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<li class="toctree-l1 current"><a class="current reference internal" href="#">Reproducing Failures</a><ul>
<li class="toctree-l2"><a class="reference internal" href="#providing-explicit-examples">Providing explicit examples</a></li>
<li class="toctree-l2"><a class="reference internal" href="#reproducing-a-test-run-with-seed">Reproducing a test run with <code class="docutils literal"><span class="pre">@seed</span></code></a></li>
<li class="toctree-l2"><a class="reference internal" href="#reproducing-an-example-with-with-reproduce-failure">Reproducing an example with with <code class="docutils literal"><span class="pre">@reproduce_failure</span></code></a></li>
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<div class="section" id="reproducing-failures">
<h1>Reproducing Failures<a class="headerlink" href="#reproducing-failures" title="Permalink to this headline">¶</a></h1>
<p>One of the things that is often concerning for people using randomized testing
like Hypothesis is the question of how to reproduce failing test cases.</p>
<p>Fortunately Hypothesis has a number of features in support of this. The one you
will use most commonly when developing locally is <cite>the example database <database></cite>,
which means that you shouldn’t have to think about the problem at all for local
use - test failures will just automatically reproduce without you having to do
anything.</p>
<p>The example database is perfectly suitable for sharing between machines, but
there currently aren’t very good work flows for that, so Hypothesis provides a
number of ways to make examples reproducible by adding them to the source code
of your tests. This is particularly useful when e.g. you are trying to run an
example that has failed on your CI, or otherwise share them between machines.</p>
<div class="section" id="providing-explicit-examples">
<span id="id1"></span><h2>Providing explicit examples<a class="headerlink" href="#providing-explicit-examples" title="Permalink to this headline">¶</a></h2>
<p>You can explicitly ask Hypothesis to try a particular example, using</p>
<dl class="function">
<dt id="hypothesis.example">
<code class="descclassname">hypothesis.</code><code class="descname">example</code><span class="sig-paren">(</span><em>*args</em>, <em>**kwargs</em><span class="sig-paren">)</span><a class="reference internal" href="_modules/hypothesis/core.html#example"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#hypothesis.example" title="Permalink to this definition">¶</a></dt>
<dd><p>A decorator which ensures a specific example is always tested.</p>
</dd></dl>
<p>Hypothesis will run all examples you’ve asked for first. If any of them fail it
will not go on to look for more examples.</p>
<p>It doesn’t matter whether you put the example decorator before or after given.
Any permutation of the decorators in the above will do the same thing.</p>
<p>Note that examples can be positional or keyword based. If they’re positional then
they will be filled in from the right when calling, so either of the following
styles will work as expected:</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="nd">@given</span><span class="p">(</span><span class="n">text</span><span class="p">())</span>
<span class="nd">@example</span><span class="p">(</span><span class="s2">"Hello world"</span><span class="p">)</span>
<span class="nd">@example</span><span class="p">(</span><span class="n">x</span><span class="o">=</span><span class="s2">"Some very long string"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">test_some_code</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>
<span class="k">assert</span> <span class="kc">True</span>
<span class="kn">from</span> <span class="nn">unittest</span> <span class="k">import</span> <span class="n">TestCase</span>
<span class="k">class</span> <span class="nc">TestThings</span><span class="p">(</span><span class="n">TestCase</span><span class="p">):</span>
<span class="nd">@given</span><span class="p">(</span><span class="n">text</span><span class="p">())</span>
<span class="nd">@example</span><span class="p">(</span><span class="s2">"Hello world"</span><span class="p">)</span>
<span class="nd">@example</span><span class="p">(</span><span class="n">x</span><span class="o">=</span><span class="s2">"Some very long string"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">test_some_code</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">):</span>
<span class="k">assert</span> <span class="kc">True</span>
</pre></div>
</div>
<p>As with <code class="docutils literal"><span class="pre">@given</span></code>, it is not permitted for a single example to be a mix of
positional and keyword arguments.
Either are fine, and you can use one in one example and the other in another
example if for some reason you really want to, but a single example must be
consistent.</p>
</div>
<div class="section" id="reproducing-a-test-run-with-seed">
<h2>Reproducing a test run with <code class="docutils literal"><span class="pre">@seed</span></code><a class="headerlink" href="#reproducing-a-test-run-with-seed" title="Permalink to this headline">¶</a></h2>
<dl class="function">
<dt id="hypothesis.seed">
<code class="descclassname">hypothesis.</code><code class="descname">seed</code><span class="sig-paren">(</span><em>seed</em><span class="sig-paren">)</span><a class="reference internal" href="_modules/hypothesis/core.html#seed"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#hypothesis.seed" title="Permalink to this definition">¶</a></dt>
<dd><p>seed: Start the test execution from a specific seed.</p>
<p>May be any hashable object. No exact meaning for seed is provided
other than that for a fixed seed value Hypothesis will try the same
actions (insofar as it can given external sources of non-
determinism. e.g. timing and hash randomization). Overrides the
derandomize setting if it is present.</p>
</dd></dl>
<p>When a test fails unexpectedly, usually due to a health check failure,
Hypothesis will print out a seed that led to that failure, if the test is not
already running with a fixed seed. You can then recreate that failure using either
the <code class="docutils literal"><span class="pre">@seed</span></code> decorator or (if you are running <a class="reference external" href="https://pypi.python.org/pypi/pytest">pytest</a>) with
<code class="docutils literal"><span class="pre">--hypothesis-seed</span></code>.</p>
</div>
<div class="section" id="reproducing-an-example-with-with-reproduce-failure">
<span id="reproduce-failure"></span><h2>Reproducing an example with with <code class="docutils literal"><span class="pre">@reproduce_failure</span></code><a class="headerlink" href="#reproducing-an-example-with-with-reproduce-failure" title="Permalink to this headline">¶</a></h2>
<p>Hypothesis has an opaque binary representation that it uses for all examples it
generates. This representation is not intended to be stable across versions or
with respect to changes in the test, but can be used to to reproduce failures
with the <code class="docutils literal"><span class="pre">@reproduce_example</span></code> decorator.</p>
<dl class="function">
<dt id="hypothesis.reproduce_failure">
<code class="descclassname">hypothesis.</code><code class="descname">reproduce_failure</code><span class="sig-paren">(</span><em>version</em>, <em>blob</em><span class="sig-paren">)</span><a class="reference internal" href="_modules/hypothesis/core.html#reproduce_failure"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#hypothesis.reproduce_failure" title="Permalink to this definition">¶</a></dt>
<dd><p>Run the example that corresponds to this data blob in order to reproduce
a failure.</p>
<p>A test with this decorator <em>always</em> runs only one example and always fails.
If the provided example does not cause a failure, or is in some way invalid
for this test, then this will fail with a DidNotReproduce error.</p>
<p>This decorator is not intended to be a permanent addition to your test
suite. It’s simply some code you can add to ease reproduction of a problem
in the event that you don’t have access to the test database. Because of
this, <em>no</em> compatibility guarantees are made between different versions of
Hypothesis - its API may change arbitrarily from version to version.</p>
</dd></dl>
<p>The intent is that you should never write this decorator by hand, but it is
instead provided by Hypothesis.
When a test fails with a falsifying example, Hypothesis may print out a
suggestion to use <code class="docutils literal"><span class="pre">@reproduce_failure</span></code> on the test to recreate the problem
as follows:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">hypothesis</span> <span class="k">import</span> <span class="n">settings</span><span class="p">,</span> <span class="n">given</span><span class="p">,</span> <span class="n">PrintSettings</span>
<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">hypothesis.strategies</span> <span class="k">as</span> <span class="nn">st</span>
<span class="gp">>>> </span><span class="nd">@given</span><span class="p">(</span><span class="n">st</span><span class="o">.</span><span class="n">floats</span><span class="p">())</span>
<span class="gp">... </span><span class="nd">@settings</span><span class="p">(</span><span class="n">print_blob</span><span class="o">=</span><span class="n">PrintSettings</span><span class="o">.</span><span class="n">ALWAYS</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">f</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">assert</span> <span class="n">f</span> <span class="o">==</span> <span class="n">f</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="k">try</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">test</span><span class="p">()</span>
<span class="gp">... </span><span class="k">except</span> <span class="ne">AssertionError</span><span class="p">:</span>
<span class="gp">... </span> <span class="k">pass</span>
<span class="go">Falsifying example: test(f=nan)</span>
<span class="go">You can reproduce this example by temporarily adding @reproduce_failure(..., b'AAD/8AAAAAAAAQA=') as a decorator on your test case</span>
</pre></div>
</div>
<p>Adding the suggested decorator to the test should reproduce the failure (as
long as everything else is the same - changing the versions of Python or
anything else involved, might of course affect the behaviour of the test! Note
that changing the version of Hypothesis will result in a different error -
each <code class="docutils literal"><span class="pre">@reproduce_failure</span></code> invocation is specific to a Hypothesis version).</p>
<p>When to do this is controlled by the <code class="xref py py-attr docutils literal"><span class="pre">print_blob</span></code>
setting, which may be one of the following values:</p>
<dl class="class">
<dt id="hypothesis.PrintSettings">
<em class="property">class </em><code class="descclassname">hypothesis.</code><code class="descname">PrintSettings</code><a class="reference internal" href="_modules/hypothesis/_settings.html#PrintSettings"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#hypothesis.PrintSettings" title="Permalink to this definition">¶</a></dt>
<dd><p>Flags to determine whether or not to print a detailed example blob to
use with <a class="reference internal" href="#hypothesis.reproduce_failure" title="hypothesis.reproduce_failure"><code class="xref py py-func docutils literal"><span class="pre">reproduce_failure()</span></code></a> for failing test cases.</p>
</dd></dl>
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