced use cases, like dynamically varying the return values
depending on what the mock is called with, <code class="docutils literal notranslate">side_effect</code> can be a function.
The function will be called with the same arguments as the mock. Whatever the
function returns is what the call returns:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; vals = {(1, 2): 1, (2, 3): 2}
&gt;&gt;&gt; def side_effect(*args):
... return vals[args]
...
&gt;&gt;&gt; mock = MagicMock(side_effect=side_effect)
&gt;&gt;&gt; mock(1, 2)
1
&gt;&gt;&gt; mock(2, 3)
2
</pre></div>
</div>
</div>
<div class="section" id="creating-a-mock-from-an-existing-object">
<h3>26.6.1.9. Creating a Mock from an Existing Object<a class="headerlink" href="#creating-a-mock-from-an-existing-object" title="Permalink to this headline">¶</a></h3>
One problem with over use of mocking is that it couples your tests to the
implementation of your mocks rather than your real code. Suppose you have a
class that implements <code class="docutils literal notranslate">some_method</code>. In a test for another class, you
provide a mock of this object that also provides <code class="docutils literal notranslate">some_method</code>. If later
you refactor the first class, so that it no longer has <code class="docutils literal notranslate">some_method</code> - then
your tests will continue to pass even though your code is now broken!
<a class="reference internal" href="unittest.mock.html#unittest.mock.Mock" title="unittest.mock.Mock"><code class="xref py py-class docutils literal notranslate">Mock</code></a> allows you to provide an object as a specification for the mock,
using the spec keyword argument. Accessing methods / attributes on the
mock that don’t exist on your specification object will immediately raise an
attribute error. If you change the implementation of your specification, then
tests that use that class will start failing immediately without you having to
instantiate the class in those tests.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = Mock(spec=SomeClass)
&gt;&gt;&gt; mock.old_method()
Traceback (most recent call last):
 ...
AttributeError: object has no attribute &#39;old_method&#39;
</pre></div>
</div>
Using a specification also enables a smarter matching of calls made to the
mock, regardless of whether some parameters were passed as positional or
named arguments:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def f(a, b, c): pass
...
&gt;&gt;&gt; mock = Mock(spec=f)
&gt;&gt;&gt; mock(1, 2, 3)
&lt;Mock name=&#39;mock()&#39; id=&#39;140161580456576&#39;&gt;
&gt;&gt;&gt; mock.assert_called_with(a=1, b=2, c=3)
</pre></div>
</div>
If you want this smarter matching to also work with method calls on the mock,
you can use <a class="reference internal" href="unittest.mock.html#auto-speccing">auto-speccing</a>.
If you want a stronger form of specification that prevents the setting
of arbitrary attributes as well as the getting of them then you can use
spec_set instead of spec.
</div>
</div>
<div class="section" id="patch-decorators">
<h2>26.6.2. Patch Decorators<a class="headerlink" href="#patch-decorators" title="Permalink to this headline">¶</a></h2>
<div class="admonition note">
Note
With <a class="reference internal" href="unittest.mock.html#unittest.mock.patch" title="unittest.mock.patch"><code class="xref py py-func docutils literal notranslate">patch()</code></a> it matters that you patch objects in the namespace where
they are looked up. This is normally straightforward, but for a quick guide
read <a class="reference internal" href="unittest.mock.html#where-to-patch">where to patch</a>.
</div>
A common need in tests is to patch a class attribute or a module attribute,
for example patching a builtin or patching a class in a module to test that it
is instantiated. Modules and classes are effectively global, so patching on
them has to be undone after the test or the patch will persist into other
tests and cause hard to diagnose problems.
mock provides three convenient decorators for this: <a class="reference internal" href="unittest.mock.html#unittest.mock.patch" title="unittest.mock.patch"><code class="xref py py-func docutils literal notranslate">patch()</code></a>, <a class="reference internal" href="unittest.mock.html#unittest.mock.patch.object" title="unittest.mock.patch.object"><code class="xref py py-func docutils literal notranslate">patch.object()</code></a> and
<a class="reference internal" href="unittest.mock.html#unittest.mock.patch.dict" title="unittest.mock.patch.dict"><code class="xref py py-func docutils literal notranslate">patch.dict()</code></a>. <code class="docutils literal notranslate">patch</code> takes a single string, of the form
<code class="docutils literal notranslate">package.module.Class.attribute</code> to specify the attribute you are patching. It
also optionally takes a value that you want the attribute (or class or
whatever) to be replaced with. ‘patch.object’ takes an object and the name of
the attribute you would like patched, plus optionally the value to patch it
with.
<code class="docutils literal notranslate">patch.object</code>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; original = SomeClass.attribute
&gt;&gt;&gt; @patch.object(SomeClass, &#39;attribute&#39;, sentinel.attribute)
... def test():
... assert SomeClass.attribute == sentinel.attribute
...
&gt;&gt;&gt; test()
&gt;&gt;&gt; assert SomeClass.attribute == original
</pre></div>
</div>
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; @patch(&#39;package.module.attribute&#39;, sentinel.attribute)
... def test():
... from package.module import attribute
... assert attribute is sentinel.attribute
...
&gt;&gt;&gt; test()
</pre></div>
</div>
If you are patching a module (including <a class="reference internal" href="builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate">builtins</code></a>) then use <a class="reference internal" href="unittest.mock.html#unittest.mock.patch" title="unittest.mock.patch"><code class="xref py py-func docutils literal notranslate">patch()</code></a>
instead of <a class="reference internal" href="unittest.mock.html#unittest.mock.patch.object" title="unittest.mock.patch.object"><code class="xref py py-func docutils literal notranslate">patch.object()</code></a>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = MagicMock(return_value=sentinel.file_handle)
&gt;&gt;&gt; with patch(&#39;builtins.open&#39;, mock):
... handle = open(&#39;filename&#39;, &#39;r&#39;)
...
&gt;&gt;&gt; mock.assert_called_with(&#39;filename&#39;, &#39;r&#39;)
&gt;&gt;&gt; assert handle == sentinel.file_handle, &quot;incorrect file handle returned&quot;
</pre></div>
</div>
The module name can be ‘dotted’, in the form <code class="docutils literal notranslate">package.module</code> if needed:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; @patch(&#39;package.module.ClassName.attribute&#39;, sentinel.attribute)
... def test():
... from package.module import ClassName
... assert ClassName.attribute == sentinel.attribute
...
&gt;&gt;&gt; test()
</pre></div>
</div>
A nice pattern is to actually decorate test methods themselves:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class MyTest(unittest.TestCase):
... @patch.object(SomeClass, &#39;attribute&#39;, sentinel.attribute)
... def test_something(self):
... self.assertEqual(SomeClass.attribute, sentinel.attribute)
...
&gt;&gt;&gt; original = SomeClass.attribute
&gt;&gt;&gt; MyTest(&#39;test_something&#39;).test_something()
&gt;&gt;&gt; assert SomeClass.attribute == original
</pre></div>
</div>
If you want to patch with a Mock, you can use <a class="reference internal" href="unittest.mock.html#unittest.mock.patch" title="unittest.mock.patch"><code class="xref py py-func docutils literal notranslate">patch()</code></a> with only one argument
(or <a class="reference internal" href="unittest.mock.html#unittest.mock.patch.object" title="unittest.mock.patch.object"><code class="xref py py-func docutils literal notranslate">patch.object()</code></a> with two arguments). The mock will be created for you and
passed into the test function / method:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class MyTest(unittest.TestCase):
... @patch.object(SomeClass, &#39;static_method&#39;)
... def test_something(self, mock_method):
... SomeClass.static_method()
... mock_method.assert_called_with()
...
&gt;&gt;&gt; MyTest(&#39;test_something&#39;).test_something()
</pre></div>
</div>
You can stack up multiple patch decorators using this pattern:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class MyTest(unittest.TestCase):
... @patch(&#39;package.module.ClassName1&#39;)
... @patch(&#39;package.module.ClassName2&#39;)
... def test_something(self, MockClass2, MockClass1):
... self.assertIs(package.module.ClassName1, MockClass1)
... self.assertIs(package.module.ClassName2, MockClass2)
...
&gt;&gt;&gt; MyTest(&#39;test_something&#39;).test_something()
</pre></div>
</div>
When you nest patch decorators the mocks are passed in to the decorated
function in the same order they applied (the normal python order that
decorators are applied). This means from the bottom up, so in the example
above the mock for <code class="docutils literal notranslate">test_module.ClassName2</code> is passed in first.
There is also <a class="reference internal" href="unittest.mock.html#unittest.mock.patch.dict" title="unittest.mock.patch.dict"><code class="xref py py-func docutils literal notranslate">patch.dict()</code></a> for setting values in a dictionary just
during a scope and restoring the dictionary to its original state when the test
ends:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; foo = {&#39;key&#39;: &#39;value&#39;}
&gt;&gt;&gt; original = foo.copy()
&gt;&gt;&gt; with patch.dict(foo, {&#39;newkey&#39;: &#39;newvalue&#39;}, clear=True):
... assert foo == {&#39;newkey&#39;: &#39;newvalue&#39;}
...
&gt;&gt;&gt; assert foo == original
</pre></div>
</div>
<code class="docutils literal notranslate">patch</code>, <code class="docutils literal notranslate">patch.object</code> and <code class="docutils literal notranslate">patch.dict</code> can all be used as context managers.
Where you use <a class="reference internal" href="unittest.mock.html#unittest.mock.patch" title="unittest.mock.patch"><code class="xref py py-func docutils literal notranslate">patch()</code></a> to create a mock for you, you can get a reference to the
mock using the “as” form of the with statement:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class ProductionClass:
... def method(self):
... pass
...
&gt;&gt;&gt; with patch.object(ProductionClass, &#39;method&#39;) as mock_method:
... mock_method.return_value = None
... real = ProductionClass()
... real.method(1, 2, 3)
...
&gt;&gt;&gt; mock_method.assert_called_with(1, 2, 3)
</pre></div>
</div>
As an alternative <code class="docutils literal notranslate">patch</code>, <code class="docutils literal notranslate">patch.object</code> and <code class="docutils literal notranslate">patch.dict</code> can be used as
class decorators. When used in this way it is the same as applying the
decorator individually to every method whose name starts with “test”.
</div>
<div class="section" id="further-examples">
<h2>26.6.3. Further Examples<a class="headerlink" href="#further-examples" title="Permalink to this headline">¶</a></h2>
Here are some more examples for some slightly more advanced scenarios.
<div class="section" id="mocking-chained-calls">
<h3>26.6.3.1. Mocking chained calls<a class="headerlink" href="#mocking-chained-calls" title="Permalink to this headline">¶</a></h3>
Mocking chained calls is actually straightforward with mock once you
understand the <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.return_value" title="unittest.mock.Mock.return_value"><code class="xref py py-attr docutils literal notranslate">return_value</code></a> attribute. When a mock is called for
the first time, or you fetch its <code class="docutils literal notranslate">return_value</code> before it has been called, a
new <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock" title="unittest.mock.Mock"><code class="xref py py-class docutils literal notranslate">Mock</code></a> is created.
This means that you can see how the object returned from a call to a mocked
object has been used by interrogating the <code class="docutils literal notranslate">return_value</code> mock:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = Mock()
&gt;&gt;&gt; mock().foo(a=2, b=3)
&lt;Mock name=&#39;mock().foo()&#39; id=&#39;...&#39;&gt;
&gt;&gt;&gt; mock.return_value.foo.assert_called_with(a=2, b=3)
</pre></div>
</div>
From here it is a simple step to configure and then make assertions about
chained calls. Of course another alternative is writing your code in a more
testable way in the first place…
So, suppose we have some code that looks a little bit like this:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class Something:
... def __init__(self):
... self.backend = BackendProvider()
... def method(self):
... response = self.backend.get_endpoint(&#39;foobar&#39;).create_call(&#39;spam&#39;, &#39;eggs&#39;).start_call()
... # more code
</pre></div>
</div>
Assuming that <code class="docutils literal notranslate">BackendProvider</code> is already well tested, how do we test
<code class="docutils literal notranslate">method()</code>? Specifically, we want to test that the code section <code class="docutils literal notranslate"># more
code</code> uses the response object in the correct way.
As this chain of calls is made from an instance attribute we can monkey patch
the <code class="docutils literal notranslate">backend</code> attribute on a <code class="docutils literal notranslate">Something</code> instance. In this particular case
we are only interested in the return value from the final call to
<code class="docutils literal notranslate">start_call</code> so we don’t have much configuration to do. Let’s assume the
object it returns is ‘file-like’, so we’ll ensure that our response object
uses the builtin <a class="reference internal" href="functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate">open()</code></a> as its <code class="docutils literal notranslate">spec</code>.
To do this we create a mock instance as our mock backend and create a mock
response object for it. To set the response as the return value for that final
<code class="docutils literal notranslate">start_call</code> we could do this:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>mock_backend.get_endpoint.return_value.create_call.return_value.start_call.return_value = mock_response
</pre></div>
</div>
We can do that in a slightly nicer way using the <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.configure_mock" title="unittest.mock.Mock.configure_mock"><code class="xref py py-meth docutils literal notranslate">configure_mock()</code></a>
method to directly set the return value for us:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; something = Something()
&gt;&gt;&gt; mock_response = Mock(spec=open)
&gt;&gt;&gt; mock_backend = Mock()
&gt;&gt;&gt; config = {&#39;get_endpoint.return_value.create_call.return_value.start_call.return_value&#39;: mock_response}
&gt;&gt;&gt; mock_backend.configure_mock(**config)
</pre></div>
</div>
With these we monkey patch the “mock backend” in place and can make the real
call:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; something.backend = mock_backend
&gt;&gt;&gt; something.method()
</pre></div>
</div>
Using <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.mock_calls" title="unittest.mock.Mock.mock_calls"><code class="xref py py-attr docutils literal notranslate">mock_calls</code></a> we can check the chained call with a single
assert. A chained call is several calls in one line of code, so there will be
several entries in <code class="docutils literal notranslate">mock_calls</code>. We can use <a class="reference internal" href="unittest.mock.html#unittest.mock.call.call_list" title="unittest.mock.call.call_list"><code class="xref py py-meth docutils literal notranslate">call.call_list()</code></a> to create
this list of calls for us:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; chained = call.get_endpoint(&#39;foobar&#39;).create_call(&#39;spam&#39;, &#39;eggs&#39;).start_call()
&gt;&gt;&gt; call_list = chained.call_list()
&gt;&gt;&gt; assert mock_backend.mock_calls == call_list
</pre></div>
</div>
</div>
<div class="section" id="partial-mocking">
<h3>26.6.3.2. Partial mocking<a class="headerlink" href="#partial-mocking" title="Permalink to this headline">¶</a></h3>
In some tests I wanted to mock out a call to <a class="reference internal" href="datetime.html#datetime.date.today" title="datetime.date.today"><code class="xref py py-meth docutils literal notranslate">datetime.date.today()</code></a>
to return a known date, but I didn’t want to prevent the code under test from
creating new date objects. Unfortunately <a class="reference internal" href="datetime.html#datetime.date" title="datetime.date"><code class="xref py py-class docutils literal notranslate">datetime.date</code></a> is written in C, and
so I couldn’t just monkey-patch out the static <code class="xref py py-meth docutils literal notranslate">date.today()</code> method.
I found a simple way of doing this that involved effectively wrapping the date
class with a mock, but passing through calls to the constructor to the real
class (and returning real instances).
The <a class="reference internal" href="unittest.mock.html#unittest.mock.patch" title="unittest.mock.patch"><code class="xref py py-func docutils literal notranslate">patch decorator</code></a> is used here to
mock out the <code class="docutils literal notranslate">date</code> class in the module under test. The <code class="xref py py-attr docutils literal notranslate">side_effect</code>
attribute on the mock date class is then set to a lambda function that returns
a real date. When the mock date class is called a real date will be
constructed and returned by <code class="docutils literal notranslate">side_effect</code>.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from datetime import date
&gt;&gt;&gt; with patch(&#39;mymodule.date&#39;) as mock_date:
... mock_date.today.return_value = date(2010, 10, 8)
... mock_date.side_effect = lambda *args, **kw: date(*args, **kw)
...
... assert mymodule.date.today() == date(2010, 10, 8)
... assert mymodule.date(2009, 6, 8) == date(2009, 6, 8)
...
</pre></div>
</div>
Note that we don’t patch <a class="reference internal" href="datetime.html#datetime.date" title="datetime.date"><code class="xref py py-class docutils literal notranslate">datetime.date</code></a> globally, we patch <code class="docutils literal notranslate">date</code> in the
module that uses it. See <a class="reference internal" href="unittest.mock.html#where-to-patch">where to patch</a>.
When <code class="docutils literal notranslate">date.today()</code> is called a known date is returned, but calls to the
<code class="docutils literal notranslate">date(...)</code> constructor still return normal dates. Without this you can find
yourself having to calculate an expected result using exactly the same
algorithm as the code under test, which is a classic testing anti-pattern.
Calls to the date constructor are recorded in the <code class="docutils literal notranslate">mock_date</code> attributes
(<code class="docutils literal notranslate">call_count</code> and friends) which may also be useful for your tests.
An alternative way of dealing with mocking dates, or other builtin classes,
is discussed in <a class="reference external" href="https://williambert.online/2011/07/how-to-unit-testing-in-django-with-mocking-and-patching/">this blog entry</a>.
</div>
<div class="section" id="mocking-a-generator-method">
<h3>26.6.3.3. Mocking a Generator Method<a class="headerlink" href="#mocking-a-generator-method" title="Permalink to this headline">¶</a></h3>
A Python generator is a function or method that uses the <a class="reference internal" href="../reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate">yield</code></a> statement
to return a series of values when iterated over <a class="footnote-reference" href="#id3" id="id2">[1]</a>.
A generator method / function is called to return the generator object. It is
the generator object that is then iterated over. The protocol method for
iteration is <a class="reference internal" href="stdtypes.html#container.__iter__" title="container.__iter__"><code class="xref py py-meth docutils literal notranslate">__iter__()</code></a>, so we can
mock this using a <a class="reference internal" href="unittest.mock.html#unittest.mock.MagicMock" title="unittest.mock.MagicMock"><code class="xref py py-class docutils literal notranslate">MagicMock</code></a>.
Here’s an example class with an “iter” method implemented as a generator:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class Foo:
... def iter(self):
... for i in [1, 2, 3]:
... yield i
...
&gt;&gt;&gt; foo = Foo()
&gt;&gt;&gt; list(foo.iter())
[1, 2, 3]
</pre></div>
</div>
How would we mock this class, and in particular its “iter” method?
To configure the values returned from the iteration (implicit in the call to
<a class="reference internal" href="stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate">list</code></a>), we need to configure the object returned by the call to <code class="docutils literal notranslate">foo.iter()</code>.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock_foo = MagicMock()
&gt;&gt;&gt; mock_foo.iter.return_value = iter([1, 2, 3])
&gt;&gt;&gt; list(mock_foo.iter())
[1, 2, 3]
</pre></div>
</div>
<table class="docutils footnote" frame="void" id="id3" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id2">[1]</a></td><td>There are also generator expressions and more <a class="reference external" href="http://www.dabeaz.com/coroutines/index.html">advanced uses</a> of generators, but we aren’t
concerned about them here. A very good introduction to generators and how
powerful they are is: <a class="reference external" href="http://www.dabeaz.com/generators/">Generator Tricks for Systems Programmers</a>.</td></tr>
</tbody>
</table>
</div>
<div class="section" id="applying-the-same-patch-to-every-test-method">
<h3>26.6.3.4. Applying the same patch to every test method<a class="headerlink" href="#applying-the-same-patch-to-every-test-method" title="Permalink to this headline">¶</a></h3>
If you want several patches in place for multiple test methods the obvious way
is to apply the patch decorators to every method. This can feel like unnecessary
repetition. For Python 2.6 or more recent you can use <a class="reference internal" href="unittest.mock.html#unittest.mock.patch" title="unittest.mock.patch"><code class="xref py py-func docutils literal notranslate">patch()</code></a> (in all its
various forms) as a class decorator. This applies the patches to all test
methods on the class. A test method is identified by methods whose names start
with <code class="docutils literal notranslate">test</code>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; @patch(&#39;mymodule.SomeClass&#39;)
... class MyTest(TestCase):
...
... def test_one(self, MockSomeClass):
... self.assertIs(mymodule.SomeClass, MockSomeClass)
...
... def test_two(self, MockSomeClass):
... self.assertIs(mymodule.SomeClass, MockSomeClass)
...
... def not_a_test(self):
... return &#39;something&#39;
...
&gt;&gt;&gt; MyTest(&#39;test_one&#39;).test_one()
&gt;&gt;&gt; MyTest(&#39;test_two&#39;).test_two()
&gt;&gt;&gt; MyTest(&#39;test_two&#39;).not_a_test()
&#39;something&#39;
</pre></div>
</div>
An alternative way of managing patches is to use the <a class="reference internal" href="unittest.mock.html#start-and-stop">patch methods: start and stop</a>.
These allow you to move the patching into your <code class="docutils literal notranslate">setUp</code> and <code class="docutils literal notranslate">tearDown</code> methods.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class MyTest(TestCase):
... def setUp(self):
... self.patcher = patch(&#39;mymodule.foo&#39;)
... self.mock_foo = self.patcher.start()
...
... def test_foo(self):
... self.assertIs(mymodule.foo, self.mock_foo)
...
... def tearDown(self):
... self.patcher.stop()
...
&gt;&gt;&gt; MyTest(&#39;test_foo&#39;).run()
</pre></div>
</div>
If you use this technique you must ensure that the patching is “undone” by
calling <code class="docutils literal notranslate">stop</code>. This can be fiddlier than you might think, because if an
exception is raised in the setUp then tearDown is not called.
<a class="reference internal" href="unittest.html#unittest.TestCase.addCleanup" title="unittest.TestCase.addCleanup"><code class="xref py py-meth docutils literal notranslate">unittest.TestCase.addCleanup()</code></a> makes this easier:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class MyTest(TestCase):
... def setUp(self):
... patcher = patch(&#39;mymodule.foo&#39;)
... self.addCleanup(patcher.stop)
... self.mock_foo = patcher.start()
...
... def test_foo(self):
... self.assertIs(mymodule.foo, self.mock_foo)
...
&gt;&gt;&gt; MyTest(&#39;test_foo&#39;).run()
</pre></div>
</div>
</div>
<div class="section" id="mocking-unbound-methods">
<h3>26.6.3.5. Mocking Unbound Methods<a class="headerlink" href="#mocking-unbound-methods" title="Permalink to this headline">¶</a></h3>
Whilst writing tests today I needed to patch an unbound method (patching the
method on the class rather than on the instance). I needed self to be passed
in as the first argument because I want to make asserts about which objects
were calling this particular method. The issue is that you can’t patch with a
mock for this, because if you replace an unbound method with a mock it doesn’t
become a bound method when fetched from the instance, and so it doesn’t get
self passed in. The workaround is to patch the unbound method with a real
function instead. The <a class="reference internal" href="unittest.mock.html#unittest.mock.patch" title="unittest.mock.patch"><code class="xref py py-func docutils literal notranslate">patch()</code></a> decorator makes it so simple to
patch out methods with a mock that having to create a real function becomes a
nuisance.
If you pass <code class="docutils literal notranslate">autospec=True</code> to patch then it does the patching with a
real function object. This function object has the same signature as the one
it is replacing, but delegates to a mock under the hood. You still get your
mock auto-created in exactly the same way as before. What it means though, is
that if you use it to patch out an unbound method on a class the mocked
function will be turned into a bound method if it is fetched from an instance.
It will have <code class="docutils literal notranslate">self</code> passed in as the first argument, which is exactly what I
wanted:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class Foo:
... def foo(self):
... pass
...
&gt;&gt;&gt; with patch.object(Foo, &#39;foo&#39;, autospec=True) as mock_foo:
... mock_foo.return_value = &#39;foo&#39;
... foo = Foo()
... foo.foo()
...
&#39;foo&#39;
&gt;&gt;&gt; mock_foo.assert_called_once_with(foo)
</pre></div>
</div>
If we don’t use <code class="docutils literal notranslate">autospec=True</code> then the unbound method is patched out
with a Mock instance instead, and isn’t called with <code class="docutils literal notranslate">self</code>.
</div>
<div class="section" id="checking-multiple-calls-with-mock">
<h3>26.6.3.6. Checking multiple calls with mock<a class="headerlink" href="#checking-multiple-calls-with-mock" title="Permalink to this headline">¶</a></h3>
mock has a nice API for making assertions about how your mock objects are used.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = Mock()
&gt;&gt;&gt; mock.foo_bar.return_value = None
&gt;&gt;&gt; mock.foo_bar(&#39;baz&#39;, spam=&#39;eggs&#39;)
&gt;&gt;&gt; mock.foo_bar.assert_called_with(&#39;baz&#39;, spam=&#39;eggs&#39;)
</pre></div>
</div>
If your mock is only being called once you can use the
<code class="xref py py-meth docutils literal notranslate">assert_called_once_with()</code> method that also asserts that the
<code class="xref py py-attr docutils literal notranslate">call_count</code> is one.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock.foo_bar.assert_called_once_with(&#39;baz&#39;, spam=&#39;eggs&#39;)
&gt;&gt;&gt; mock.foo_bar()
&gt;&gt;&gt; mock.foo_bar.assert_called_once_with(&#39;baz&#39;, spam=&#39;eggs&#39;)
Traceback (most recent call last):
 ...
AssertionError: Expected to be called once. Called 2 times.
</pre></div>
</div>
Both <code class="docutils literal notranslate">assert_called_with</code> and <code class="docutils literal notranslate">assert_called_once_with</code> make assertions about
the most recent call. If your mock is going to be called several times, and
you want to make assertions about all those calls you can use
<a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.call_args_list" title="unittest.mock.Mock.call_args_list"><code class="xref py py-attr docutils literal notranslate">call_args_list</code></a>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = Mock(return_value=None)
&gt;&gt;&gt; mock(1, 2, 3)
&gt;&gt;&gt; mock(4, 5, 6)
&gt;&gt;&gt; mock()
&gt;&gt;&gt; mock.call_args_list
[call(1, 2, 3), call(4, 5, 6), call()]
</pre></div>
</div>
The <a class="reference internal" href="unittest.mock.html#unittest.mock.call" title="unittest.mock.call"><code class="xref py py-data docutils literal notranslate">call</code></a> helper makes it easy to make assertions about these calls. You
can build up a list of expected calls and compare it to <code class="docutils literal notranslate">call_args_list</code>. This
looks remarkably similar to the repr of the <code class="docutils literal notranslate">call_args_list</code>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; expected = [call(1, 2, 3), call(4, 5, 6), call()]
&gt;&gt;&gt; mock.call_args_list == expected
True
</pre></div>
</div>
</div>
<div class="section" id="coping-with-mutable-arguments">
<h3>26.6.3.7. Coping with mutable arguments<a class="headerlink" href="#coping-with-mutable-arguments" title="Permalink to this headline">¶</a></h3>
Another situation is rare, but can bite you, is when your mock is called with
mutable arguments. <code class="docutils literal notranslate">call_args</code> and <code class="docutils literal notranslate">call_args_list</code> store references to the
arguments. If the arguments are mutated by the code under test then you can no
longer make assertions about what the values were when the mock was called.
Here’s some example code that shows the problem. Imagine the following functions
defined in ‘mymodule’:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>def frob(val):
 pass

def grob(val):
 &quot;First frob and then clear val&quot;
 frob(val)
 val.clear()
</pre></div>
</div>
When we try to test that <code class="docutils literal notranslate">grob</code> calls <code class="docutils literal notranslate">frob</code> with the correct argument look
what happens:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; with patch(&#39;mymodule.frob&#39;) as mock_frob:
... val = {6}
... mymodule.grob(val)
...
&gt;&gt;&gt; val
set()
&gt;&gt;&gt; mock_frob.assert_called_with({6})
Traceback (most recent call last):
 ...
AssertionError: Expected: (({6},), {})
Called with: ((set(),), {})
</pre></div>
</div>
One possibility would be for mock to copy the arguments you pass in. This
could then cause problems if you do assertions that rely on object identity
for equality.
Here’s one solution that uses the <code class="xref py py-attr docutils literal notranslate">side_effect</code>
functionality. If you provide a <code class="docutils literal notranslate">side_effect</code> function for a mock then
<code class="docutils literal notranslate">side_effect</code> will be called with the same args as the mock. This gives us an
opportunity to copy the arguments and store them for later assertions. In this
example I’m using another mock to store the arguments so that I can use the
mock methods for doing the assertion. Again a helper function sets this up for
me.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from copy import deepcopy
&gt;&gt;&gt; from unittest.mock import Mock, patch, DEFAULT
&gt;&gt;&gt; def copy_call_args(mock):
... new_mock = Mock()
... def side_effect(*args, **kwargs):
... args = deepcopy(args)
... kwargs = deepcopy(kwargs)
... new_mock(*args, **kwargs)
... return DEFAULT
... mock.side_effect = side_effect
... return new_mock
...
&gt;&gt;&gt; with patch(&#39;mymodule.frob&#39;) as mock_frob:
... new_mock = copy_call_args(mock_frob)
... val = {6}
... mymodule.grob(val)
...
&gt;&gt;&gt; new_mock.assert_called_with({6})
&gt;&gt;&gt; new_mock.call_args
call({6})
</pre></div>
</div>
<code class="docutils literal notranslate">copy_call_args</code> is called with the mock that will be called. It returns a new
mock that we do the assertion on. The <code class="docutils literal notranslate">side_effect</code> function makes a copy of
the args and calls our <code class="docutils literal notranslate">new_mock</code> with the copy.
<div class="admonition note">
Note
If your mock is only going to be used once there is an easier way of
checking arguments at the point they are called. You can simply do the
checking inside a <code class="docutils literal notranslate">side_effect</code> function.
<div class="last highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def side_effect(arg):
... assert arg == {6}
...
&gt;&gt;&gt; mock = Mock(side_effect=side_effect)
&gt;&gt;&gt; mock({6})
&gt;&gt;&gt; mock(set())
Traceback (most recent call last):
 ...
AssertionError
</pre></div>
</div>
</div>
An alternative approach is to create a subclass of <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock" title="unittest.mock.Mock"><code class="xref py py-class docutils literal notranslate">Mock</code></a> or
<a class="reference internal" href="unittest.mock.html#unittest.mock.MagicMock" title="unittest.mock.MagicMock"><code class="xref py py-class docutils literal notranslate">MagicMock</code></a> that copies (using <a class="reference internal" href="copy.html#copy.deepcopy" title="copy.deepcopy"><code class="xref py py-func docutils literal notranslate">copy.deepcopy()</code></a>) the arguments.
Here’s an example implementation:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from copy import deepcopy
&gt;&gt;&gt; class CopyingMock(MagicMock):
... def __call__(self, *args, **kwargs):
... args = deepcopy(args)
... kwargs = deepcopy(kwargs)
... return super(CopyingMock, self).__call__(*args, **kwargs)
...
&gt;&gt;&gt; c = CopyingMock(return_value=None)
&gt;&gt;&gt; arg = set()
&gt;&gt;&gt; c(arg)
&gt;&gt;&gt; arg.add(1)
&gt;&gt;&gt; c.assert_called_with(set())
&gt;&gt;&gt; c.assert_called_with(arg)
Traceback (most recent call last):
 ...
AssertionError: Expected call: mock({1})
Actual call: mock(set())
&gt;&gt;&gt; c.foo
&lt;CopyingMock name=&#39;mock.foo&#39; id=&#39;...&#39;&gt;
</pre></div>
</div>
When you subclass <code class="docutils literal notranslate">Mock</code> or <code class="docutils literal notranslate">MagicMock</code> all dynamically created attributes,
and the <code class="docutils literal notranslate">return_value</code> will use your subclass automatically. That means all
children of a <code class="docutils literal notranslate">CopyingMock</code> will also have the type <code class="docutils literal notranslate">CopyingMock</code>.
</div>
<div class="section" id="nesting-patches">
<h3>26.6.3.8. Nesting Patches<a class="headerlink" href="#nesting-patches" title="Permalink to this headline">¶</a></h3>
Using patch as a context manager is nice, but if you do multiple patches you
can end up with nested with statements indenting further and further to the
right:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class MyTest(TestCase):
...
... def test_foo(self):
... with patch(&#39;mymodule.Foo&#39;) as mock_foo:
... with patch(&#39;mymodule.Bar&#39;) as mock_bar:
... with patch(&#39;mymodule.Spam&#39;) as mock_spam:
... assert mymodule.Foo is mock_foo
... assert mymodule.Bar is mock_bar
... assert mymodule.Spam is mock_spam
...
&gt;&gt;&gt; original = mymodule.Foo
&gt;&gt;&gt; MyTest(&#39;test_foo&#39;).test_foo()
&gt;&gt;&gt; assert mymodule.Foo is original
</pre></div>
</div>
With unittest <code class="docutils literal notranslate">cleanup</code> functions and the <a class="reference internal" href="unittest.mock.html#start-and-stop">patch methods: start and stop</a> we can
achieve the same effect without the nested indentation. A simple helper
method, <code class="docutils literal notranslate">create_patch</code>, puts the patch in place and returns the created mock
for us:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class MyTest(TestCase):
...
... def create_patch(self, name):
... patcher = patch(name)
... thing = patcher.start()
... self.addCleanup(patcher.stop)
... return thing
...
... def test_foo(self):
... mock_foo = self.create_patch(&#39;mymodule.Foo&#39;)
... mock_bar = self.create_patch(&#39;mymodule.Bar&#39;)
... mock_spam = self.create_patch(&#39;mymodule.Spam&#39;)
...
... assert mymodule.Foo is mock_foo
... assert mymodule.Bar is mock_bar
... assert mymodule.Spam is mock_spam
...
&gt;&gt;&gt; original = mymodule.Foo
&gt;&gt;&gt; MyTest(&#39;test_foo&#39;).run()
&gt;&gt;&gt; assert mymodule.Foo is original
</pre></div>
</div>
</div>
<div class="section" id="mocking-a-dictionary-with-magicmock">
<h3>26.6.3.9. Mocking a dictionary with MagicMock<a class="headerlink" href="#mocking-a-dictionary-with-magicmock" title="Permalink to this headline">¶</a></h3>
You may want to mock a dictionary, or other container object, recording all
access to it whilst having it still behave like a dictionary.
We can do this with <a class="reference internal" href="unittest.mock.html#unittest.mock.MagicMock" title="unittest.mock.MagicMock"><code class="xref py py-class docutils literal notranslate">MagicMock</code></a>, which will behave like a dictionary,
and using <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.side_effect" title="unittest.mock.Mock.side_effect"><code class="xref py py-data docutils literal notranslate">side_effect</code></a> to delegate dictionary access to a real
underlying dictionary that is under our control.
When the <a class="reference internal" href="../reference/datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate">__getitem__()</code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__setitem__" title="object.__setitem__"><code class="xref py py-meth docutils literal notranslate">__setitem__()</code></a> methods of our <code class="docutils literal notranslate">MagicMock</code> are called
(normal dictionary access) then <code class="docutils literal notranslate">side_effect</code> is called with the key (and in
the case of <code class="docutils literal notranslate">__setitem__</code> the value too). We can also control what is returned.
After the <code class="docutils literal notranslate">MagicMock</code> has been used we can use attributes like
<a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.call_args_list" title="unittest.mock.Mock.call_args_list"><code class="xref py py-data docutils literal notranslate">call_args_list</code></a> to assert about how the dictionary was used:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; my_dict = {&#39;a&#39;: 1, &#39;b&#39;: 2, &#39;c&#39;: 3}
&gt;&gt;&gt; def getitem(name):
... return my_dict[name]
...
&gt;&gt;&gt; def setitem(name, val):
... my_dict[name] = val
...
&gt;&gt;&gt; mock = MagicMock()
&gt;&gt;&gt; mock.__getitem__.side_effect = getitem
&gt;&gt;&gt; mock.__setitem__.side_effect = setitem
</pre></div>
</div>
<div class="admonition note">
Note
An alternative to using <code class="docutils literal notranslate">MagicMock</code> is to use <code class="docutils literal notranslate">Mock</code> and only provide
the magic methods you specifically want:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = Mock()
&gt;&gt;&gt; mock.__getitem__ = Mock(side_effect=getitem)
&gt;&gt;&gt; mock.__setitem__ = Mock(side_effect=setitem)
</pre></div>
</div>
A third option is to use <code class="docutils literal notranslate">MagicMock</code> but passing in <code class="docutils literal notranslate">dict</code> as the spec
(or spec_set) argument so that the <code class="docutils literal notranslate">MagicMock</code> created only has
dictionary magic methods available:
<div class="last highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = MagicMock(spec_set=dict)
&gt;&gt;&gt; mock.__getitem__.side_effect = getitem
&gt;&gt;&gt; mock.__setitem__.side_effect = setitem
</pre></div>
</div>
</div>
With these side effect functions in place, the <code class="docutils literal notranslate">mock</code> will behave like a normal
dictionary but recording the access. It even raises a <a class="reference internal" href="exceptions.html#KeyError" title="KeyError"><code class="xref py py-exc docutils literal notranslate">KeyError</code></a> if you try
to access a key that doesn’t exist.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock[&#39;a&#39;]
1
&gt;&gt;&gt; mock[&#39;c&#39;]
3
&gt;&gt;&gt; mock[&#39;d&#39;]
Traceback (most recent call last):
 ...
KeyError: &#39;d&#39;
&gt;&gt;&gt; mock[&#39;b&#39;] = &#39;fish&#39;
&gt;&gt;&gt; mock[&#39;d&#39;] = &#39;eggs&#39;
&gt;&gt;&gt; mock[&#39;b&#39;]
&#39;fish&#39;
&gt;&gt;&gt; mock[&#39;d&#39;]
&#39;eggs&#39;
</pre></div>
</div>
After it has been used you can make assertions about the access using the normal
mock methods and attributes:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock.__getitem__.call_args_list
[call(&#39;a&#39;), call(&#39;c&#39;), call(&#39;d&#39;), call(&#39;b&#39;), call(&#39;d&#39;)]
&gt;&gt;&gt; mock.__setitem__.call_args_list
[call(&#39;b&#39;, &#39;fish&#39;), call(&#39;d&#39;, &#39;eggs&#39;)]
&gt;&gt;&gt; my_dict
{&#39;a&#39;: 1, &#39;c&#39;: 3, &#39;b&#39;: &#39;fish&#39;, &#39;d&#39;: &#39;eggs&#39;}
</pre></div>
</div>
</div>
<div class="section" id="mock-subclasses-and-their-attributes">
<h3>26.6.3.10. Mock subclasses and their attributes<a class="headerlink" href="#mock-subclasses-and-their-attributes" title="Permalink to this headline">¶</a></h3>
There are various reasons why you might want to subclass <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock" title="unittest.mock.Mock"><code class="xref py py-class docutils literal notranslate">Mock</code></a>. One
reason might be to add helper methods. Here’s a silly example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class MyMock(MagicMock):
... def has_been_called(self):
... return self.called
...
&gt;&gt;&gt; mymock = MyMock(return_value=None)
&gt;&gt;&gt; mymock
&lt;MyMock id=&#39;...&#39;&gt;
&gt;&gt;&gt; mymock.has_been_called()
False
&gt;&gt;&gt; mymock()
&gt;&gt;&gt; mymock.has_been_called()
True
</pre></div>
</div>
The standard behaviour for <code class="docutils literal notranslate">Mock</code> instances is that attributes and the return
value mocks are of the same type as the mock they are accessed on. This ensures
that <code class="docutils literal notranslate">Mock</code> attributes are <code class="docutils literal notranslate">Mocks</code> and <code class="docutils literal notranslate">MagicMock</code> attributes are <code class="docutils literal notranslate">MagicMocks</code>
<a class="footnote-reference" href="#id5" id="id4">[2]</a>. So if you’re subclassing to add helper methods then they’ll also be
available on the attributes and return value mock of instances of your
subclass.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mymock.foo
&lt;MyMock name=&#39;mock.foo&#39; id=&#39;...&#39;&gt;
&gt;&gt;&gt; mymock.foo.has_been_called()
False
&gt;&gt;&gt; mymock.foo()
&lt;MyMock name=&#39;mock.foo()&#39; id=&#39;...&#39;&gt;
&gt;&gt;&gt; mymock.foo.has_been_called()
True
</pre></div>
</div>
Sometimes this is inconvenient. For example, <a class="reference external" href="https://code.google.com/p/mock/issues/detail?id=105">one user</a> is subclassing mock to
created a <a class="reference external" href="https://twistedmatrix.com/documents/11.0.0/api/twisted.python.components.html">Twisted adaptor</a>.
Having this applied to attributes too actually causes errors.
<code class="docutils literal notranslate">Mock</code> (in all its flavours) uses a method called <code class="docutils literal notranslate">_get_child_mock</code> to create
these “sub-mocks” for attributes and return values. You can prevent your
subclass being used for attributes by overriding this method. The signature is
that it takes arbitrary keyword arguments (<code class="docutils literal notranslate">**kwargs</code>) which are then passed
onto the mock constructor:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class Subclass(MagicMock):
... def _get_child_mock(self, **kwargs):
... return MagicMock(**kwargs)
...
&gt;&gt;&gt; mymock = Subclass()
&gt;&gt;&gt; mymock.foo
&lt;MagicMock name=&#39;mock.foo&#39; id=&#39;...&#39;&gt;
&gt;&gt;&gt; assert isinstance(mymock, Subclass)
&gt;&gt;&gt; assert not isinstance(mymock.foo, Subclass)
&gt;&gt;&gt; assert not isinstance(mymock(), Subclass)
</pre></div>
</div>
<table class="docutils footnote" frame="void" id="id5" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id4">[2]</a></td><td>An exception to this rule are the non-callable mocks. Attributes use the
callable variant because otherwise non-callable mocks couldn’t have callable
methods.</td></tr>
</tbody>
</table>
</div>
<div class="section" id="mocking-imports-with-patch-dict">
<h3>26.6.3.11. Mocking imports with patch.dict<a class="headerlink" href="#mocking-imports-with-patch-dict" title="Permalink to this headline">¶</a></h3>
One situation where mocking can be hard is where you have a local import inside
a function. These are harder to mock because they aren’t using an object from
the module namespace that we can patch out.
Generally local imports are to be avoided. They are sometimes done to prevent
circular dependencies, for which there is usually a much better way to solve
the problem (refactor the code) or to prevent “up front costs” by delaying the
import. This can also be solved in better ways than an unconditional local
import (store the module as a class or module attribute and only do the import
on first use).
That aside there is a way to use <code class="docutils literal notranslate">mock</code> to affect the results of an import.
Importing fetches an object from the <a class="reference internal" href="sys.html#sys.modules" title="sys.modules"><code class="xref py py-data docutils literal notranslate">sys.modules</code></a> dictionary. Note that it
fetches an object, which need not be a module. Importing a module for the
first time results in a module object being put in <cite>sys.modules</cite>, so usually
when you import something you get a module back. This need not be the case
however.
This means you can use <a class="reference internal" href="unittest.mock.html#unittest.mock.patch.dict" title="unittest.mock.patch.dict"><code class="xref py py-func docutils literal notranslate">patch.dict()</code></a> to temporarily put a mock in place
in <a class="reference internal" href="sys.html#sys.modules" title="sys.modules"><code class="xref py py-data docutils literal notranslate">sys.modules</code></a>. Any imports whilst this patch is active will fetch the mock.
When the patch is complete (the decorated function exits, the with statement
body is complete or <code class="docutils literal notranslate">patcher.stop()</code> is called) then whatever was there
previously will be restored safely.
Here’s an example that mocks out the ‘fooble’ module.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = Mock()
&gt;&gt;&gt; with patch.dict(&#39;sys.modules&#39;, {&#39;fooble&#39;: mock}):
... import fooble
... fooble.blob()
...
&lt;Mock name=&#39;mock.blob()&#39; id=&#39;...&#39;&gt;
&gt;&gt;&gt; assert &#39;fooble&#39; not in sys.modules
&gt;&gt;&gt; mock.blob.assert_called_once_with()
</pre></div>
</div>
As you can see the <code class="docutils literal notranslate">import fooble</code> succeeds, but on exit there is no ‘fooble’
left in <a class="reference internal" href="sys.html#sys.modules" title="sys.modules"><code class="xref py py-data docutils literal notranslate">sys.modules</code></a>.
This also works for the <code class="docutils literal notranslate">from module import name</code> form:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = Mock()
&gt;&gt;&gt; with patch.dict(&#39;sys.modules&#39;, {&#39;fooble&#39;: mock}):
... from fooble import blob
... blob.blip()
...
&lt;Mock name=&#39;mock.blob.blip()&#39; id=&#39;...&#39;&gt;
&gt;&gt;&gt; mock.blob.blip.assert_called_once_with()
</pre></div>
</div>
With slightly more work you can also mock package imports:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock = Mock()
&gt;&gt;&gt; modules = {&#39;package&#39;: mock, &#39;package.module&#39;: mock.module}
&gt;&gt;&gt; with patch.dict(&#39;sys.modules&#39;, modules):
... from package.module import fooble
... fooble()
...
&lt;Mock name=&#39;mock.module.fooble()&#39; id=&#39;...&#39;&gt;
&gt;&gt;&gt; mock.module.fooble.assert_called_once_with()
</pre></div>
</div>
</div>
<div class="section" id="tracking-order-of-calls-and-less-verbose-call-assertions">
<h3>26.6.3.12. Tracking order of calls and less verbose call assertions<a class="headerlink" href="#tracking-order-of-calls-and-less-verbose-call-assertions" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock" title="unittest.mock.Mock"><code class="xref py py-class docutils literal notranslate">Mock</code></a> class allows you to track the order of method calls on
your mock objects through the <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.method_calls" title="unittest.mock.Mock.method_calls"><code class="xref py py-attr docutils literal notranslate">method_calls</code></a> attribute. This
doesn’t allow you to track the order of calls between separate mock objects,
however we can use <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.mock_calls" title="unittest.mock.Mock.mock_calls"><code class="xref py py-attr docutils literal notranslate">mock_calls</code></a> to achieve the same effect.
Because mocks track calls to child mocks in <code class="docutils literal notranslate">mock_calls</code>, and accessing an
arbitrary attribute of a mock creates a child mock, we can create our separate
mocks from a parent one. Calls to those child mock will then all be recorded,
in order, in the <code class="docutils literal notranslate">mock_calls</code> of the parent:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; manager = Mock()
&gt;&gt;&gt; mock_foo = manager.foo
&gt;&gt;&gt; mock_bar = manager.bar
</pre></div>
</div>
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; mock_foo.something()
&lt;Mock name=&#39;mock.foo.something()&#39; id=&#39;...&#39;&gt;
&gt;&gt;&gt; mock_bar.other.thing()
&lt;Mock name=&#39;mock.bar.other.thing()&#39; id=&#39;...&#39;&gt;
</pre></div>
</div>
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; manager.mock_calls
[call.foo.something(), call.bar.other.thing()]
</pre></div>
</div>
We can then assert about the calls, including the order, by comparing with
the <code class="docutils literal notranslate">mock_calls</code> attribute on the manager mock:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; expected_calls = [call.foo.something(), call.bar.other.thing()]
&gt;&gt;&gt; manager.mock_calls == expected_calls
True
</pre></div>
</div>
If <code class="docutils literal notranslate">patch</code> is creating, and putting in place, your mocks then you can attach
them to a manager mock using the <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.attach_mock" title="unittest.mock.Mock.attach_mock"><code class="xref py py-meth docutils literal notranslate">attach_mock()</code></a> method. After
attaching calls will be recorded in <code class="docutils literal notranslate">mock_calls</code> of the manager.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; manager = MagicMock()
&gt;&gt;&gt; with patch(&#39;mymodule.Class1&#39;) as MockClass1:
... with patch(&#39;mymodule.Class2&#39;) as MockClass2:
... manager.attach_mock(MockClass1, &#39;MockClass1&#39;)
... manager.attach_mock(MockClass2, &#39;MockClass2&#39;)
... MockClass1().foo()
... MockClass2().bar()
...
&lt;MagicMock name=&#39;mock.MockClass1().foo()&#39; id=&#39;...&#39;&gt;
&lt;MagicMock name=&#39;mock.MockClass2().bar()&#39; id=&#39;...&#39;&gt;
&gt;&gt;&gt; manager.mock_calls
[call.MockClass1(),
 call.MockClass1().foo(),
 call.MockClass2(),
 call.MockClass2().bar()]
</pre></div>
</div>
If many calls have been made, but you’re only interested in a particular
sequence of them then an alternative is to use the
<a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.assert_has_calls" title="unittest.mock.Mock.assert_has_calls"><code class="xref py py-meth docutils literal notranslate">assert_has_calls()</code></a> method. This takes a list of calls (constructed
with the <a class="reference internal" href="unittest.mock.html#unittest.mock.call" title="unittest.mock.call"><code class="xref py py-data docutils literal notranslate">call</code></a> object). If that sequence of calls are in
<a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.mock_calls" title="unittest.mock.Mock.mock_calls"><code class="xref py py-attr docutils literal notranslate">mock_calls</code></a> then the assert succeeds.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; m = MagicMock()
&gt;&gt;&gt; m().foo().bar().baz()
&lt;MagicMock name=&#39;mock().foo().bar().baz()&#39; id=&#39;...&#39;&gt;
&gt;&gt;&gt; m.one().two().three()
&lt;MagicMock name=&#39;mock.one().two().three()&#39; id=&#39;...&#39;&gt;
&gt;&gt;&gt; calls = call.one().two().three().call_list()
&gt;&gt;&gt; m.assert_has_calls(calls)
</pre></div>
</div>
Even though the chained call <code class="docutils literal notranslate">m.one().two().three()</code> aren’t the only calls that
have been made to the mock, the assert still succeeds.
Sometimes a mock may have several calls made to it, and you are only interested
in asserting about some of those calls. You may not even care about the
order. In this case you can pass <code class="docutils literal notranslate">any_order=True</code> to <code class="docutils literal notranslate">assert_has_calls</code>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; m = MagicMock()
&gt;&gt;&gt; m(1), m.two(2, 3), m.seven(7), m.fifty(&#39;50&#39;)
(...)
&gt;&gt;&gt; calls = [call.fifty(&#39;50&#39;), call(1), call.seven(7)]
&gt;&gt;&gt; m.assert_has_calls(calls, any_order=True)
</pre></div>
</div>
</div>
<div class="section" id="more-complex-argument-matching">
<h3>26.6.3.13. More complex argument matching<a class="headerlink" href="#more-complex-argument-matching" title="Permalink to this headline">¶</a></h3>
Using the same basic concept as <a class="reference internal" href="unittest.mock.html#unittest.mock.ANY" title="unittest.mock.ANY"><code class="xref py py-data docutils literal notranslate">ANY</code></a> we can implement matchers to do more
complex assertions on objects used as arguments to mocks.
Suppose we expect some object to be passed to a mock that by default
compares equal based on object identity (which is the Python default for user
defined classes). To use <a class="reference internal" href="unittest.mock.html#unittest.mock.Mock.assert_called_with" title="unittest.mock.Mock.assert_called_with"><code class="xref py py-meth docutils literal notranslate">assert_called_with()</code></a> we would need to pass
in the exact same object. If we are only interested in some of the attributes
of this object then we can create a matcher that will check these attributes
for us.
You can see in this example how a ‘standard’ call to <code class="docutils literal notranslate">assert_called_with</code> isn’t
sufficient:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class Foo:
... def __init__(self, a, b):
... self.a, self.b = a, b
...
&gt;&gt;&gt; mock = Mock(return_value=None)
&gt;&gt;&gt; mock(Foo(1, 2))
&gt;&gt;&gt; mock.assert_called_with(Foo(1, 2))
Traceback (most recent call last):
 ...
AssertionError: Expected: call(&lt;__main__.Foo object at 0x...&gt;)
Actual call: call(&lt;__main__.Foo object at 0x...&gt;)
</pre></div>
</div>
A comparison function for our <code class="docutils literal notranslate">Foo</code> class might look something like this:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def compare(self, other):
... if not type(self) == type(other):
... return False
... if self.a != other.a:
... return False
... if self.b != other.b:
... return False
... return True
...
</pre></div>
</div>
And a matcher object that can use comparison functions like this for its
equality operation would look something like this:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class Matcher:
... def __init__(self, compare, some_obj):
... self.compare = compare
... self.some_obj = some_obj
... def __eq__(self, other):
... return self.compare(self.some_obj, other)
...
</pre></div>
</div>
Putting all this together:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; match_foo = Matcher(compare, Foo(1, 2))
&gt;&gt;&gt; mock.assert_called_with(match_foo)
</pre></div>
</div>
The <code class="docutils literal notranslate">Matcher</code> is instantiated with our compare function and the <code class="docutils literal notranslate">Foo</code> object
we want to compare against. In <code class="docutils literal notranslate">assert_called_with</code> the <code class="docutils literal notranslate">Matcher</code> equality
method will be called, which compares the object the mock was called with
against the one we created our matcher with. If they match then
<code class="docutils literal notranslate">assert_called_with</code> passes, and if they don’t an <a class="reference internal" href="exceptions.html#AssertionError" title="AssertionError"><code class="xref py py-exc docutils literal notranslate">AssertionError</code></a> is raised:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; match_wrong = Matcher(compare, Foo(3, 4))
&gt;&gt;&gt; mock.assert_called_with(match_wrong)
Traceback (most recent call last):
 ...
AssertionError: Expected: ((&lt;Matcher object at 0x...&gt;,), {})
Called with: ((&lt;Foo object at 0x...&gt;,), {})
</pre></div>
</div>
With a bit of tweaking you could have the comparison function raise the
<a class="reference internal" href="exceptions.html#AssertionError" title="AssertionError"><code class="xref py py-exc docutils literal notranslate">AssertionError</code></a> directly and provide a more useful failure message.
As of version 1.5, the Python testing library <a class="reference external" href="https://pyhamcrest.readthedocs.org/">PyHamcrest</a> provides similar functionality,
that may be useful here, in the form of its equality matcher
(<a class="reference external" href="https://pyhamcrest.readthedocs.org/en/release-1.8/integration/#module-hamcrest.library.integration.match_equality">hamcrest.library.integration.match_equality</a>).
</div>
</div>
</div>

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 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">26.6. <code class="docutils literal notranslate">unittest.mock</code> — getting started</a><ul>
<li><a class="reference internal" href="#using-mock">26.6.1. Using Mock</a><ul>
<li><a class="reference internal" href="#mock-patching-methods">26.6.1.1. Mock Patching Methods</a></li>
<li><a class="reference internal" href="#mock-for-method-calls-on-an-object">26.6.1.2. Mock for Method Calls on an Object</a></li>
<li><a class="reference internal" href="#mocking-classes">26.6.1.3. Mocking Classes</a></li>
<li><a class="reference internal" href="#naming-your-mocks">26.6.1.4. Naming your mocks</a></li>
<li><a class="reference internal" href="#tracking-all-calls">26.6.1.5. Tracking all Calls</a></li>
<li><a class="reference internal" href="#setting-return-values-and-attributes">26.6.1.6. Setting Return Values and Attributes</a></li>
<li><a class="reference internal" href="#raising-exceptions-with-mocks">26.6.1.7. Raising exceptions with mocks</a></li>
<li><a class="reference internal" href="#side-effect-functions-and-iterables">26.6.1.8. Side effect functions and iterables</a></li>
<li><a class="reference internal" href="#creating-a-mock-from-an-existing-object">26.6.1.9. Creating a Mock from an Existing Object</a></li>
</ul>
</li>
<li><a class="reference internal" href="#patch-decorators">26.6.2. Patch Decorators</a></li>
<li><a class="reference internal" href="#further-examples">26.6.3. Further Examples</a><ul>
<li><a class="reference internal" href="#mocking-chained-calls">26.6.3.1. Mocking chained calls</a></li>
<li><a class="reference internal" href="#partial-mocking">26.6.3.2. Partial mocking</a></li>
<li><a class="reference internal" href="#mocking-a-generator-method">26.6.3.3. Mocking a Generator Method</a></li>
<li><a class="reference internal" href="#applying-the-same-patch-to-every-test-method">26.6.3.4. Applying the same patch to every test method</a></li>
<li><a class="reference internal" href="#mocking-unbound-methods">26.6.3.5. Mocking Unbound Methods</a></li>
<li><a class="reference internal" href="#checking-multiple-calls-with-mock">26.6.3.6. Checking multiple calls with mock</a></li>
<li><a class="reference internal" href="#coping-with-mutable-arguments">26.6.3.7. Coping with mutable arguments</a></li>
<li><a class="reference internal" href="#nesting-patches">26.6.3.8. Nesting Patches</a></li>
<li><a class="reference internal" href="#mocking-a-dictionary-with-magicmock">26.6.3.9. Mocking a dictionary with MagicMock</a></li>
<li><a class="reference internal" href="#mock-subclasses-and-their-attributes">26.6.3.10. Mock subclasses and their attributes</a></li>
<li><a class="reference internal" href="#mocking-imports-with-patch-dict">26.6.3.11. Mocking imports with patch.dict</a></li>
<li><a class="reference internal" href="#tracking-order-of-calls-and-less-verbose-call-assertions">26.6.3.12. Tracking order of calls and less verbose call assertions</a></li>
<li><a class="reference internal" href="#more-complex-argument-matching">26.6.3.13. More complex argument matching</a></li>
</ul>
</li>
</ul>
</li>
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