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<div class="section" id="the-writing-and-reporting-of-assertions-in-tests">
<h1>The writing and reporting of assertions in tests<a class="headerlink" href="#the-writing-and-reporting-of-assertions-in-tests" title="Permalink to this headline">¶</a></h1>
<div class="section" id="asserting-with-the-assert-statement">
<span id="assert"></span><span id="assert-with-the-assert-statement"></span><span id="assertfeedback"></span><h2>Asserting with the <code class="docutils literal"><span class="pre">assert</span></code> statement<a class="headerlink" href="#asserting-with-the-assert-statement" title="Permalink to this headline">¶</a></h2>
<p><code class="docutils literal"><span class="pre">pytest</span></code> allows you to use the standard python <code class="docutils literal"><span class="pre">assert</span></code> for verifying
expectations and values in Python tests. For example, you can write the
following:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="c1"># content of test_assert1.py</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">():</span>
<span class="k">return</span> <span class="mi">3</span>
<span class="k">def</span> <span class="nf">test_function</span><span class="p">():</span>
<span class="k">assert</span> <span class="n">f</span><span class="p">()</span> <span class="o">==</span> <span class="mi">4</span>
</pre></div>
</div>
<p>to assert that your function returns a certain value. If this assertion fails
you will see the return value of the function call:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>$ pytest test_assert1.py
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-3.x.y, py-1.x.y, pluggy-0.x.y
rootdir: $REGENDOC_TMPDIR, inifile:
collected 1 item
test_assert1.py F [100%]
================================= FAILURES =================================
______________________________ test_function _______________________________
def test_function():
> assert f() == 4
E assert 3 == 4
E + where 3 = f()
test_assert1.py:5: AssertionError
========================= 1 failed in 0.12 seconds =========================
</pre></div>
</div>
<p><code class="docutils literal"><span class="pre">pytest</span></code> has support for showing the values of the most common subexpressions
including calls, attributes, comparisons, and binary and unary
operators. (See <a class="reference internal" href="example/reportingdemo.html#tbreportdemo"><span class="std std-ref">Demo of Python failure reports with pytest</span></a>). This allows you to use the
idiomatic python constructs without boilerplate code while not losing
introspection information.</p>
<p>However, if you specify a message with the assertion like this:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="k">assert</span> <span class="n">a</span> <span class="o">%</span> <span class="mi">2</span> <span class="o">==</span> <span class="mi">0</span><span class="p">,</span> <span class="s2">"value was odd, should be even"</span>
</pre></div>
</div>
<p>then no assertion introspection takes places at all and the message
will be simply shown in the traceback.</p>
<p>See <a class="reference internal" href="#assert-details"><span class="std std-ref">Advanced assertion introspection</span></a> for more information on assertion introspection.</p>
</div>
<div class="section" id="assertions-about-expected-exceptions">
<span id="assertraises"></span><h2>Assertions about expected exceptions<a class="headerlink" href="#assertions-about-expected-exceptions" title="Permalink to this headline">¶</a></h2>
<p>In order to write assertions about raised exceptions, you can use
<code class="docutils literal"><span class="pre">pytest.raises</span></code> as a context manager like this:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">pytest</span>
<span class="k">def</span> <span class="nf">test_zero_division</span><span class="p">():</span>
<span class="k">with</span> <span class="n">pytest</span><span class="o">.</span><span class="n">raises</span><span class="p">(</span><span class="ne">ZeroDivisionError</span><span class="p">):</span>
<span class="mi">1</span> <span class="o">/</span> <span class="mi">0</span>
</pre></div>
</div>
<p>and if you need to have access to the actual exception info you may use:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">test_recursion_depth</span><span class="p">():</span>
<span class="k">with</span> <span class="n">pytest</span><span class="o">.</span><span class="n">raises</span><span class="p">(</span><span class="ne">RuntimeError</span><span class="p">)</span> <span class="k">as</span> <span class="n">excinfo</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">():</span>
<span class="n">f</span><span class="p">()</span>
<span class="n">f</span><span class="p">()</span>
<span class="k">assert</span> <span class="s1">'maximum recursion'</span> <span class="ow">in</span> <span class="nb">str</span><span class="p">(</span><span class="n">excinfo</span><span class="o">.</span><span class="n">value</span><span class="p">)</span>
</pre></div>
</div>
<p><code class="docutils literal"><span class="pre">excinfo</span></code> is a <code class="docutils literal"><span class="pre">ExceptionInfo</span></code> instance, which is a wrapper around
the actual exception raised. The main attributes of interest are
<code class="docutils literal"><span class="pre">.type</span></code>, <code class="docutils literal"><span class="pre">.value</span></code> and <code class="docutils literal"><span class="pre">.traceback</span></code>.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.0.</span></p>
</div>
<p>In the context manager form you may use the keyword argument
<code class="docutils literal"><span class="pre">message</span></code> to specify a custom failure message:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">with</span> <span class="n">raises</span><span class="p">(</span><span class="ne">ZeroDivisionError</span><span class="p">,</span> <span class="n">message</span><span class="o">=</span><span class="s2">"Expecting ZeroDivisionError"</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">pass</span>
<span class="gp">... </span><span class="n">Failed</span><span class="p">:</span> <span class="n">Expecting</span> <span class="ne">ZeroDivisionError</span>
</pre></div>
</div>
<p>If you want to write test code that works on Python 2.4 as well,
you may also use two other ways to test for an expected exception:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">pytest</span><span class="o">.</span><span class="n">raises</span><span class="p">(</span><span class="n">ExpectedException</span><span class="p">,</span> <span class="n">func</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
<span class="n">pytest</span><span class="o">.</span><span class="n">raises</span><span class="p">(</span><span class="n">ExpectedException</span><span class="p">,</span> <span class="s2">"func(*args, **kwargs)"</span><span class="p">)</span>
</pre></div>
</div>
<p>both of which execute the specified function with args and kwargs and
asserts that the given <code class="docutils literal"><span class="pre">ExpectedException</span></code> is raised. The reporter will
provide you with helpful output in case of failures such as <em>no
exception</em> or <em>wrong exception</em>.</p>
<p>Note that it is also possible to specify a "raises" argument to
<code class="docutils literal"><span class="pre">pytest.mark.xfail</span></code>, which checks that the test is failing in a more
specific way than just having any exception raised:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="nd">@pytest</span><span class="o">.</span><span class="n">mark</span><span class="o">.</span><span class="n">xfail</span><span class="p">(</span><span class="n">raises</span><span class="o">=</span><span class="ne">IndexError</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">test_f</span><span class="p">():</span>
<span class="n">f</span><span class="p">()</span>
</pre></div>
</div>
<p>Using <code class="docutils literal"><span class="pre">pytest.raises</span></code> is likely to be better for cases where you are testing
exceptions your own code is deliberately raising, whereas using
<code class="docutils literal"><span class="pre">@pytest.mark.xfail</span></code> with a check function is probably better for something
like documenting unfixed bugs (where the test describes what "should" happen)
or bugs in dependencies.</p>
<p>Also, the context manager form accepts a <code class="docutils literal"><span class="pre">match</span></code> keyword parameter to test
that a regular expression matches on the string representation of an exception
(like the <code class="docutils literal"><span class="pre">TestCase.assertRaisesRegexp</span></code> method from <code class="docutils literal"><span class="pre">unittest</span></code>):</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">pytest</span>
<span class="k">def</span> <span class="nf">myfunc</span><span class="p">():</span>
<span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s2">"Exception 123 raised"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">test_match</span><span class="p">():</span>
<span class="k">with</span> <span class="n">pytest</span><span class="o">.</span><span class="n">raises</span><span class="p">(</span><span class="ne">ValueError</span><span class="p">,</span> <span class="n">match</span><span class="o">=</span><span class="sa">r</span><span class="s1">'.* 123 .*'</span><span class="p">):</span>
<span class="n">myfunc</span><span class="p">()</span>
</pre></div>
</div>
<p>The regexp parameter of the <code class="docutils literal"><span class="pre">match</span></code> method is matched with the <code class="docutils literal"><span class="pre">re.search</span></code>
function. So in the above example <code class="docutils literal"><span class="pre">match='123'</span></code> would have worked as
well.</p>
</div>
<div class="section" id="assertions-about-expected-warnings">
<span id="assertwarns"></span><h2>Assertions about expected warnings<a class="headerlink" href="#assertions-about-expected-warnings" title="Permalink to this headline">¶</a></h2>
<div class="versionadded">
<p><span class="versionmodified">New in version 2.8.</span></p>
</div>
<p>You can check that code raises a particular warning using
<a class="reference internal" href="warnings.html#warns"><span class="std std-ref">pytest.warns</span></a>.</p>
</div>
<div class="section" id="making-use-of-context-sensitive-comparisons">
<span id="newreport"></span><h2>Making use of context-sensitive comparisons<a class="headerlink" href="#making-use-of-context-sensitive-comparisons" title="Permalink to this headline">¶</a></h2>
<div class="versionadded">
<p><span class="versionmodified">New in version 2.0.</span></p>
</div>
<p><code class="docutils literal"><span class="pre">pytest</span></code> has rich support for providing context-sensitive information
when it encounters comparisons. For example:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="c1"># content of test_assert2.py</span>
<span class="k">def</span> <span class="nf">test_set_comparison</span><span class="p">():</span>
<span class="n">set1</span> <span class="o">=</span> <span class="nb">set</span><span class="p">(</span><span class="s2">"1308"</span><span class="p">)</span>
<span class="n">set2</span> <span class="o">=</span> <span class="nb">set</span><span class="p">(</span><span class="s2">"8035"</span><span class="p">)</span>
<span class="k">assert</span> <span class="n">set1</span> <span class="o">==</span> <span class="n">set2</span>
</pre></div>
</div>
<p>if you run this module:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>$ pytest test_assert2.py
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-3.x.y, py-1.x.y, pluggy-0.x.y
rootdir: $REGENDOC_TMPDIR, inifile:
collected 1 item
test_assert2.py F [100%]
================================= FAILURES =================================
___________________________ test_set_comparison ____________________________
def test_set_comparison():
set1 = set("1308")
set2 = set("8035")
> assert set1 == set2
E AssertionError: assert {'0', '1', '3', '8'} == {'0', '3', '5', '8'}
E Extra items in the left set:
E '1'
E Extra items in the right set:
E '5'
E Use -v to get the full diff
test_assert2.py:5: AssertionError
========================= 1 failed in 0.12 seconds =========================
</pre></div>
</div>
<p>Special comparisons are done for a number of cases:</p>
<ul class="simple">
<li>comparing long strings: a context diff is shown</li>
<li>comparing long sequences: first failing indices</li>
<li>comparing dicts: different entries</li>
</ul>
<p>See the <a class="reference internal" href="example/reportingdemo.html#tbreportdemo"><span class="std std-ref">reporting demo</span></a> for many more examples.</p>
</div>
<div class="section" id="defining-your-own-assertion-comparison">
<h2>Defining your own assertion comparison<a class="headerlink" href="#defining-your-own-assertion-comparison" title="Permalink to this headline">¶</a></h2>
<p>It is possible to add your own detailed explanations by implementing
the <code class="docutils literal"><span class="pre">pytest_assertrepr_compare</span></code> hook.</p>
<dl class="function">
<dt>
<code class="descname">pytest_assertrepr_compare</code><span class="sig-paren">(</span><em>config</em>, <em>op</em>, <em>left</em>, <em>right</em><span class="sig-paren">)</span><a class="reference internal" href="_modules/_pytest/hookspec.html#pytest_assertrepr_compare"><span class="viewcode-link">[source]</span></a></dt>
<dd><p>return explanation for comparisons in failing assert expressions.</p>
<p>Return None for no custom explanation, otherwise return a list
of strings. The strings will be joined by newlines but any newlines
<em>in</em> a string will be escaped. Note that all but the first line will
be indented slightly, the intention is for the first line to be a summary.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><strong>config</strong> (<a class="reference internal" href="writing_plugins.html#_pytest.config.Config" title="_pytest.config.Config"><em>_pytest.config.Config</em></a>) -- pytest config object</td>
</tr>
</tbody>
</table>
</dd></dl>
<p>As an example consider adding the following hook in a <a class="reference internal" href="fixture.html#conftest-py"><span class="std std-ref">conftest.py</span></a>
file which provides an alternative explanation for <code class="docutils literal"><span class="pre">Foo</span></code> objects:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="c1"># content of conftest.py</span>
<span class="kn">from</span> <span class="nn">test_foocompare</span> <span class="k">import</span> <span class="n">Foo</span>
<span class="k">def</span> <span class="nf">pytest_assertrepr_compare</span><span class="p">(</span><span class="n">op</span><span class="p">,</span> <span class="n">left</span><span class="p">,</span> <span class="n">right</span><span class="p">):</span>
<span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">left</span><span class="p">,</span> <span class="n">Foo</span><span class="p">)</span> <span class="ow">and</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">right</span><span class="p">,</span> <span class="n">Foo</span><span class="p">)</span> <span class="ow">and</span> <span class="n">op</span> <span class="o">==</span> <span class="s2">"=="</span><span class="p">:</span>
<span class="k">return</span> <span class="p">[</span><span class="s1">'Comparing Foo instances:'</span><span class="p">,</span>
<span class="s1">' vals: </span><span class="si">%s</span><span class="s1"> != </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">left</span><span class="o">.</span><span class="n">val</span><span class="p">,</span> <span class="n">right</span><span class="o">.</span><span class="n">val</span><span class="p">)]</span>
</pre></div>
</div>
<p>now, given this test module:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="c1"># content of test_foocompare.py</span>
<span class="k">class</span> <span class="nc">Foo</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">val</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">val</span> <span class="o">=</span> <span class="n">val</span>
<span class="k">def</span> <span class="nf">__eq__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">val</span> <span class="o">==</span> <span class="n">other</span><span class="o">.</span><span class="n">val</span>
<span class="k">def</span> <span class="nf">test_compare</span><span class="p">():</span>
<span class="n">f1</span> <span class="o">=</span> <span class="n">Foo</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="n">f2</span> <span class="o">=</span> <span class="n">Foo</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="k">assert</span> <span class="n">f1</span> <span class="o">==</span> <span class="n">f2</span>
</pre></div>
</div>
<p>you can run the test module and get the custom output defined in
the conftest file:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>$ pytest -q test_foocompare.py
F [100%]
================================= FAILURES =================================
_______________________________ test_compare _______________________________
def test_compare():
f1 = Foo(1)
f2 = Foo(2)
> assert f1 == f2
E assert Comparing Foo instances:
E vals: 1 != 2
test_foocompare.py:11: AssertionError
1 failed in 0.12 seconds
</pre></div>
</div>
</div>
<div class="section" id="advanced-assertion-introspection">
<span id="assert-introspection"></span><span id="assert-details"></span><h2>Advanced assertion introspection<a class="headerlink" href="#advanced-assertion-introspection" title="Permalink to this headline">¶</a></h2>
<div class="versionadded">
<p><span class="versionmodified">New in version 2.1.</span></p>
</div>
<p>Reporting details about a failing assertion is achieved by rewriting assert
statements before they are run. Rewritten assert statements put introspection
information into the assertion failure message. <code class="docutils literal"><span class="pre">pytest</span></code> only rewrites test
modules directly discovered by its test collection process, so asserts in
supporting modules which are not themselves test modules will not be rewritten.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p><code class="docutils literal"><span class="pre">pytest</span></code> rewrites test modules on import by using an import
hook to write new <code class="docutils literal"><span class="pre">pyc</span></code> files. Most of the time this works transparently.
However, if you are messing with import yourself, the import hook may
interfere.</p>
<p>If this is the case you have two options:</p>
<ul class="simple">
<li>Disable rewriting for a specific module by adding the string
<code class="docutils literal"><span class="pre">PYTEST_DONT_REWRITE</span></code> to its docstring.</li>
<li>Disable rewriting for all modules by using <code class="docutils literal"><span class="pre">--assert=plain</span></code>.</li>
</ul>
<p class="last">Additionally, rewriting will fail silently if it cannot write new <code class="docutils literal"><span class="pre">.pyc</span></code> files,
i.e. in a read-only filesystem or a zipfile.</p>
</div>
<p>For further information, Benjamin Peterson wrote up <a class="reference external" href="http://pybites.blogspot.com/2011/07/behind-scenes-of-pytests-new-assertion.html">Behind the scenes of pytest's new assertion rewriting</a>.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 2.1: </span>Add assert rewriting as an alternate introspection technique.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 2.1: </span>Introduce the <code class="docutils literal"><span class="pre">--assert</span></code> option. Deprecate <code class="docutils literal"><span class="pre">--no-assert</span></code> and
<code class="docutils literal"><span class="pre">--nomagic</span></code>.</p>
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<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.0: </span>Removes the <code class="docutils literal"><span class="pre">--no-assert</span></code> and <code class="docutils literal"><span class="pre">--nomagic</span></code> options.
Removes the <code class="docutils literal"><span class="pre">--assert=reinterp</span></code> option.</p>
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<li><a class="reference internal" href="#">The writing and reporting of assertions in tests</a><ul>
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