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<div class="section" id="hypothesis-for-django-users">
<span id="hypothesis-django"></span><h1>Hypothesis for Django users<a class="headerlink" href="#hypothesis-for-django-users" title="Permalink to this headline">¶</a></h1>
<p>Hypothesis offers a number of features specific for Django testing, available
in the <code class="xref py py-mod docutils literal"><span class="pre">hypothesis[django]</span></code> <a class="reference internal" href="extras.html"><span class="doc">extra</span></a>. This is tested
against each supported series with mainstream or extended support -
if you’re still getting security patches, you can test with Hypothesis.</p>
<p>Using it is quite straightforward: All you need to do is subclass
<code class="xref py py-class docutils literal"><span class="pre">hypothesis.extra.django.TestCase</span></code> or
<code class="xref py py-class docutils literal"><span class="pre">hypothesis.extra.django.TransactionTestCase</span></code>
and you can use <a class="reference internal" href="details.html#hypothesis.given" title="hypothesis.given"><code class="xref py py-func docutils literal"><span class="pre">@given</span></code></a> as normal,
and the transactions will be per example
rather than per test function as they would be if you used <a class="reference internal" href="details.html#hypothesis.given" title="hypothesis.given"><code class="xref py py-func docutils literal"><span class="pre">@given</span></code></a> with a normal
django test suite (this is important because your test function will be called
multiple times and you don’t want them to interfere with each other). Test cases
on these classes that do not use
<a class="reference internal" href="details.html#hypothesis.given" title="hypothesis.given"><code class="xref py py-func docutils literal"><span class="pre">@given</span></code></a> will be run as normal.</p>
<p>I strongly recommend not using
<code class="xref py py-class docutils literal"><span class="pre">TransactionTestCase</span></code>
unless you really have to.
Because Hypothesis runs this in a loop the performance problems it normally has
are significantly exacerbated and your tests will be really slow.
If you are using <code class="xref py py-class docutils literal"><span class="pre">TransactionTestCase</span></code>,
you may need to use <code class="docutils literal"><span class="pre">@settings(suppress_health_check=[HealthCheck.too_slow])</span></code>
to avoid <a class="reference internal" href="healthchecks.html"><span class="doc">errors due to slow example generation</span></a>.</p>
<p>In addition to the above, Hypothesis has some support for automatically
deriving strategies for your model types, which you can then customize further.</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">Hypothesis creates saved models. This will run inside your testing
transaction when using the test runner, but if you use the dev console this
will leave debris in your database.</p>
</div>
<p>For example, using the trivial django project I have for testing:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">hypothesis.extra.django.models</span> <span class="kn">import</span> <span class="n">models</span>
<span class="gp">>>> </span><span class="kn">from</span> <span class="nn">toystore.models</span> <span class="kn">import</span> <span class="n">Customer</span>
<span class="gp">>>> </span><span class="n">c</span> <span class="o">=</span> <span class="n">models</span><span class="p">(</span><span class="n">Customer</span><span class="p">)</span><span class="o">.</span><span class="n">example</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">c</span>
<span class="go"><Customer: Customer object></span>
<span class="gp">>>> </span><span class="n">c</span><span class="o">.</span><span class="n">email</span>
<span class="go">'jaime.urbina@gmail.com'</span>
<span class="gp">>>> </span><span class="n">c</span><span class="o">.</span><span class="n">name</span>
<span class="go">'\U00109d3d\U000e07be\U000165f8\U0003fabf\U000c12cd\U000f1910\U00059f12\U000519b0\U0003fabf\U000f1910\U000423fb\U000423fb\U00059f12\U000e07be\U000c12cd\U000e07be\U000519b0\U000165f8\U0003fabf\U0007bc31'</span>
<span class="gp">>>> </span><span class="n">c</span><span class="o">.</span><span class="n">age</span>
<span class="go">-873375803</span>
</pre></div>
</div>
<p>Hypothesis has just created this with whatever the relevant type of data is.</p>
<p>Obviously the customer’s age is implausible, so lets fix that:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">hypothesis.strategies</span> <span class="kn">import</span> <span class="n">integers</span>
<span class="gp">>>> </span><span class="n">c</span> <span class="o">=</span> <span class="n">models</span><span class="p">(</span><span class="n">Customer</span><span class="p">,</span> <span class="n">age</span><span class="o">=</span><span class="n">integers</span><span class="p">(</span><span class="n">min_value</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">max_value</span><span class="o">=</span><span class="mi">120</span><span class="p">))</span><span class="o">.</span><span class="n">example</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">c</span>
<span class="go"><Customer: Customer object></span>
<span class="gp">>>> </span><span class="n">c</span><span class="o">.</span><span class="n">age</span>
<span class="go">5</span>
</pre></div>
</div>
<p>You can use this to override any fields you like. Sometimes this will be
mandatory: If you have a non-nullable field of a type Hypothesis doesn’t know
how to create (e.g. a foreign key) then the models function will error unless
you explicitly pass a strategy to use there.</p>
<p>Foreign keys are not automatically derived. If they’re nullable they will default
to always being null, otherwise you always have to specify them. e.g. suppose
we had a Shop type with a foreign key to company, we would define a strategy
for it as:</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="n">shop_strategy</span> <span class="o">=</span> <span class="n">models</span><span class="p">(</span><span class="n">Shop</span><span class="p">,</span> <span class="n">company</span><span class="o">=</span><span class="n">models</span><span class="p">(</span><span class="n">Company</span><span class="p">))</span>
</pre></div>
</div>
<div class="section" id="tips-and-tricks">
<h2>Tips and tricks<a class="headerlink" href="#tips-and-tricks" title="Permalink to this headline">¶</a></h2>
<div class="section" id="custom-field-types">
<h3>Custom field types<a class="headerlink" href="#custom-field-types" title="Permalink to this headline">¶</a></h3>
<p>If you have a custom Django field type you can register it with Hypothesis’s
model deriving functionality by registering a default strategy for it:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">toystore.models</span> <span class="kn">import</span> <span class="n">CustomishField</span><span class="p">,</span> <span class="n">Customish</span>
<span class="gp">>>> </span><span class="n">models</span><span class="p">(</span><span class="n">Customish</span><span class="p">)</span><span class="o">.</span><span class="n">example</span><span class="p">()</span>
<span class="go">hypothesis.errors.InvalidArgument: Missing arguments for mandatory field</span>
<span class="go"> customish for model Customish</span>
<span class="gp">>>> </span><span class="kn">from</span> <span class="nn">hypothesis.extra.django.models</span> <span class="kn">import</span> <span class="n">add_default_field_mapping</span>
<span class="gp">>>> </span><span class="kn">from</span> <span class="nn">hypothesis.strategies</span> <span class="kn">import</span> <span class="n">just</span>
<span class="gp">>>> </span><span class="n">add_default_field_mapping</span><span class="p">(</span><span class="n">CustomishField</span><span class="p">,</span> <span class="n">just</span><span class="p">(</span><span class="s2">"hi"</span><span class="p">))</span>
<span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="n">models</span><span class="p">(</span><span class="n">Customish</span><span class="p">)</span><span class="o">.</span><span class="n">example</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">x</span><span class="o">.</span><span class="n">customish</span>
<span class="go">'hi'</span>
</pre></div>
</div>
<p>Note that this mapping is on exact type. Subtypes will not inherit it.</p>
</div>
<div class="section" id="generating-child-models">
<h3>Generating child models<a class="headerlink" href="#generating-child-models" title="Permalink to this headline">¶</a></h3>
<p>For the moment there’s no explicit support in hypothesis-django for generating
dependent models. i.e. a Company model will generate no Shops. However if you
want to generate some dependent models as well, you can emulate this by using
the <em>flatmap</em> function as follows:</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">hypothesis.strategies</span> <span class="k">import</span> <span class="n">lists</span><span class="p">,</span> <span class="n">just</span>
<span class="k">def</span> <span class="nf">generate_with_shops</span><span class="p">(</span><span class="n">company</span><span class="p">):</span>
<span class="k">return</span> <span class="n">lists</span><span class="p">(</span><span class="n">models</span><span class="p">(</span><span class="n">Shop</span><span class="p">,</span> <span class="n">company</span><span class="o">=</span><span class="n">just</span><span class="p">(</span><span class="n">company</span><span class="p">)))</span><span class="o">.</span><span class="n">map</span><span class="p">(</span><span class="k">lambda</span> <span class="n">_</span><span class="p">:</span> <span class="n">company</span><span class="p">)</span>
<span class="n">company_with_shops_strategy</span> <span class="o">=</span> <span class="n">models</span><span class="p">(</span><span class="n">Company</span><span class="p">)</span><span class="o">.</span><span class="n">flatmap</span><span class="p">(</span><span class="n">generate_with_shops</span><span class="p">)</span>
</pre></div>
</div>
<p>Lets unpack what this is doing:</p>
<p>The way flatmap works is that we draw a value from the original strategy, then
apply a function to it which gives us a new strategy. We then draw a value from
<em>that</em> strategy. So in this case we’re first drawing a company, and then we’re
drawing a list of shops belonging to that company: The <em>just</em> strategy is a
strategy such that drawing it always produces the individual value, so
<code class="docutils literal"><span class="pre">models(Shop,</span> <span class="pre">company=just(company))</span></code> is a strategy that generates a Shop belonging
to the original company.</p>
<p>So the following code would give us a list of shops all belonging to the same
company:</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="n">models</span><span class="p">(</span><span class="n">Company</span><span class="p">)</span><span class="o">.</span><span class="n">flatmap</span><span class="p">(</span><span class="k">lambda</span> <span class="n">c</span><span class="p">:</span> <span class="n">lists</span><span class="p">(</span><span class="n">models</span><span class="p">(</span><span class="n">Shop</span><span class="p">,</span> <span class="n">company</span><span class="o">=</span><span class="n">just</span><span class="p">(</span><span class="n">c</span><span class="p">))))</span>
</pre></div>
</div>
<p>The only difference from this and the above is that we want the company, not
the shops. This is where the inner map comes in. We build the list of shops
and then throw it away, instead returning the company we started for. This
works because the models that Hypothesis generates are saved in the database,
so we’re essentially running the inner strategy purely for the side effect of
creating those children in the database.</p>
</div>
<div class="section" id="using-default-field-values">
<h3>Using default field values<a class="headerlink" href="#using-default-field-values" title="Permalink to this headline">¶</a></h3>
<p>Hypothesis ignores field defaults and always tries to generate values, even if
it doesn’t know how to. You can tell it to use the default value for a field
instead of generating one by passing <code class="docutils literal"><span class="pre">fieldname=default_value</span></code> to
<code class="docutils literal"><span class="pre">models()</span></code>:</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">toystore.models</span> <span class="k">import</span> <span class="n">DefaultCustomish</span>
<span class="gp">>>> </span><span class="n">models</span><span class="p">(</span><span class="n">DefaultCustomish</span><span class="p">)</span><span class="o">.</span><span class="n">example</span><span class="p">()</span>
<span class="go">hypothesis.errors.InvalidArgument: Missing arguments for mandatory field</span>
<span class="go"> customish for model DefaultCustomish</span>
<span class="gp">>>> </span><span class="kn">from</span> <span class="nn">hypothesis.extra.django.models</span> <span class="k">import</span> <span class="n">default_value</span>
<span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="n">models</span><span class="p">(</span><span class="n">DefaultCustomish</span><span class="p">,</span> <span class="n">customish</span><span class="o">=</span><span class="n">default_value</span><span class="p">)</span><span class="o">.</span><span class="n">example</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">x</span><span class="o">.</span><span class="n">customish</span>
<span class="go">'b'</span>
</pre></div>
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