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<div class="section" id="mocking-out-objects-and-methods">
<h1>Mocking out objects and methods<a class="headerlink" href="#mocking-out-objects-and-methods" title="Permalink to this headline">¶</a></h1>
<p>Mocking is the process of replacing chunks of complex functionality
that aren’t the subject of the test with mock objects that allow you
to check that the mocked out functionality is being used as expected.</p>
<p>In this way, you can break down testing of a complicated set of
interacting components into testing of each individual component.
The behaviour of components can then be tested individually,
irrespective of the behaviour of the components around it.</p>
<p>There are a few implementations of mock objects in the python world. An
excellent example and the one recommended for use with TestFixtures is
the Mock package: <a class="reference external" href="http://pypi.python.org/pypi/mock/">http://pypi.python.org/pypi/mock/</a></p>
<div class="section" id="methods-of-replacement">
<h2>Methods of replacement<a class="headerlink" href="#methods-of-replacement" title="Permalink to this headline">¶</a></h2>
<p>TestFixtures provides three different methods of mocking out
functionality that can be used to replace functions, classes
or even individual methods on a class. Consider the following module:</p>
<div class="module topic">
<p class="topic-title first">testfixtures.tests.sample1</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">X</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">y</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="s2">"original y"</span>
<span class="nd">@classmethod</span>
<span class="k">def</span> <span class="nf">aMethod</span><span class="p">(</span><span class="bp">cls</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">cls</span>
<span class="nd">@staticmethod</span>
<span class="k">def</span> <span class="nf">bMethod</span><span class="p">():</span>
<span class="k">return</span> <span class="mi">2</span>
</pre></div>
</div>
</div>
<p>We want to mock out the <code class="docutils literal"><span class="pre">y</span></code> method of the <code class="docutils literal"><span class="pre">X</span></code> class, with,
for example, the following function:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">mock_y</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="s1">'mock y'</span>
</pre></div>
</div>
<div class="section" id="the-context-manager">
<h3>The context manager<a class="headerlink" href="#the-context-manager" title="Permalink to this headline">¶</a></h3>
<p>For replacement of a single thing, it’s easiest to use the
<a class="reference internal" href="api.html#testfixtures.Replace" title="testfixtures.Replace"><code class="xref py py-class docutils literal"><span class="pre">Replace</span></code></a> context manager:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">testfixtures</span> <span class="kn">import</span> <span class="n">Replace</span>
<span class="k">def</span> <span class="nf">test_function</span><span class="p">():</span>
<span class="k">with</span> <span class="n">Replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample1.X.y'</span><span class="p">,</span> <span class="n">mock_y</span><span class="p">):</span>
<span class="k">print</span><span class="p">(</span><span class="n">X</span><span class="p">()</span><span class="o">.</span><span class="n">y</span><span class="p">())</span>
</pre></div>
</div>
<p>For the duration of the <code class="docutils literal"><span class="pre">with</span></code> block, the replacement is used:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">test_function</span><span class="p">()</span>
<span class="go">mock y</span>
</pre></div>
</div>
<p>For multiple replacements to do, or where the you need access to the replacement
within the code block under test, the <a class="reference internal" href="api.html#testfixtures.Replacer" title="testfixtures.Replacer"><code class="xref py py-class docutils literal"><span class="pre">Replacer</span></code></a> context
manager can be used instead:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">mock</span> <span class="kn">import</span> <span class="n">Mock</span>
<span class="kn">from</span> <span class="nn">testfixtures</span> <span class="kn">import</span> <span class="n">Replacer</span>
<span class="k">def</span> <span class="nf">test_function</span><span class="p">():</span>
<span class="k">with</span> <span class="n">Replacer</span><span class="p">()</span> <span class="k">as</span> <span class="n">replace</span><span class="p">:</span>
<span class="n">mock_y</span> <span class="o">=</span> <span class="n">replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample1.X.y'</span><span class="p">,</span> <span class="n">Mock</span><span class="p">())</span>
<span class="n">mock_y</span><span class="o">.</span><span class="n">return_value</span> <span class="o">=</span> <span class="s1">'mock y'</span>
<span class="k">print</span><span class="p">(</span><span class="n">X</span><span class="p">()</span><span class="o">.</span><span class="n">y</span><span class="p">())</span>
</pre></div>
</div>
<p>For the duration of the <code class="docutils literal"><span class="pre">with</span></code> block, the replacement is used:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">test_function</span><span class="p">()</span>
<span class="go">mock y</span>
</pre></div>
</div>
</div>
<div class="section" id="the-decorator">
<h3>The decorator<a class="headerlink" href="#the-decorator" title="Permalink to this headline">¶</a></h3>
<p>If you are working in a traditional <a class="reference external" href="https://docs.python.org/3.5/library/unittest.html#module-unittest" title="(in Python v3.5)"><code class="xref py py-mod docutils literal"><span class="pre">unittest</span></code></a> environment and
want to replace different things in different test functions, you may
find the decorator suits your needs better:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">testfixtures</span> <span class="kn">import</span> <span class="n">replace</span>
<span class="nd">@replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample1.X.y'</span><span class="p">,</span> <span class="n">mock_y</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">test_function</span><span class="p">():</span>
<span class="k">print</span><span class="p">(</span><span class="n">X</span><span class="p">()</span><span class="o">.</span><span class="n">y</span><span class="p">())</span>
</pre></div>
</div>
<p>When using the decorator, the replacement is used for the duration of
the decorated callable’s execution:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">test_function</span><span class="p">()</span>
<span class="go">mock y</span>
</pre></div>
</div>
<p>If you need to manipulate or inspect the object that’s used as a
replacement, you can add an extra parameter to your function. The
decorator will see this and pass the replacement in it’s place:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">mock</span> <span class="kn">import</span> <span class="n">Mock</span><span class="p">,</span> <span class="n">call</span>
<span class="kn">from</span> <span class="nn">testfixtures</span> <span class="kn">import</span> <span class="n">compare</span><span class="p">,</span><span class="n">replace</span>
<span class="nd">@replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample1.X.y'</span><span class="p">,</span> <span class="n">Mock</span><span class="p">())</span>
<span class="k">def</span> <span class="nf">test_function</span><span class="p">(</span><span class="n">mock_y</span><span class="p">):</span>
<span class="n">mock_y</span><span class="o">.</span><span class="n">return_value</span> <span class="o">=</span> <span class="s1">'mock y'</span>
<span class="k">print</span><span class="p">(</span><span class="n">X</span><span class="p">()</span><span class="o">.</span><span class="n">y</span><span class="p">())</span>
<span class="n">compare</span><span class="p">(</span><span class="n">mock_y</span><span class="o">.</span><span class="n">mock_calls</span><span class="p">,</span> <span class="n">expected</span><span class="o">=</span><span class="p">[</span><span class="n">call</span><span class="p">()])</span>
</pre></div>
</div>
<p>The above still results in the same output:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">test_function</span><span class="p">()</span>
<span class="go">mock y</span>
</pre></div>
</div>
</div>
<div class="section" id="manual-usage">
<h3>Manual usage<a class="headerlink" href="#manual-usage" title="Permalink to this headline">¶</a></h3>
<p>If you want to replace something for the duration of a doctest or you
want to replace something for every test in a
<a class="reference external" href="https://docs.python.org/3.5/library/unittest.html#unittest.TestCase" title="(in Python v3.5)"><code class="xref py py-class docutils literal"><span class="pre">TestCase</span></code></a>, then you can use the
<a class="reference internal" href="api.html#testfixtures.Replacer" title="testfixtures.Replacer"><code class="xref py py-class docutils literal"><span class="pre">Replacer</span></code></a> manually.</p>
<p>The instantiation and replacement are done in the <code class="docutils literal"><span class="pre">setUp</span></code> function
of the <a class="reference external" href="https://docs.python.org/3.5/library/unittest.html#unittest.TestCase" title="(in Python v3.5)"><code class="xref py py-class docutils literal"><span class="pre">TestCase</span></code></a> or passed to the
<code class="xref py py-class docutils literal"><span class="pre">DocTestSuite</span></code> constructor:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">testfixtures</span> <span class="k">import</span> <span class="n">Replacer</span>
<span class="gp">>>> </span><span class="n">replace</span> <span class="o">=</span> <span class="n">Replacer</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample1.X.y'</span><span class="p">,</span> <span class="n">mock_y</span><span class="p">)</span>
<span class="go"><...></span>
</pre></div>
</div>
<p>The replacement then stays in place until removed:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">X</span><span class="p">()</span><span class="o">.</span><span class="n">y</span><span class="p">()</span>
<span class="go">'mock y'</span>
</pre></div>
</div>
<p>Then, in the <code class="docutils literal"><span class="pre">tearDown</span></code> function
of the <a class="reference external" href="https://docs.python.org/3.5/library/unittest.html#unittest.TestCase" title="(in Python v3.5)"><code class="xref py py-class docutils literal"><span class="pre">TestCase</span></code></a> or passed to the
<code class="xref py py-class docutils literal"><span class="pre">DocTestSuite</span></code> constructor, the replacement is
removed:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">replace</span><span class="o">.</span><span class="n">restore</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">X</span><span class="p">()</span><span class="o">.</span><span class="n">y</span><span class="p">()</span>
<span class="go">'original y'</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="api.html#testfixtures.Replacer.restore" title="testfixtures.Replacer.restore"><code class="xref py py-meth docutils literal"><span class="pre">restore()</span></code></a> method can also be added as an
<a class="reference external" href="https://docs.python.org/3.5/library/unittest.html#unittest.TestCase.addCleanup" title="(in Python v3.5)"><code class="xref py py-meth docutils literal"><span class="pre">addCleanup()</span></code></a> if that is easier or more compact in your test
suite.</p>
</div>
</div>
<div class="section" id="replacing-more-than-one-thing">
<h2>Replacing more than one thing<a class="headerlink" href="#replacing-more-than-one-thing" title="Permalink to this headline">¶</a></h2>
<p>Both the <a class="reference internal" href="api.html#testfixtures.Replacer" title="testfixtures.Replacer"><code class="xref py py-class docutils literal"><span class="pre">Replacer</span></code></a> and the
<a class="reference internal" href="api.html#testfixtures.replace" title="testfixtures.replace"><code class="xref py py-func docutils literal"><span class="pre">replace()</span></code></a> decorator can be used to replace more
than one thing at a time. For the former, this is fairly obvious:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">test_function</span><span class="p">():</span>
<span class="k">with</span> <span class="n">Replacer</span><span class="p">()</span> <span class="k">as</span> <span class="n">replace</span><span class="p">:</span>
<span class="n">y</span> <span class="o">=</span> <span class="n">replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample1.X.y'</span><span class="p">,</span> <span class="n">Mock</span><span class="p">())</span>
<span class="n">y</span><span class="o">.</span><span class="n">return_value</span> <span class="o">=</span> <span class="s1">'mock y'</span>
<span class="n">aMethod</span> <span class="o">=</span> <span class="n">replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample1.X.aMethod'</span><span class="p">,</span> <span class="n">Mock</span><span class="p">())</span>
<span class="n">aMethod</span><span class="o">.</span><span class="n">return_value</span> <span class="o">=</span> <span class="s1">'mock method'</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">X</span><span class="p">()</span>
<span class="k">print</span><span class="p">(</span><span class="n">x</span><span class="o">.</span><span class="n">y</span><span class="p">(),</span> <span class="n">x</span><span class="o">.</span><span class="n">aMethod</span><span class="p">())</span>
</pre></div>
</div>
<p>For the decorator, it’s less obvious but still pretty easy:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">testfixtures</span> <span class="kn">import</span> <span class="n">replace</span>
<span class="nd">@replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample1.X.y'</span><span class="p">,</span> <span class="n">Mock</span><span class="p">())</span>
<span class="nd">@replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample1.X.aMethod'</span><span class="p">,</span> <span class="n">Mock</span><span class="p">())</span>
<span class="k">def</span> <span class="nf">test_function</span><span class="p">(</span><span class="n">aMethod</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span>
<span class="k">print</span><span class="p">(</span><span class="n">aMethod</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span>
<span class="n">aMethod</span><span class="p">()</span><span class="o">.</span><span class="n">return_value</span> <span class="o">=</span> <span class="s1">'mock method'</span>
<span class="n">y</span><span class="p">()</span><span class="o">.</span><span class="n">return_value</span> <span class="o">=</span> <span class="s1">'mock y'</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">X</span><span class="p">()</span>
<span class="k">print</span><span class="p">(</span><span class="n">aMethod</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span>
<span class="k">print</span><span class="p">(</span><span class="n">x</span><span class="o">.</span><span class="n">y</span><span class="p">(),</span> <span class="n">x</span><span class="o">.</span><span class="n">aMethod</span><span class="p">())</span>
</pre></div>
</div>
<p>You’ll notice that you can still get access to the replacements, even
though there are several of them.</p>
</div>
<div class="section" id="replacing-things-that-may-not-be-there">
<h2>Replacing things that may not be there<a class="headerlink" href="#replacing-things-that-may-not-be-there" title="Permalink to this headline">¶</a></h2>
<p>The following code shows a situation where <code class="docutils literal"><span class="pre">hpy</span></code> may or may not be
present depending on whether the <code class="docutils literal"><span class="pre">guppy</span></code> package is installed or
not.</p>
<div class="module topic">
<p class="topic-title first">testfixtures.tests.sample2</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span>
<span class="kn">from</span> <span class="nn">guppy</span> <span class="k">import</span> <span class="n">hpy</span>
<span class="n">guppy</span> <span class="o">=</span> <span class="kc">True</span>
<span class="k">except</span> <span class="ne">ImportError</span><span class="p">:</span>
<span class="n">guppy</span> <span class="o">=</span> <span class="kc">False</span>
<span class="k">def</span> <span class="nf">dump</span><span class="p">(</span><span class="n">path</span><span class="p">):</span>
<span class="k">if</span> <span class="n">guppy</span><span class="p">:</span>
<span class="n">hpy</span><span class="p">()</span><span class="o">.</span><span class="n">heap</span><span class="p">()</span><span class="o">.</span><span class="n">stat</span><span class="o">.</span><span class="n">dump</span><span class="p">(</span><span class="n">path</span><span class="p">)</span>
</pre></div>
</div>
</div>
<p>To test the behaviour of the code that uses <code class="docutils literal"><span class="pre">hpy</span></code> in both of
these cases, regardless of whether or not the <code class="docutils literal"><span class="pre">guppy</span></code> package is
actually installed, we need to be able to mock out both <code class="docutils literal"><span class="pre">hpy</span></code> and the
<code class="docutils literal"><span class="pre">guppy</span></code> global. This is done by doing non-strict replacement, as
shown in the following <a class="reference external" href="https://docs.python.org/3.5/library/unittest.html#unittest.TestCase" title="(in Python v3.5)"><code class="xref py py-class docutils literal"><span class="pre">TestCase</span></code></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">testfixtures.tests.sample2</span> <span class="kn">import</span> <span class="n">dump</span>
<span class="kn">from</span> <span class="nn">testfixtures</span> <span class="kn">import</span> <span class="n">replace</span>
<span class="kn">from</span> <span class="nn">mock</span> <span class="kn">import</span> <span class="n">Mock</span><span class="p">,</span> <span class="n">call</span>
<span class="k">class</span> <span class="nc">Tests</span><span class="p">(</span><span class="n">unittest</span><span class="o">.</span><span class="n">TestCase</span><span class="p">):</span>
<span class="nd">@replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample2.guppy'</span><span class="p">,</span> <span class="bp">True</span><span class="p">)</span>
<span class="nd">@replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample2.hpy'</span><span class="p">,</span> <span class="n">Mock</span><span class="p">(),</span> <span class="n">strict</span><span class="o">=</span><span class="bp">False</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">test_method</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">hpy</span><span class="p">):</span>
<span class="n">dump</span><span class="p">(</span><span class="s1">'somepath'</span><span class="p">)</span>
<span class="n">compare</span><span class="p">([</span>
<span class="n">call</span><span class="p">(),</span>
<span class="n">call</span><span class="p">()</span><span class="o">.</span><span class="n">heap</span><span class="p">(),</span>
<span class="n">call</span><span class="p">()</span><span class="o">.</span><span class="n">heap</span><span class="p">()</span><span class="o">.</span><span class="n">stat</span><span class="o">.</span><span class="n">dump</span><span class="p">(</span><span class="s1">'somepath'</span><span class="p">)</span>
<span class="p">],</span> <span class="n">hpy</span><span class="o">.</span><span class="n">mock_calls</span><span class="p">)</span>
<span class="nd">@replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample2.guppy'</span><span class="p">,</span> <span class="bp">False</span><span class="p">)</span>
<span class="nd">@replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample2.hpy'</span><span class="p">,</span> <span class="n">Mock</span><span class="p">(),</span> <span class="n">strict</span><span class="o">=</span><span class="bp">False</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">test_method_no_heapy</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span><span class="n">hpy</span><span class="p">):</span>
<span class="n">dump</span><span class="p">(</span><span class="s1">'somepath'</span><span class="p">)</span>
<span class="n">compare</span><span class="p">(</span><span class="n">hpy</span><span class="o">.</span><span class="n">mock_calls</span><span class="p">,[])</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="api.html#testfixtures.Replacer.replace" title="testfixtures.Replacer.replace"><code class="xref py py-meth docutils literal"><span class="pre">replace()</span></code></a> method and calling
a <code class="xref py py-class docutils literal"><span class="pre">Replacer</span></code> also supports non-strict replacement using the same
keyword parameter.</p>
</div>
<div class="section" id="replacing-items-in-dictionaries-and-lists">
<h2>Replacing items in dictionaries and lists<a class="headerlink" href="#replacing-items-in-dictionaries-and-lists" title="Permalink to this headline">¶</a></h2>
<p><a class="reference internal" href="api.html#testfixtures.Replace" title="testfixtures.Replace"><code class="xref py py-class docutils literal"><span class="pre">Replace</span></code></a>, <a class="reference internal" href="api.html#testfixtures.Replacer" title="testfixtures.Replacer"><code class="xref py py-class docutils literal"><span class="pre">Replacer</span></code></a> and the
<a class="reference internal" href="api.html#testfixtures.replace" title="testfixtures.replace"><code class="xref py py-func docutils literal"><span class="pre">replace()</span></code></a> decorator can be used to replace items
in dictionaries and lists.</p>
<p>For example, suppose you have a data structure like the following:</p>
<div class="module topic">
<p class="topic-title first">testfixtures.tests.sample1</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">someDict</span> <span class="o">=</span> <span class="nb">dict</span><span class="p">(</span>
<span class="n">key</span><span class="o">=</span><span class="s1">'value'</span><span class="p">,</span>
<span class="n">complex_key</span><span class="o">=</span><span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">],</span>
<span class="p">)</span>
</pre></div>
</div>
</div>
<p>You can mock out the value associated with <code class="docutils literal"><span class="pre">key</span></code> and the second
element in the <code class="docutils literal"><span class="pre">complex_key</span></code> list as follows:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">pprint</span> <span class="kn">import</span> <span class="n">pprint</span>
<span class="kn">from</span> <span class="nn">testfixtures</span> <span class="kn">import</span> <span class="n">Replacer</span>
<span class="kn">from</span> <span class="nn">testfixtures.tests.sample1</span> <span class="kn">import</span> <span class="n">someDict</span>
<span class="k">def</span> <span class="nf">test_function</span><span class="p">():</span>
<span class="k">with</span> <span class="n">Replacer</span><span class="p">()</span> <span class="k">as</span> <span class="n">replace</span><span class="p">:</span>
<span class="n">replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample1.someDict.key'</span><span class="p">,</span> <span class="s1">'foo'</span><span class="p">)</span>
<span class="n">replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample1.someDict.complex_key.1'</span><span class="p">,</span> <span class="mi">42</span><span class="p">)</span>
<span class="n">pprint</span><span class="p">(</span><span class="n">someDict</span><span class="p">)</span>
</pre></div>
</div>
<p>While the replacement is in effect, the new items are in place:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">test_function</span><span class="p">()</span>
<span class="go">{'complex_key': [1, 42, 3], 'key': 'foo'}</span>
</pre></div>
</div>
<p>When it is no longer in effect, the originals are returned:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">pprint</span><span class="p">(</span><span class="n">someDict</span><span class="p">)</span>
<span class="go">{'complex_key': [1, 2, 3], 'key': 'value'}</span>
</pre></div>
</div>
</div>
<div class="section" id="removing-attributes-and-dictionary-items">
<span id="removing-attr-and-item"></span><h2>Removing attributes and dictionary items<a class="headerlink" href="#removing-attributes-and-dictionary-items" title="Permalink to this headline">¶</a></h2>
<p><a class="reference internal" href="api.html#testfixtures.Replace" title="testfixtures.Replace"><code class="xref py py-class docutils literal"><span class="pre">Replace</span></code></a>, <a class="reference internal" href="api.html#testfixtures.Replacer" title="testfixtures.Replacer"><code class="xref py py-class docutils literal"><span class="pre">Replacer</span></code></a> and the
<a class="reference internal" href="api.html#testfixtures.replace" title="testfixtures.replace"><code class="xref py py-func docutils literal"><span class="pre">replace()</span></code></a> decorator can be used to remove
attributes from objects and remove items from dictionaries.</p>
<p>For example, suppose you have a data structure like the following:</p>
<div class="module topic">
<p class="topic-title first">testfixtures.tests.sample1</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">someDict</span> <span class="o">=</span> <span class="nb">dict</span><span class="p">(</span>
<span class="n">key</span><span class="o">=</span><span class="s1">'value'</span><span class="p">,</span>
<span class="n">complex_key</span><span class="o">=</span><span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">],</span>
<span class="p">)</span>
</pre></div>
</div>
</div>
<p>If you want to remove the <code class="docutils literal"><span class="pre">key</span></code> for the duration of a test, you can
do so as follows:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">testfixtures</span> <span class="kn">import</span> <span class="n">Replace</span><span class="p">,</span> <span class="n">not_there</span>
<span class="kn">from</span> <span class="nn">testfixtures.tests.sample1</span> <span class="kn">import</span> <span class="n">someDict</span>
<span class="k">def</span> <span class="nf">test_function</span><span class="p">():</span>
<span class="k">with</span> <span class="n">Replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample1.someDict.key'</span><span class="p">,</span> <span class="n">not_there</span><span class="p">):</span>
<span class="n">pprint</span><span class="p">(</span><span class="n">someDict</span><span class="p">)</span>
</pre></div>
</div>
<p>While the replacement is in effect, <code class="docutils literal"><span class="pre">key</span></code> is gone:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">test_function</span><span class="p">()</span>
<span class="go">{'complex_key': [1, 2, 3]}</span>
</pre></div>
</div>
<p>When it is no longer in effect, <code class="docutils literal"><span class="pre">key</span></code> is returned:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">pprint</span><span class="p">(</span><span class="n">someDict</span><span class="p">)</span>
<span class="go">{'complex_key': [1, 2, 3], 'key': 'value'}</span>
</pre></div>
</div>
<p>If you want the whole <code class="docutils literal"><span class="pre">someDict</span></code> dictionary to be removed for the
duration of a test, you would do so as follows:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">testfixtures</span> <span class="kn">import</span> <span class="n">Replace</span><span class="p">,</span> <span class="n">not_there</span>
<span class="kn">from</span> <span class="nn">testfixtures.tests</span> <span class="kn">import</span> <span class="n">sample1</span>
<span class="k">def</span> <span class="nf">test_function</span><span class="p">():</span>
<span class="k">with</span> <span class="n">Replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample1.someDict'</span><span class="p">,</span> <span class="n">not_there</span><span class="p">):</span>
<span class="k">print</span><span class="p">(</span><span class="nb">hasattr</span><span class="p">(</span><span class="n">sample1</span><span class="p">,</span> <span class="s1">'someDict'</span><span class="p">))</span>
</pre></div>
</div>
<p>While the replacement is in effect, <code class="docutils literal"><span class="pre">key</span></code> is gone:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">test_function</span><span class="p">()</span>
<span class="go">False</span>
</pre></div>
</div>
<p>When it is no longer in effect, <code class="docutils literal"><span class="pre">key</span></code> is returned:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">pprint</span><span class="p">(</span><span class="n">sample1</span><span class="o">.</span><span class="n">someDict</span><span class="p">)</span>
<span class="go">{'complex_key': [1, 2, 3], 'key': 'value'}</span>
</pre></div>
</div>
</div>
<div class="section" id="gotchas">
<h2>Gotchas<a class="headerlink" href="#gotchas" title="Permalink to this headline">¶</a></h2>
<ul>
<li><p class="first">Make sure you replace the object where it’s used and not where it’s
defined. For example, with the following code from the
<code class="docutils literal"><span class="pre">testfixtures.tests.sample1</span></code> package:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">time</span> <span class="k">import</span> <span class="n">time</span>
<span class="k">def</span> <span class="nf">str_time</span><span class="p">():</span>
<span class="k">return</span> <span class="nb">str</span><span class="p">(</span><span class="n">time</span><span class="p">())</span>
</pre></div>
</div>
<p>You might be tempted to mock things as follows:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">replace</span> <span class="o">=</span> <span class="n">Replacer</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">replace</span><span class="p">(</span><span class="s1">'time.time'</span><span class="p">,</span> <span class="n">Mock</span><span class="p">())</span>
<span class="go"><...></span>
</pre></div>
</div>
<p>But this won’t work:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">testfixtures.tests.sample1</span> <span class="k">import</span> <span class="n">str_time</span>
<span class="gp">>>> </span><span class="nb">type</span><span class="p">(</span><span class="nb">float</span><span class="p">(</span><span class="n">str_time</span><span class="p">()))</span>
<span class="go"><... 'float'></span>
</pre></div>
</div>
<p>You need to replace <a class="reference external" href="https://docs.python.org/3.5/library/time.html#time.time" title="(in Python v3.5)"><code class="xref py py-func docutils literal"><span class="pre">time()</span></code></a> where it’s used, not where
it’s defined:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">replace</span><span class="p">(</span><span class="s1">'testfixtures.tests.sample1.time'</span><span class="p">,</span> <span class="n">Mock</span><span class="p">())</span>
<span class="go"><...></span>
<span class="gp">>>> </span><span class="n">str_time</span><span class="p">()</span>
<span class="go">"<...Mock...>"</span>
</pre></div>
</div>
<p>A corollary of this is that you need to replace <em>all</em> occurrences of
an original to safely be able to test. This can be tricky when an
original is imported into many modules that may be used by a
particular test.</p>
</li>
<li><p class="first">You can’t replace whole top level modules, and nor should you want
to! The reason being that everything up to the last dot in the
replacement target specifies where the replacement will take place,
and the part after the last dot is used as the name of the thing to
be replaced:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">Replacer</span><span class="p">()</span><span class="o">.</span><span class="n">replace</span><span class="p">(</span><span class="s1">'sys'</span><span class="p">,</span> <span class="n">Mock</span><span class="p">())</span>
<span class="gt">Traceback (most recent call last):</span>
<span class="c">...</span>
<span class="gr">ValueError</span>: <span class="n">target must contain at least one dot!</span>
</pre></div>
</div>
</li>
</ul>
</div>
</div>
</div>
</div>
</div>
<div class="sphinxsidebar" role="navigation" aria-label="main navigation">
<div class="sphinxsidebarwrapper">
<h3><a href="index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Mocking out objects and methods</a><ul>
<li><a class="reference internal" href="#methods-of-replacement">Methods of replacement</a><ul>
<li><a class="reference internal" href="#the-context-manager">The context manager</a></li>
<li><a class="reference internal" href="#the-decorator">The decorator</a></li>
<li><a class="reference internal" href="#manual-usage">Manual usage</a></li>
</ul>
</li>
<li><a class="reference internal" href="#replacing-more-than-one-thing">Replacing more than one thing</a></li>
<li><a class="reference internal" href="#replacing-things-that-may-not-be-there">Replacing things that may not be there</a></li>
<li><a class="reference internal" href="#replacing-items-in-dictionaries-and-lists">Replacing items in dictionaries and lists</a></li>
<li><a class="reference internal" href="#removing-attributes-and-dictionary-items">Removing attributes and dictionary items</a></li>
<li><a class="reference internal" href="#gotchas">Gotchas</a></li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
<p class="topless"><a href="comparing.html"
title="previous chapter">Comparing objects and sequences</a></p>
<h4>Next topic</h4>
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title="next chapter">Mocking dates and times</a></p>
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