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<div class="section" id="monkeypatching-mocking-modules-and-environments">
<h1>Monkeypatching/mocking modules and environments<a class="headerlink" href="#monkeypatching-mocking-modules-and-environments" title="Permalink to this headline">¶</a></h1>
<p>Sometimes tests need to invoke functionality which depends
on global settings or which invokes code which cannot be easily
tested such as network access. The <code class="docutils literal"><span class="pre">monkeypatch</span></code> fixture
helps you to safely set/delete an attribute, dictionary item or
environment variable or to modify <code class="docutils literal"><span class="pre">sys.path</span></code> for importing.
See the <a class="reference external" href="http://tetamap.wordpress.com/2009/03/03/monkeypatching-in-unit-tests-done-right/">monkeypatch blog post</a> for some introduction material
and a discussion of its motivation.</p>
<div class="section" id="simple-example-monkeypatching-functions">
<h2>Simple example: monkeypatching functions<a class="headerlink" href="#simple-example-monkeypatching-functions" title="Permalink to this headline">¶</a></h2>
<p>If you want to pretend that <code class="docutils literal"><span class="pre">os.expanduser</span></code> returns a certain
directory, you can use the <code class="xref py py-meth docutils literal"><span class="pre">monkeypatch.setattr()</span></code> method to
patch this function before calling into a function which uses it:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="c1"># content of test_module.py</span>
<span class="kn">import</span> <span class="nn">os.path</span>
<span class="k">def</span> <span class="nf">getssh</span><span class="p">():</span> <span class="c1"># pseudo application code</span>
<span class="k">return</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">expanduser</span><span class="p">(</span><span class="s2">"~admin"</span><span class="p">),</span> <span class="s1">'.ssh'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">test_mytest</span><span class="p">(</span><span class="n">monkeypatch</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">mockreturn</span><span class="p">(</span><span class="n">path</span><span class="p">):</span>
<span class="k">return</span> <span class="s1">'/abc'</span>
<span class="n">monkeypatch</span><span class="o">.</span><span class="n">setattr</span><span class="p">(</span><span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="p">,</span> <span class="s1">'expanduser'</span><span class="p">,</span> <span class="n">mockreturn</span><span class="p">)</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">getssh</span><span class="p">()</span>
<span class="k">assert</span> <span class="n">x</span> <span class="o">==</span> <span class="s1">'/abc/.ssh'</span>
</pre></div>
</div>
<p>Here our test function monkeypatches <code class="docutils literal"><span class="pre">os.path.expanduser</span></code> and
then calls into a function that calls it. After the test function
finishes the <code class="docutils literal"><span class="pre">os.path.expanduser</span></code> modification will be undone.</p>
</div>
<div class="section" id="example-preventing-requests-from-remote-operations">
<h2>example: preventing "requests" from remote operations<a class="headerlink" href="#example-preventing-requests-from-remote-operations" title="Permalink to this headline">¶</a></h2>
<p>If you want to prevent the "requests" library from performing http
requests in all your tests, you can do:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="c1"># content of conftest.py</span>
<span class="kn">import</span> <span class="nn">pytest</span>
<span class="nd">@pytest</span><span class="o">.</span><span class="n">fixture</span><span class="p">(</span><span class="n">autouse</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">no_requests</span><span class="p">(</span><span class="n">monkeypatch</span><span class="p">):</span>
<span class="n">monkeypatch</span><span class="o">.</span><span class="n">delattr</span><span class="p">(</span><span class="s2">"requests.sessions.Session.request"</span><span class="p">)</span>
</pre></div>
</div>
<p>This autouse fixture will be executed for each test function and it
will delete the method <code class="docutils literal"><span class="pre">request.session.Session.request</span></code>
so that any attempts within tests to create http requests will fail.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Be advised that it is not recommended to patch builtin functions such as <code class="docutils literal"><span class="pre">open</span></code>,
<code class="docutils literal"><span class="pre">compile</span></code>, etc., because it might break pytest's internals. If that's
unavoidable, passing <code class="docutils literal"><span class="pre">--tb=native</span></code>, <code class="docutils literal"><span class="pre">--assert=plain</span></code> and <code class="docutils literal"><span class="pre">--capture=no</span></code> might
help although there's no guarantee.</p>
</div>
</div>
<div class="section" id="method-reference-of-the-monkeypatch-fixture">
<h2>Method reference of the monkeypatch fixture<a class="headerlink" href="#method-reference-of-the-monkeypatch-fixture" title="Permalink to this headline">¶</a></h2>
<dl class="class">
<dt id="_pytest.monkeypatch.MonkeyPatch">
<em class="property">class </em><code class="descname">MonkeyPatch</code><a class="reference internal" href="_modules/_pytest/monkeypatch.html#MonkeyPatch"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#_pytest.monkeypatch.MonkeyPatch" title="Permalink to this definition">¶</a></dt>
<dd><p>Object returned by the <code class="docutils literal"><span class="pre">monkeypatch</span></code> fixture keeping a record of setattr/item/env/syspath changes.</p>
<dl class="method">
<dt id="_pytest.monkeypatch.MonkeyPatch.setattr">
<code class="descname">setattr</code><span class="sig-paren">(</span><em>target</em>, <em>name</em>, <em>value=<notset></em>, <em>raising=True</em><span class="sig-paren">)</span><a class="reference internal" href="_modules/_pytest/monkeypatch.html#MonkeyPatch.setattr"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#_pytest.monkeypatch.MonkeyPatch.setattr" title="Permalink to this definition">¶</a></dt>
<dd><p>Set attribute value on target, memorizing the old value.
By default raise AttributeError if the attribute did not exist.</p>
<p>For convenience you can specify a string as <code class="docutils literal"><span class="pre">target</span></code> which
will be interpreted as a dotted import path, with the last part
being the attribute name. Example:
<code class="docutils literal"><span class="pre">monkeypatch.setattr("os.getcwd",</span> <span class="pre">lambda</span> <span class="pre">x:</span> <span class="pre">"/")</span></code>
would set the <code class="docutils literal"><span class="pre">getcwd</span></code> function of the <code class="docutils literal"><span class="pre">os</span></code> module.</p>
<p>The <code class="docutils literal"><span class="pre">raising</span></code> value determines if the setattr should fail
if the attribute is not already present (defaults to True
which means it will raise).</p>
</dd></dl>
<dl class="method">
<dt id="_pytest.monkeypatch.MonkeyPatch.delattr">
<code class="descname">delattr</code><span class="sig-paren">(</span><em>target</em>, <em>name=<notset></em>, <em>raising=True</em><span class="sig-paren">)</span><a class="reference internal" href="_modules/_pytest/monkeypatch.html#MonkeyPatch.delattr"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#_pytest.monkeypatch.MonkeyPatch.delattr" title="Permalink to this definition">¶</a></dt>
<dd><p>Delete attribute <code class="docutils literal"><span class="pre">name</span></code> from <code class="docutils literal"><span class="pre">target</span></code>, by default raise
AttributeError it the attribute did not previously exist.</p>
<p>If no <code class="docutils literal"><span class="pre">name</span></code> is specified and <code class="docutils literal"><span class="pre">target</span></code> is a string
it will be interpreted as a dotted import path with the
last part being the attribute name.</p>
<p>If <code class="docutils literal"><span class="pre">raising</span></code> is set to False, no exception will be raised if the
attribute is missing.</p>
</dd></dl>
<dl class="method">
<dt id="_pytest.monkeypatch.MonkeyPatch.setitem">
<code class="descname">setitem</code><span class="sig-paren">(</span><em>dic</em>, <em>name</em>, <em>value</em><span class="sig-paren">)</span><a class="reference internal" href="_modules/_pytest/monkeypatch.html#MonkeyPatch.setitem"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#_pytest.monkeypatch.MonkeyPatch.setitem" title="Permalink to this definition">¶</a></dt>
<dd><p>Set dictionary entry <code class="docutils literal"><span class="pre">name</span></code> to value.</p>
</dd></dl>
<dl class="method">
<dt id="_pytest.monkeypatch.MonkeyPatch.delitem">
<code class="descname">delitem</code><span class="sig-paren">(</span><em>dic</em>, <em>name</em>, <em>raising=True</em><span class="sig-paren">)</span><a class="reference internal" href="_modules/_pytest/monkeypatch.html#MonkeyPatch.delitem"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#_pytest.monkeypatch.MonkeyPatch.delitem" title="Permalink to this definition">¶</a></dt>
<dd><p>Delete <code class="docutils literal"><span class="pre">name</span></code> from dict. Raise KeyError if it doesn't exist.</p>
<p>If <code class="docutils literal"><span class="pre">raising</span></code> is set to False, no exception will be raised if the
key is missing.</p>
</dd></dl>
<dl class="method">
<dt id="_pytest.monkeypatch.MonkeyPatch.setenv">
<code class="descname">setenv</code><span class="sig-paren">(</span><em>name</em>, <em>value</em>, <em>prepend=None</em><span class="sig-paren">)</span><a class="reference internal" href="_modules/_pytest/monkeypatch.html#MonkeyPatch.setenv"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#_pytest.monkeypatch.MonkeyPatch.setenv" title="Permalink to this definition">¶</a></dt>
<dd><p>Set environment variable <code class="docutils literal"><span class="pre">name</span></code> to <code class="docutils literal"><span class="pre">value</span></code>. If <code class="docutils literal"><span class="pre">prepend</span></code>
is a character, read the current environment variable value
and prepend the <code class="docutils literal"><span class="pre">value</span></code> adjoined with the <code class="docutils literal"><span class="pre">prepend</span></code> character.</p>
</dd></dl>
<dl class="method">
<dt id="_pytest.monkeypatch.MonkeyPatch.delenv">
<code class="descname">delenv</code><span class="sig-paren">(</span><em>name</em>, <em>raising=True</em><span class="sig-paren">)</span><a class="reference internal" href="_modules/_pytest/monkeypatch.html#MonkeyPatch.delenv"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#_pytest.monkeypatch.MonkeyPatch.delenv" title="Permalink to this definition">¶</a></dt>
<dd><p>Delete <code class="docutils literal"><span class="pre">name</span></code> from the environment. Raise KeyError it does not
exist.</p>
<p>If <code class="docutils literal"><span class="pre">raising</span></code> is set to False, no exception will be raised if the
environment variable is missing.</p>
</dd></dl>
<dl class="method">
<dt id="_pytest.monkeypatch.MonkeyPatch.syspath_prepend">
<code class="descname">syspath_prepend</code><span class="sig-paren">(</span><em>path</em><span class="sig-paren">)</span><a class="reference internal" href="_modules/_pytest/monkeypatch.html#MonkeyPatch.syspath_prepend"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#_pytest.monkeypatch.MonkeyPatch.syspath_prepend" title="Permalink to this definition">¶</a></dt>
<dd><p>Prepend <code class="docutils literal"><span class="pre">path</span></code> to <code class="docutils literal"><span class="pre">sys.path</span></code> list of import locations.</p>
</dd></dl>
<dl class="method">
<dt id="_pytest.monkeypatch.MonkeyPatch.chdir">
<code class="descname">chdir</code><span class="sig-paren">(</span><em>path</em><span class="sig-paren">)</span><a class="reference internal" href="_modules/_pytest/monkeypatch.html#MonkeyPatch.chdir"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#_pytest.monkeypatch.MonkeyPatch.chdir" title="Permalink to this definition">¶</a></dt>
<dd><p>Change the current working directory to the specified path.
Path can be a string or a py.path.local object.</p>
</dd></dl>
<dl class="method">
<dt id="_pytest.monkeypatch.MonkeyPatch.undo">
<code class="descname">undo</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="reference internal" href="_modules/_pytest/monkeypatch.html#MonkeyPatch.undo"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#_pytest.monkeypatch.MonkeyPatch.undo" title="Permalink to this definition">¶</a></dt>
<dd><p>Undo previous changes. This call consumes the
undo stack. Calling it a second time has no effect unless
you do more monkeypatching after the undo call.</p>
<p>There is generally no need to call <cite>undo()</cite>, since it is
called automatically during tear-down.</p>
<p>Note that the same <cite>monkeypatch</cite> fixture is used across a
single test function invocation. If <cite>monkeypatch</cite> is used both by
the test function itself and one of the test fixtures,
calling <cite>undo()</cite> will undo all of the changes made in
both functions.</p>
</dd></dl>
</dd></dl>
<p><code class="docutils literal"><span class="pre">monkeypatch.setattr/delattr/delitem/delenv()</span></code> all
by default raise an Exception if the target does not exist.
Pass <code class="docutils literal"><span class="pre">raising=False</span></code> if you want to skip this check.</p>
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<li><a class="reference internal" href="#">Monkeypatching/mocking modules and environments</a><ul>
<li><a class="reference internal" href="#simple-example-monkeypatching-functions">Simple example: monkeypatching functions</a></li>
<li><a class="reference internal" href="#example-preventing-requests-from-remote-operations">example: preventing "requests" from remote operations</a></li>
<li><a class="reference internal" href="#method-reference-of-the-monkeypatch-fixture">Method reference of the monkeypatch fixture</a></li>
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