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<div class="section" id="patch-decorators">
<h1>Patch Decorators<a class="headerlink" href="#patch-decorators" title="Permalink to this headline">¶</a></h1>
<p>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.</p>
<div class="section" id="patch">
<h2>patch<a class="headerlink" href="#patch" title="Permalink to this headline">¶</a></h2>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last"><cite>patch</cite> is straightforward to use. The key is to do the patching in the
right namespace. See the section <a class="reference internal" href="#id1">where to patch</a>.</p>
</div>
<dl class="function">
<dt id="mock.patch">
<tt class="descname">patch</tt><big>(</big><em>target</em>, <em>new=DEFAULT</em>, <em>spec=None</em>, <em>create=False</em>, <em>spec_set=None</em>, <em>autospec=None</em>, <em>new_callable=None</em>, <em>**kwargs</em><big>)</big><a class="headerlink" href="#mock.patch" title="Permalink to this definition">¶</a></dt>
<dd><p><cite>patch</cite> acts as a function decorator, class decorator or a context
manager. Inside the body of the function or with statement, the <cite>target</cite>
is patched with a <cite>new</cite> object. When the function/with statement exits
the patch is undone.</p>
<p>If <cite>new</cite> is omitted, then the target is replaced with a
<a class="reference internal" href="magicmock.html#mock.MagicMock" title="mock.MagicMock"><tt class="xref py py-class docutils literal"><span class="pre">MagicMock</span></tt></a>. If <cite>patch</cite> is used as a decorator and <cite>new</cite> is
omitted, the created mock is passed in as an extra argument to the
decorated function. If <cite>patch</cite> is used as a context manager the created
mock is returned by the context manager.</p>
<p><cite>target</cite> should be a string in the form <cite>‘package.module.ClassName’</cite>. The
<cite>target</cite> is imported and the specified object replaced with the <cite>new</cite>
object, so the <cite>target</cite> must be importable from the environment you are
calling <cite>patch</cite> from. The target is imported when the decorated function
is executed, not at decoration time.</p>
<p>The <cite>spec</cite> and <cite>spec_set</cite> keyword arguments are passed to the <cite>MagicMock</cite>
if patch is creating one for you.</p>
<p>In addition you can pass <cite>spec=True</cite> or <cite>spec_set=True</cite>, which causes
patch to pass in the object being mocked as the spec/spec_set object.</p>
<p><cite>new_callable</cite> allows you to specify a different class, or callable object,
that will be called to create the <cite>new</cite> object. By default <cite>MagicMock</cite> is
used.</p>
<p>A more powerful form of <cite>spec</cite> is <cite>autospec</cite>. If you set <cite>autospec=True</cite>
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 <cite>TypeError</cite> 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 <a class="reference internal" href="helpers.html#mock.create_autospec" title="mock.create_autospec"><tt class="xref py py-func docutils literal"><span class="pre">create_autospec()</span></tt></a> function and
<a class="reference internal" href="helpers.html#auto-speccing"><em>Autospeccing</em></a>.</p>
<p>Instead of <cite>autospec=True</cite> you can pass <cite>autospec=some_object</cite> to use an
arbitrary object as the spec instead of the one being replaced.</p>
<p>By default <cite>patch</cite> will fail to replace attributes that don’t exist. If
you pass in <cite>create=True</cite>, 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!</p>
<p>Patch can be used as a <cite>TestCase</cite> 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. <cite>patch</cite> finds
tests by looking for method names that start with <cite>patch.TEST_PREFIX</cite>.
By default this is <cite>test</cite>, which matches the way <cite>unittest</cite> finds tests.
You can specify an alternative prefix by setting <cite>patch.TEST_PREFIX</cite>.</p>
<p>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 <cite>patch</cite> is creating a mock object for you.</p>
<p><cite>patch</cite> takes arbitrary keyword arguments. These will be passed to
the <cite>Mock</cite> (or <cite>new_callable</cite>) on construction.</p>
<p><cite>patch.dict(...)</cite>, <cite>patch.multiple(...)</cite> and <cite>patch.object(...)</cite> are
available for alternate use-cases.</p>
</dd></dl>
<p><cite>patch</cite> as function decorator, creating the mock for you and passing it into
the decorated function:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="nd">@patch</span><span class="p">(</span><span class="s">'__main__.SomeClass'</span><span class="p">)</span>
<span class="gp">... </span><span class="k">def</span> <span class="nf">function</span><span class="p">(</span><span class="n">normal_argument</span><span class="p">,</span> <span class="n">mock_class</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">print</span> <span class="n">mock_class</span> <span class="ow">is</span> <span class="n">SomeClass</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">function</span><span class="p">(</span><span class="bp">None</span><span class="p">)</span>
<span class="go">True</span>
</pre></div>
</div>
<p>Patching a class replaces the class with a <cite>MagicMock</cite> <em>instance</em>. If the
class is instantiated in the code under test then it will be the
<a class="reference internal" href="mock.html#mock.Mock.return_value" title="mock.Mock.return_value"><tt class="xref py py-attr docutils literal"><span class="pre">return_value</span></tt></a> of the mock that will be used.</p>
<p>If the class is instantiated multiple times you could use
<a class="reference internal" href="mock.html#mock.Mock.side_effect" title="mock.Mock.side_effect"><tt class="xref py py-attr docutils literal"><span class="pre">side_effect</span></tt></a> to return a new mock each time. Alternatively you
can set the <cite>return_value</cite> to be anything you want.</p>
<p>To configure return values on methods of <em>instances</em> on the patched class
you must do this on the <cite>return_value</cite>. For example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">class</span> <span class="nc">Class</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">method</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">pass</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="k">with</span> <span class="n">patch</span><span class="p">(</span><span class="s">'__main__.Class'</span><span class="p">)</span> <span class="k">as</span> <span class="n">MockClass</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">instance</span> <span class="o">=</span> <span class="n">MockClass</span><span class="o">.</span><span class="n">return_value</span>
<span class="gp">... </span> <span class="n">instance</span><span class="o">.</span><span class="n">method</span><span class="o">.</span><span class="n">return_value</span> <span class="o">=</span> <span class="s">'foo'</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">Class</span><span class="p">()</span> <span class="ow">is</span> <span class="n">instance</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">Class</span><span class="p">()</span><span class="o">.</span><span class="n">method</span><span class="p">()</span> <span class="o">==</span> <span class="s">'foo'</span>
<span class="gp">...</span>
</pre></div>
</div>
<p>If you use <cite>spec</cite> or <cite>spec_set</cite> and <cite>patch</cite> is replacing a <em>class</em>, then the
return value of the created mock will have the same spec.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">Original</span> <span class="o">=</span> <span class="n">Class</span>
<span class="gp">>>> </span><span class="n">patcher</span> <span class="o">=</span> <span class="n">patch</span><span class="p">(</span><span class="s">'__main__.Class'</span><span class="p">,</span> <span class="n">spec</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">MockClass</span> <span class="o">=</span> <span class="n">patcher</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">instance</span> <span class="o">=</span> <span class="n">MockClass</span><span class="p">()</span>
<span class="gp">>>> </span><span class="k">assert</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">instance</span><span class="p">,</span> <span class="n">Original</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">patcher</span><span class="o">.</span><span class="n">stop</span><span class="p">()</span>
</pre></div>
</div>
<p>The <cite>new_callable</cite> argument is useful where you want to use an alternative
class to the default <a class="reference internal" href="magicmock.html#mock.MagicMock" title="mock.MagicMock"><tt class="xref py py-class docutils literal"><span class="pre">MagicMock</span></tt></a> for the created mock. For example, if
you wanted a <a class="reference internal" href="mock.html#mock.NonCallableMock" title="mock.NonCallableMock"><tt class="xref py py-class docutils literal"><span class="pre">NonCallableMock</span></tt></a> to be used:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">thing</span> <span class="o">=</span> <span class="nb">object</span><span class="p">()</span>
<span class="gp">>>> </span><span class="k">with</span> <span class="n">patch</span><span class="p">(</span><span class="s">'__main__.thing'</span><span class="p">,</span> <span class="n">new_callable</span><span class="o">=</span><span class="n">NonCallableMock</span><span class="p">)</span> <span class="k">as</span> <span class="n">mock_thing</span><span class="p">:</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">thing</span> <span class="ow">is</span> <span class="n">mock_thing</span>
<span class="gp">... </span> <span class="n">thing</span><span class="p">()</span>
<span class="gp">...</span>
<span class="gt">Traceback (most recent call last):</span>
<span class="c">...</span>
<span class="gr">TypeError</span>: <span class="n">'NonCallableMock' object is not callable</span>
</pre></div>
</div>
<p>Another use case might be to replace an object with a <cite>StringIO</cite> instance:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">StringIO</span> <span class="kn">import</span> <span class="n">StringIO</span>
<span class="gp">>>> </span><span class="k">def</span> <span class="nf">foo</span><span class="p">():</span>
<span class="gp">... </span> <span class="k">print</span> <span class="s">'Something'</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="nd">@patch</span><span class="p">(</span><span class="s">'sys.stdout'</span><span class="p">,</span> <span class="n">new_callable</span><span class="o">=</span><span class="n">StringIO</span><span class="p">)</span>
<span class="gp">... </span><span class="k">def</span> <span class="nf">test</span><span class="p">(</span><span class="n">mock_stdout</span><span class="p">):</span>
<span class="gp">... </span> <span class="n">foo</span><span class="p">()</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">mock_stdout</span><span class="o">.</span><span class="n">getvalue</span><span class="p">()</span> <span class="o">==</span> <span class="s">'Something</span><span class="se">\n</span><span class="s">'</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">test</span><span class="p">()</span>
</pre></div>
</div>
<p>When <cite>patch</cite> 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:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">patcher</span> <span class="o">=</span> <span class="n">patch</span><span class="p">(</span><span class="s">'__main__.thing'</span><span class="p">,</span> <span class="n">first</span><span class="o">=</span><span class="s">'one'</span><span class="p">,</span> <span class="n">second</span><span class="o">=</span><span class="s">'two'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">mock_thing</span> <span class="o">=</span> <span class="n">patcher</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">mock_thing</span><span class="o">.</span><span class="n">first</span>
<span class="go">'one'</span>
<span class="gp">>>> </span><span class="n">mock_thing</span><span class="o">.</span><span class="n">second</span>
<span class="go">'two'</span>
</pre></div>
</div>
<p>As well as attributes on the created mock attributes, like the
<a class="reference internal" href="mock.html#mock.Mock.return_value" title="mock.Mock.return_value"><tt class="xref py py-attr docutils literal"><span class="pre">return_value</span></tt></a> and <a class="reference internal" href="mock.html#mock.Mock.side_effect" title="mock.Mock.side_effect"><tt class="xref py py-attr docutils literal"><span class="pre">side_effect</span></tt></a>, 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 <cite>patch</cite> call using <cite>**</cite>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">config</span> <span class="o">=</span> <span class="p">{</span><span class="s">'method.return_value'</span><span class="p">:</span> <span class="mi">3</span><span class="p">,</span> <span class="s">'other.side_effect'</span><span class="p">:</span> <span class="ne">KeyError</span><span class="p">}</span>
<span class="gp">>>> </span><span class="n">patcher</span> <span class="o">=</span> <span class="n">patch</span><span class="p">(</span><span class="s">'__main__.thing'</span><span class="p">,</span> <span class="o">**</span><span class="n">config</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">mock_thing</span> <span class="o">=</span> <span class="n">patcher</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">mock_thing</span><span class="o">.</span><span class="n">method</span><span class="p">()</span>
<span class="go">3</span>
<span class="gp">>>> </span><span class="n">mock_thing</span><span class="o">.</span><span class="n">other</span><span class="p">()</span>
<span class="gt">Traceback (most recent call last):</span>
<span class="c">...</span>
<span class="gr">KeyError</span>
</pre></div>
</div>
</div>
<div class="section" id="patch-object">
<h2>patch.object<a class="headerlink" href="#patch-object" title="Permalink to this headline">¶</a></h2>
<dl class="function">
<dt id="mock.patch.object">
<tt class="descclassname">patch.</tt><tt class="descname">object</tt><big>(</big><em>target</em>, <em>attribute</em>, <em>new=DEFAULT</em>, <em>spec=None</em>, <em>create=False</em>, <em>spec_set=None</em>, <em>autospec=None</em>, <em>new_callable=None</em>, <em>**kwargs</em><big>)</big><a class="headerlink" href="#mock.patch.object" title="Permalink to this definition">¶</a></dt>
<dd><p>patch the named member (<cite>attribute</cite>) on an object (<cite>target</cite>) with a mock
object.</p>
<p><cite>patch.object</cite> can be used as a decorator, class decorator or a context
manager. Arguments <cite>new</cite>, <cite>spec</cite>, <cite>create</cite>, <cite>spec_set</cite>, <cite>autospec</cite> and
<cite>new_callable</cite> have the same meaning as for <cite>patch</cite>. Like <cite>patch</cite>,
<cite>patch.object</cite> takes arbitrary keyword arguments for configuring the mock
object it creates.</p>
<p>When used as a class decorator <cite>patch.object</cite> honours <cite>patch.TEST_PREFIX</cite>
for choosing which methods to wrap.</p>
</dd></dl>
<p>You can either call <cite>patch.object</cite> 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.</p>
<p>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:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="nd">@patch.object</span><span class="p">(</span><span class="n">SomeClass</span><span class="p">,</span> <span class="s">'class_method'</span><span class="p">)</span>
<span class="gp">... </span><span class="k">def</span> <span class="nf">test</span><span class="p">(</span><span class="n">mock_method</span><span class="p">):</span>
<span class="gp">... </span> <span class="n">SomeClass</span><span class="o">.</span><span class="n">class_method</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>
<span class="gp">... </span> <span class="n">mock_method</span><span class="o">.</span><span class="n">assert_called_with</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">test</span><span class="p">()</span>
</pre></div>
</div>
<p><cite>spec</cite>, <cite>create</cite> and the other arguments to <cite>patch.object</cite> have the same
meaning as they do for <cite>patch</cite>.</p>
</div>
<div class="section" id="patch-dict">
<h2>patch.dict<a class="headerlink" href="#patch-dict" title="Permalink to this headline">¶</a></h2>
<dl class="function">
<dt id="mock.patch.dict">
<tt class="descclassname">patch.</tt><tt class="descname">dict</tt><big>(</big><em>in_dict</em>, <em>values=()</em>, <em>clear=False</em>, <em>**kwargs</em><big>)</big><a class="headerlink" href="#mock.patch.dict" title="Permalink to this definition">¶</a></dt>
<dd><p>Patch a dictionary, or dictionary like object, and restore the dictionary
to its original state after the test.</p>
<p><cite>in_dict</cite> 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.</p>
<p><cite>in_dict</cite> can also be a string specifying the name of the dictionary, which
will then be fetched by importing it.</p>
<p><cite>values</cite> can be a dictionary of values to set in the dictionary. <cite>values</cite>
can also be an iterable of <cite>(key, value)</cite> pairs.</p>
<p>If <cite>clear</cite> is True then the dictionary will be cleared before the new
values are set.</p>
<p><cite>patch.dict</cite> can also be called with arbitrary keyword arguments to set
values in the dictionary.</p>
<p><cite>patch.dict</cite> can be used as a context manager, decorator or class
decorator. When used as a class decorator <cite>patch.dict</cite> honours
<cite>patch.TEST_PREFIX</cite> for choosing which methods to wrap.</p>
</dd></dl>
<p><cite>patch.dict</cite> 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.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">mock</span> <span class="kn">import</span> <span class="n">patch</span>
<span class="gp">>>> </span><span class="n">foo</span> <span class="o">=</span> <span class="p">{}</span>
<span class="gp">>>> </span><span class="k">with</span> <span class="n">patch</span><span class="o">.</span><span class="n">dict</span><span class="p">(</span><span class="n">foo</span><span class="p">,</span> <span class="p">{</span><span class="s">'newkey'</span><span class="p">:</span> <span class="s">'newvalue'</span><span class="p">}):</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">foo</span> <span class="o">==</span> <span class="p">{</span><span class="s">'newkey'</span><span class="p">:</span> <span class="s">'newvalue'</span><span class="p">}</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="k">assert</span> <span class="n">foo</span> <span class="o">==</span> <span class="p">{}</span>
<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">os</span>
<span class="gp">>>> </span><span class="k">with</span> <span class="n">patch</span><span class="o">.</span><span class="n">dict</span><span class="p">(</span><span class="s">'os.environ'</span><span class="p">,</span> <span class="p">{</span><span class="s">'newkey'</span><span class="p">:</span> <span class="s">'newvalue'</span><span class="p">}):</span>
<span class="gp">... </span> <span class="k">print</span> <span class="n">os</span><span class="o">.</span><span class="n">environ</span><span class="p">[</span><span class="s">'newkey'</span><span class="p">]</span>
<span class="gp">...</span>
<span class="go">newvalue</span>
<span class="gp">>>> </span><span class="k">assert</span> <span class="s">'newkey'</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">os</span><span class="o">.</span><span class="n">environ</span>
</pre></div>
</div>
<p>Keywords can be used in the <cite>patch.dict</cite> call to set values in the dictionary:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">mymodule</span> <span class="o">=</span> <span class="n">MagicMock</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">mymodule</span><span class="o">.</span><span class="n">function</span><span class="o">.</span><span class="n">return_value</span> <span class="o">=</span> <span class="s">'fish'</span>
<span class="gp">>>> </span><span class="k">with</span> <span class="n">patch</span><span class="o">.</span><span class="n">dict</span><span class="p">(</span><span class="s">'sys.modules'</span><span class="p">,</span> <span class="n">mymodule</span><span class="o">=</span><span class="n">mymodule</span><span class="p">):</span>
<span class="gp">... </span> <span class="kn">import</span> <span class="nn">mymodule</span>
<span class="gp">... </span> <span class="n">mymodule</span><span class="o">.</span><span class="n">function</span><span class="p">(</span><span class="s">'some'</span><span class="p">,</span> <span class="s">'args'</span><span class="p">)</span>
<span class="gp">...</span>
<span class="go">'fish'</span>
</pre></div>
</div>
<p><cite>patch.dict</cite> 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 <cite>__getitem__</cite>, <cite>__setitem__</cite>, <cite>__delitem__</cite> and either
<cite>__iter__</cite> or <cite>__contains__</cite>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">class</span> <span class="nc">Container</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">values</span> <span class="o">=</span> <span class="p">{}</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">__getitem__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">values</span><span class="p">[</span><span class="n">name</span><span class="p">]</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">__setitem__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">values</span><span class="p">[</span><span class="n">name</span><span class="p">]</span> <span class="o">=</span> <span class="n">value</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">__delitem__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">del</span> <span class="bp">self</span><span class="o">.</span><span class="n">values</span><span class="p">[</span><span class="n">name</span><span class="p">]</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">__iter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">return</span> <span class="nb">iter</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">values</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">thing</span> <span class="o">=</span> <span class="n">Container</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">thing</span><span class="p">[</span><span class="s">'one'</span><span class="p">]</span> <span class="o">=</span> <span class="mi">1</span>
<span class="gp">>>> </span><span class="k">with</span> <span class="n">patch</span><span class="o">.</span><span class="n">dict</span><span class="p">(</span><span class="n">thing</span><span class="p">,</span> <span class="n">one</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">two</span><span class="o">=</span><span class="mi">3</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">thing</span><span class="p">[</span><span class="s">'one'</span><span class="p">]</span> <span class="o">==</span> <span class="mi">2</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">thing</span><span class="p">[</span><span class="s">'two'</span><span class="p">]</span> <span class="o">==</span> <span class="mi">3</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="k">assert</span> <span class="n">thing</span><span class="p">[</span><span class="s">'one'</span><span class="p">]</span> <span class="o">==</span> <span class="mi">1</span>
<span class="gp">>>> </span><span class="k">assert</span> <span class="nb">list</span><span class="p">(</span><span class="n">thing</span><span class="p">)</span> <span class="o">==</span> <span class="p">[</span><span class="s">'one'</span><span class="p">]</span>
</pre></div>
</div>
</div>
<div class="section" id="patch-multiple">
<h2>patch.multiple<a class="headerlink" href="#patch-multiple" title="Permalink to this headline">¶</a></h2>
<dl class="function">
<dt id="mock.patch.multiple">
<tt class="descclassname">patch.</tt><tt class="descname">multiple</tt><big>(</big><em>target</em>, <em>spec=None</em>, <em>create=False</em>, <em>spec_set=None</em>, <em>autospec=None</em>, <em>new_callable=None</em>, <em>**kwargs</em><big>)</big><a class="headerlink" href="#mock.patch.multiple" title="Permalink to this definition">¶</a></dt>
<dd><p>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:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">with</span> <span class="n">patch</span><span class="o">.</span><span class="n">multiple</span><span class="p">(</span><span class="n">settings</span><span class="p">,</span> <span class="n">FIRST_PATCH</span><span class="o">=</span><span class="s">'one'</span><span class="p">,</span> <span class="n">SECOND_PATCH</span><span class="o">=</span><span class="s">'two'</span><span class="p">):</span>
<span class="o">...</span>
</pre></div>
</div>
<p>Use <a class="reference internal" href="sentinel.html#mock.DEFAULT" title="mock.DEFAULT"><tt class="xref py py-data docutils literal"><span class="pre">DEFAULT</span></tt></a> as the value if you want <cite>patch.multiple</cite> 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 <cite>patch.multiple</cite> is
used as a context manager.</p>
<p><cite>patch.multiple</cite> can be used as a decorator, class decorator or a context
manager. The arguments <cite>spec</cite>, <cite>spec_set</cite>, <cite>create</cite>, <cite>autospec</cite> and
<cite>new_callable</cite> have the same meaning as for <cite>patch</cite>. These arguments will
be applied to <em>all</em> patches done by <cite>patch.multiple</cite>.</p>
<p>When used as a class decorator <cite>patch.multiple</cite> honours <cite>patch.TEST_PREFIX</cite>
for choosing which methods to wrap.</p>
</dd></dl>
<p>If you want <cite>patch.multiple</cite> to create mocks for you, then you can use
<a class="reference internal" href="sentinel.html#mock.DEFAULT" title="mock.DEFAULT"><tt class="xref py py-data docutils literal"><span class="pre">DEFAULT</span></tt></a> as the value. If you use <cite>patch.multiple</cite> as a decorator
then the created mocks are passed into the decorated function by keyword.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">thing</span> <span class="o">=</span> <span class="nb">object</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">other</span> <span class="o">=</span> <span class="nb">object</span><span class="p">()</span>
<span class="gp">>>> </span><span class="nd">@patch.multiple</span><span class="p">(</span><span class="s">'__main__'</span><span class="p">,</span> <span class="n">thing</span><span class="o">=</span><span class="n">DEFAULT</span><span class="p">,</span> <span class="n">other</span><span class="o">=</span><span class="n">DEFAULT</span><span class="p">)</span>
<span class="gp">... </span><span class="k">def</span> <span class="nf">test_function</span><span class="p">(</span><span class="n">thing</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">thing</span><span class="p">,</span> <span class="n">MagicMock</span><span class="p">)</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">other</span><span class="p">,</span> <span class="n">MagicMock</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">test_function</span><span class="p">()</span>
</pre></div>
</div>
<p><cite>patch.multiple</cite> can be nested with other <cite>patch</cite> decorators, but put arguments
passed by keyword <em>after</em> any of the standard arguments created by <cite>patch</cite>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="nd">@patch</span><span class="p">(</span><span class="s">'sys.exit'</span><span class="p">)</span>
<span class="gp">... </span><span class="nd">@patch.multiple</span><span class="p">(</span><span class="s">'__main__'</span><span class="p">,</span> <span class="n">thing</span><span class="o">=</span><span class="n">DEFAULT</span><span class="p">,</span> <span class="n">other</span><span class="o">=</span><span class="n">DEFAULT</span><span class="p">)</span>
<span class="gp">... </span><span class="k">def</span> <span class="nf">test_function</span><span class="p">(</span><span class="n">mock_exit</span><span class="p">,</span> <span class="n">other</span><span class="p">,</span> <span class="n">thing</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="s">'other'</span> <span class="ow">in</span> <span class="nb">repr</span><span class="p">(</span><span class="n">other</span><span class="p">)</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="s">'thing'</span> <span class="ow">in</span> <span class="nb">repr</span><span class="p">(</span><span class="n">thing</span><span class="p">)</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="s">'exit'</span> <span class="ow">in</span> <span class="nb">repr</span><span class="p">(</span><span class="n">mock_exit</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">test_function</span><span class="p">()</span>
</pre></div>
</div>
<p>If <cite>patch.multiple</cite> is used as a context manager, the value returned by the
context manger is a dictionary where created mocks are keyed by name:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">with</span> <span class="n">patch</span><span class="o">.</span><span class="n">multiple</span><span class="p">(</span><span class="s">'__main__'</span><span class="p">,</span> <span class="n">thing</span><span class="o">=</span><span class="n">DEFAULT</span><span class="p">,</span> <span class="n">other</span><span class="o">=</span><span class="n">DEFAULT</span><span class="p">)</span> <span class="k">as</span> <span class="n">values</span><span class="p">:</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="s">'other'</span> <span class="ow">in</span> <span class="nb">repr</span><span class="p">(</span><span class="n">values</span><span class="p">[</span><span class="s">'other'</span><span class="p">])</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="s">'thing'</span> <span class="ow">in</span> <span class="nb">repr</span><span class="p">(</span><span class="n">values</span><span class="p">[</span><span class="s">'thing'</span><span class="p">])</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">values</span><span class="p">[</span><span class="s">'thing'</span><span class="p">]</span> <span class="ow">is</span> <span class="n">thing</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">values</span><span class="p">[</span><span class="s">'other'</span><span class="p">]</span> <span class="ow">is</span> <span class="n">other</span>
<span class="gp">...</span>
</pre></div>
</div>
</div>
<div class="section" id="patch-methods-start-and-stop">
<span id="start-and-stop"></span><h2>patch methods: start and stop<a class="headerlink" href="#patch-methods-start-and-stop" title="Permalink to this headline">¶</a></h2>
<p>All the patchers have <cite>start</cite> and <cite>stop</cite> methods. These make it simpler to do
patching in <cite>setUp</cite> methods or where you want to do multiple patches without
nesting decorators or with statements.</p>
<p>To use them call <cite>patch</cite>, <cite>patch.object</cite> or <cite>patch.dict</cite> as normal and keep a
reference to the returned <cite>patcher</cite> object. You can then call <cite>start</cite> to put
the patch in place and <cite>stop</cite> to undo it.</p>
<p>If you are using <cite>patch</cite> to create a mock for you then it will be returned by
the call to <cite>patcher.start</cite>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">patcher</span> <span class="o">=</span> <span class="n">patch</span><span class="p">(</span><span class="s">'package.module.ClassName'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="kn">from</span> <span class="nn">package</span> <span class="kn">import</span> <span class="n">module</span>
<span class="gp">>>> </span><span class="n">original</span> <span class="o">=</span> <span class="n">module</span><span class="o">.</span><span class="n">ClassName</span>
<span class="gp">>>> </span><span class="n">new_mock</span> <span class="o">=</span> <span class="n">patcher</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="gp">>>> </span><span class="k">assert</span> <span class="n">module</span><span class="o">.</span><span class="n">ClassName</span> <span class="ow">is</span> <span class="ow">not</span> <span class="n">original</span>
<span class="gp">>>> </span><span class="k">assert</span> <span class="n">module</span><span class="o">.</span><span class="n">ClassName</span> <span class="ow">is</span> <span class="n">new_mock</span>
<span class="gp">>>> </span><span class="n">patcher</span><span class="o">.</span><span class="n">stop</span><span class="p">()</span>
<span class="gp">>>> </span><span class="k">assert</span> <span class="n">module</span><span class="o">.</span><span class="n">ClassName</span> <span class="ow">is</span> <span class="n">original</span>
<span class="gp">>>> </span><span class="k">assert</span> <span class="n">module</span><span class="o">.</span><span class="n">ClassName</span> <span class="ow">is</span> <span class="ow">not</span> <span class="n">new_mock</span>
</pre></div>
</div>
<p>A typical use case for this might be for doing multiple patches in the <cite>setUp</cite>
method of a <cite>TestCase</cite>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">class</span> <span class="nc">MyTest</span><span class="p">(</span><span class="n">TestCase</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">setUp</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">patcher1</span> <span class="o">=</span> <span class="n">patch</span><span class="p">(</span><span class="s">'package.module.Class1'</span><span class="p">)</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">patcher2</span> <span class="o">=</span> <span class="n">patch</span><span class="p">(</span><span class="s">'package.module.Class2'</span><span class="p">)</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">MockClass1</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">patcher1</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">MockClass2</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">patcher2</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="gp">...</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">tearDown</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">patcher1</span><span class="o">.</span><span class="n">stop</span><span class="p">()</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">patcher2</span><span class="o">.</span><span class="n">stop</span><span class="p">()</span>
<span class="gp">...</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">test_something</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">package</span><span class="o">.</span><span class="n">module</span><span class="o">.</span><span class="n">Class1</span> <span class="ow">is</span> <span class="bp">self</span><span class="o">.</span><span class="n">MockClass1</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">package</span><span class="o">.</span><span class="n">module</span><span class="o">.</span><span class="n">Class2</span> <span class="ow">is</span> <span class="bp">self</span><span class="o">.</span><span class="n">MockClass2</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">MyTest</span><span class="p">(</span><span class="s">'test_something'</span><span class="p">)</span><span class="o">.</span><span class="n">run</span><span class="p">()</span>
</pre></div>
</div>
<div class="admonition caution">
<p class="first admonition-title">Caution</p>
<p>If you use this technique you must ensure that the patching is “undone” by
calling <cite>stop</cite>. 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 external" href="http://pypi.python.org/pypi/unittest2">unittest2</a> cleanup functions make this
easier.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">class</span> <span class="nc">MyTest</span><span class="p">(</span><span class="n">TestCase</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">setUp</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span> <span class="n">patcher</span> <span class="o">=</span> <span class="n">patch</span><span class="p">(</span><span class="s">'package.module.Class'</span><span class="p">)</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">MockClass</span> <span class="o">=</span> <span class="n">patcher</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">addCleanup</span><span class="p">(</span><span class="n">patcher</span><span class="o">.</span><span class="n">stop</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">test_something</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">package</span><span class="o">.</span><span class="n">module</span><span class="o">.</span><span class="n">Class</span> <span class="ow">is</span> <span class="bp">self</span><span class="o">.</span><span class="n">MockClass</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">MyTest</span><span class="p">(</span><span class="s">'test_something'</span><span class="p">)</span><span class="o">.</span><span class="n">run</span><span class="p">()</span>
</pre></div>
</div>
<p class="last">As an added bonus you no longer need to keep a reference to the <cite>patcher</cite>
object.</p>
</div>
<p>It is also possible to stop all patches which have been started by using
<cite>patch.stopall</cite>.</p>
<dl class="function">
<dt id="mock.patch.stopall">
<tt class="descclassname">patch.</tt><tt class="descname">stopall</tt><big>(</big><big>)</big><a class="headerlink" href="#mock.patch.stopall" title="Permalink to this definition">¶</a></dt>
<dd><p>Stop all active patches. Only stops patches started with <cite>start</cite>.</p>
</dd></dl>
</div>
<div class="section" id="test-prefix">
<h2>TEST_PREFIX<a class="headerlink" href="#test-prefix" title="Permalink to this headline">¶</a></h2>
<p>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 <cite>test</cite> as being test methods. This is the same way that the
<cite>unittest.TestLoader</cite> finds test methods by default.</p>
<p>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 <cite>patch.TEST_PREFIX</cite>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">patch</span><span class="o">.</span><span class="n">TEST_PREFIX</span> <span class="o">=</span> <span class="s">'foo'</span>
<span class="gp">>>> </span><span class="n">value</span> <span class="o">=</span> <span class="mi">3</span>
<span class="go">>>></span>
<span class="gp">>>> </span><span class="nd">@patch</span><span class="p">(</span><span class="s">'__main__.value'</span><span class="p">,</span> <span class="s">'not three'</span><span class="p">)</span>
<span class="gp">... </span><span class="k">class</span> <span class="nc">Thing</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">foo_one</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">print</span> <span class="n">value</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">foo_two</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">print</span> <span class="n">value</span>
<span class="gp">...</span>
<span class="go">>>></span>
<span class="gp">>>> </span><span class="n">Thing</span><span class="p">()</span><span class="o">.</span><span class="n">foo_one</span><span class="p">()</span>
<span class="go">not three</span>
<span class="gp">>>> </span><span class="n">Thing</span><span class="p">()</span><span class="o">.</span><span class="n">foo_two</span><span class="p">()</span>
<span class="go">not three</span>
<span class="gp">>>> </span><span class="n">value</span>
<span class="go">3</span>
</pre></div>
</div>
</div>
<div class="section" id="nesting-patch-decorators">
<h2>Nesting Patch Decorators<a class="headerlink" href="#nesting-patch-decorators" title="Permalink to this headline">¶</a></h2>
<p>If you want to perform multiple patches then you can simply stack up the
decorators.</p>
<p>You can stack up multiple patch decorators using this pattern:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="nd">@patch.object</span><span class="p">(</span><span class="n">SomeClass</span><span class="p">,</span> <span class="s">'class_method'</span><span class="p">)</span>
<span class="gp">... </span><span class="nd">@patch.object</span><span class="p">(</span><span class="n">SomeClass</span><span class="p">,</span> <span class="s">'static_method'</span><span class="p">)</span>
<span class="gp">... </span><span class="k">def</span> <span class="nf">test</span><span class="p">(</span><span class="n">mock1</span><span class="p">,</span> <span class="n">mock2</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">SomeClass</span><span class="o">.</span><span class="n">static_method</span> <span class="ow">is</span> <span class="n">mock1</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">SomeClass</span><span class="o">.</span><span class="n">class_method</span> <span class="ow">is</span> <span class="n">mock2</span>
<span class="gp">... </span> <span class="n">SomeClass</span><span class="o">.</span><span class="n">static_method</span><span class="p">(</span><span class="s">'foo'</span><span class="p">)</span>
<span class="gp">... </span> <span class="n">SomeClass</span><span class="o">.</span><span class="n">class_method</span><span class="p">(</span><span class="s">'bar'</span><span class="p">)</span>
<span class="gp">... </span> <span class="k">return</span> <span class="n">mock1</span><span class="p">,</span> <span class="n">mock2</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">mock1</span><span class="p">,</span> <span class="n">mock2</span> <span class="o">=</span> <span class="n">test</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">mock1</span><span class="o">.</span><span class="n">assert_called_once_with</span><span class="p">(</span><span class="s">'foo'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">mock2</span><span class="o">.</span><span class="n">assert_called_once_with</span><span class="p">(</span><span class="s">'bar'</span><span class="p">)</span>
</pre></div>
</div>
<p>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.</p>
<p>Like all context-managers patches can be nested using contextlib’s nested
function; <em>every</em> patching will appear in the tuple after “as”:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">nested</span>
<span class="gp">>>> </span><span class="k">with</span> <span class="n">nested</span><span class="p">(</span>
<span class="gp">... </span> <span class="n">patch</span><span class="p">(</span><span class="s">'package.module.ClassName1'</span><span class="p">),</span>
<span class="gp">... </span> <span class="n">patch</span><span class="p">(</span><span class="s">'package.module.ClassName2'</span><span class="p">)</span>
<span class="gp">... </span> <span class="p">)</span> <span class="k">as</span> <span class="p">(</span><span class="n">MockClass1</span><span class="p">,</span> <span class="n">MockClass2</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">package</span><span class="o">.</span><span class="n">module</span><span class="o">.</span><span class="n">ClassName1</span> <span class="ow">is</span> <span class="n">MockClass1</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">package</span><span class="o">.</span><span class="n">module</span><span class="o">.</span><span class="n">ClassName2</span> <span class="ow">is</span> <span class="n">MockClass2</span>
<span class="gp">...</span>
</pre></div>
</div>
</div>
<div class="section" id="where-to-patch">
<span id="id1"></span><h2>Where to patch<a class="headerlink" href="#where-to-patch" title="Permalink to this headline">¶</a></h2>
<p><cite>patch</cite> works by (temporarily) changing the object that a <em>name</em> 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.</p>
<p>The basic principle is that you patch where an object is <em>looked up</em>, which
is not necessarily the same place as where it is defined. A couple of
examples will help to clarify this.</p>
<p>Imagine we have a project that we want to test with the following structure:</p>
<div class="highlight-python"><pre>a.py
-> Defines SomeClass
b.py
-> from a import SomeClass
-> some_function instantiates SomeClass</pre>
</div>
<p>Now we want to test <cite>some_function</cite> but we want to mock out <cite>SomeClass</cite> using
<cite>patch</cite>. The problem is that when we import module b, which we will have to
do then it imports <cite>SomeClass</cite> from module a. If we use <cite>patch</cite> to mock out
<cite>a.SomeClass</cite> then it will have no effect on our test; module b already has a
reference to the <em>real</em> <cite>SomeClass</cite> and it looks like our patching had no
effect.</p>
<p>The key is to patch out <cite>SomeClass</cite> where it is used (or where it is looked up
). In this case <cite>some_function</cite> will actually look up <cite>SomeClass</cite> in module b,
where we have imported it. The patching should look like:</p>
<blockquote>
<div><cite>@patch(‘b.SomeClass’)</cite></div></blockquote>
<p>However, consider the alternative scenario where instead of <cite>from a import
SomeClass</cite> module b does <cite>import a</cite> and <cite>some_function</cite> uses <cite>a.SomeClass</cite>. 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 <cite>a.SomeClass</cite> instead:</p>
<blockquote>
<div><cite>@patch(‘a.SomeClass’)</cite></div></blockquote>
</div>
<div class="section" id="patching-descriptors-and-proxy-objects">
<h2>Patching Descriptors and Proxy Objects<a class="headerlink" href="#patching-descriptors-and-proxy-objects" title="Permalink to this headline">¶</a></h2>
<p>Since version 0.6.0 both <a class="reference internal" href="#patch">patch</a> and <a class="reference internal" href="#patch-object">patch.object</a> have been able to correctly
patch and restore descriptors: class methods, static methods and properties.
You should patch these on the <em>class</em> rather than an instance.</p>
<p>Since version 0.7.0 <a class="reference internal" href="#patch">patch</a> and <a class="reference internal" href="#patch-object">patch.object</a> work correctly with some objects
that proxy attribute access, like the <a class="reference external" href="http://www.voidspace.org.uk/python/weblog/arch_d7_2010_12_04.shtml#e1198">django setttings object</a>.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">In django <cite>import settings</cite> and <cite>from django.conf import settings</cite>
return different objects. If you are using libraries / apps that do both you
may have to patch both. Grrr...</p>
</div>
</div>
</div>
</div>
</div>
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<h3><a href="index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Patch Decorators</a><ul>
<li><a class="reference internal" href="#patch">patch</a></li>
<li><a class="reference internal" href="#patch-object">patch.object</a></li>
<li><a class="reference internal" href="#patch-dict">patch.dict</a></li>
<li><a class="reference internal" href="#patch-multiple">patch.multiple</a></li>
<li><a class="reference internal" href="#patch-methods-start-and-stop">patch methods: start and stop</a></li>
<li><a class="reference internal" href="#test-prefix">TEST_PREFIX</a></li>
<li><a class="reference internal" href="#nesting-patch-decorators">Nesting Patch Decorators</a></li>
<li><a class="reference internal" href="#where-to-patch">Where to patch</a></li>
<li><a class="reference internal" href="#patching-descriptors-and-proxy-objects">Patching Descriptors and Proxy Objects</a></li>
</ul>
</li>
</ul>
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