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<div class="yui-g" id="topics-testing-doctests">
<div class="section" id="s-django-and-doctests">
<span id="django-and-doctests"></span><h1>Django and doctests<a class="headerlink" href="#django-and-doctests" title="Permalink to this headline">ΒΆ</a></h1>
<p>Doctests use Python’s standard <a class="reference external" href="http://docs.python.org/2.7/library/doctest.html#module-doctest" title="(in Python v2.7)"><tt class="xref py py-mod docutils literal"><span class="pre">doctest</span></tt></a> module, which searches your
docstrings for statements that resemble a session of the Python interactive
interpreter. A full explanation of how <a class="reference external" href="http://docs.python.org/2.7/library/doctest.html#module-doctest" title="(in Python v2.7)"><tt class="xref py py-mod docutils literal"><span class="pre">doctest</span></tt></a> works is out of the scope
of this document; read Python’s official documentation for the details.</p>
<div class="admonition-what-s-a-docstring admonition">
<p class="first admonition-title">What’s a <strong>docstring</strong>?</p>
<p>A good explanation of docstrings (and some guidelines for using them
effectively) can be found in <span class="target" id="index-0"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-0257"><strong>PEP 257</strong></a>:</p>
<blockquote>
<div>A docstring is a string literal that occurs as the first statement in
a module, function, class, or method definition. Such a docstring
becomes the <tt class="docutils literal"><span class="pre">__doc__</span></tt> special attribute of that object.</div></blockquote>
<p>For example, this function has a docstring that describes what it does:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">add_two</span><span class="p">(</span><span class="n">num</span><span class="p">):</span>
<span class="s">"Return the result of adding two to the provided number."</span>
<span class="k">return</span> <span class="n">num</span> <span class="o">+</span> <span class="mi">2</span>
</pre></div>
</div>
<p class="last">Because tests often make great documentation, putting tests directly in
your docstrings is an effective way to document <em>and</em> test your code.</p>
</div>
<p>As with unit tests, for a given Django application, the test runner looks for
doctests in two places:</p>
<ul class="simple">
<li>The <tt class="docutils literal"><span class="pre">models.py</span></tt> file. You can define module-level doctests and/or a
doctest for individual models. It’s common practice to put
application-level doctests in the module docstring and model-level
doctests in the model docstrings.</li>
<li>A file called <tt class="docutils literal"><span class="pre">tests.py</span></tt> in the application directory – i.e., the
directory that holds <tt class="docutils literal"><span class="pre">models.py</span></tt>. This file is a hook for any and all
doctests you want to write that aren’t necessarily related to models.</li>
</ul>
<p>This example doctest is equivalent to the example given in the unittest section
above:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># models.py</span>
<span class="kn">from</span> <span class="nn">django.db</span> <span class="kn">import</span> <span class="n">models</span>
<span class="k">class</span> <span class="nc">Animal</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="sd">"""</span>
<span class="sd"> An animal that knows how to make noise</span>
<span class="sd"> # Create some animals</span>
<span class="sd"> >>> lion = Animal.objects.create(name="lion", sound="roar")</span>
<span class="sd"> >>> cat = Animal.objects.create(name="cat", sound="meow")</span>
<span class="sd"> # Make 'em speak</span>
<span class="sd"> >>> lion.speak()</span>
<span class="sd"> 'The lion says "roar"'</span>
<span class="sd"> >>> cat.speak()</span>
<span class="sd"> 'The cat says "meow"'</span>
<span class="sd"> """</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">max_length</span><span class="o">=</span><span class="mi">20</span><span class="p">)</span>
<span class="n">sound</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">max_length</span><span class="o">=</span><span class="mi">20</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">speak</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="s">'The </span><span class="si">%s</span><span class="s"> says "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">sound</span><span class="p">)</span>
</pre></div>
</div>
<p>When you <a class="reference internal" href="overview.html#running-tests"><em>run your tests</em></a>, the test runner will find this
docstring, notice that portions of it look like an interactive Python session,
and execute those lines while checking that the results match.</p>
<p>In the case of model tests, note that the test runner takes care of creating
its own test database. That is, any test that accesses a database – by
creating and saving model instances, for example – will not affect your
production database. However, the database is not refreshed between doctests,
so if your doctest requires a certain state you should consider flushing the
database or loading a fixture. (See the section on <a class="reference internal" href="overview.html#topics-testing-fixtures"><em>fixtures</em></a> for more on this.) Note that to use this feature,
the database user Django is connecting as must have <tt class="docutils literal"><span class="pre">CREATE</span> <span class="pre">DATABASE</span></tt>
rights.</p>
<p>For more details about <a class="reference external" href="http://docs.python.org/2.7/library/doctest.html#module-doctest" title="(in Python v2.7)"><tt class="xref py py-mod docutils literal"><span class="pre">doctest</span></tt></a>, see the Python documentation.</p>
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