Hykilpikonna
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<header id="title-block-header">
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<h1 class="title">B.2 <code>pytest</code></h1>
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</header>
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<section>
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<p><code>pytest</code> is a Python library used to run tests for your code. In this section, we’ll describe how to write tests that are automatically discovered and run by <code>pytest</code>, how to actually run <code>pytest</code> in your code, and some tips and tricks for making the most of <code>pytest</code>.</p>
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<h2 id="how-do-you-write-a-pytest-test">How do you write a <code>pytest</code> test?</h2>
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<p>A <strong>test</strong> in <code>pytest</code> is a Python function whose name starts with <code>test_</code>. Inside a test function, we use <code>assert</code> statements to verify expected values or behaviours of a function.</p>
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<p>For example:</p>
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<div class="sourceCode" id="cb1"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb1-1"><a href="#cb1-1"></a><span class="co"># This is the function to test</span></span>
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<span id="cb1-2"><a href="#cb1-2"></a><span class="kw">def</span> has_more_trues(booleans: <span class="bu">list</span>) <span class="op">-></span> <span class="bu">bool</span>:</span>
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<span id="cb1-3"><a href="#cb1-3"></a> <span class="co">"""Return whether booleans contains more True values than False values.</span></span>
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<span id="cb1-4"><a href="#cb1-4"></a></span>
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<span id="cb1-5"><a href="#cb1-5"></a><span class="co"> >>> has_more_trues([True, False, True])</span></span>
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<span id="cb1-6"><a href="#cb1-6"></a><span class="co"> True</span></span>
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<span id="cb1-7"><a href="#cb1-7"></a><span class="co"> >>> has_more_trues([True, False, False])</span></span>
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<span id="cb1-8"><a href="#cb1-8"></a><span class="co"> False</span></span>
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<span id="cb1-9"><a href="#cb1-9"></a><span class="co"> """</span></span>
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<span id="cb1-10"><a href="#cb1-10"></a> <span class="co"># Function body omitted</span></span>
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<span id="cb1-11"><a href="#cb1-11"></a></span>
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<span id="cb1-12"><a href="#cb1-12"></a></span>
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<span id="cb1-13"><a href="#cb1-13"></a><span class="co"># This the test</span></span>
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<span id="cb1-14"><a href="#cb1-14"></a><span class="kw">def</span> test_mixture_one_more_true() <span class="op">-></span> <span class="va">None</span>:</span>
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<span id="cb1-15"><a href="#cb1-15"></a> <span class="co">"""Test has_more_trues on a list with a mixture of True and False,</span></span>
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<span id="cb1-16"><a href="#cb1-16"></a><span class="co"> with one more True than False.</span></span>
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<span id="cb1-17"><a href="#cb1-17"></a><span class="co"> """</span></span>
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<span id="cb1-18"><a href="#cb1-18"></a> <span class="cf">assert</span> has_more_trues([<span class="va">True</span>, <span class="va">False</span>, <span class="va">True</span>])</span></code></pre></div>
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<p>A single test can have multiple <code>assert</code> statements, although it is generally recommended to separate each <code>assert</code> statement into a separate test. A single Python file can have multiple tests; when <code>pytest</code> is run on a file, it (by default) runs all the tests in that file.</p>
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<h2 id="running-pytest">Running <code>pytest</code></h2>
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<p>The simplest way of running <code>pytest</code> is to add the following <code>if __name__ == '__main__'</code> block to the bottom of a test file:</p>
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<div class="sourceCode" id="cb2"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb2-1"><a href="#cb2-1"></a><span class="cf">if</span> <span class="va">__name__</span> <span class="op">==</span> <span class="st">'__main__'</span>:</span>
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<span id="cb2-2"><a href="#cb2-2"></a> <span class="im">import</span> pytest</span>
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<span id="cb2-3"><a href="#cb2-3"></a> pytest.main()</span></code></pre></div>
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<p>When you run this file, <code>pytest.main</code> will run all test functions in the file. <em>Note</em>: by default, <code>pytest.main</code> actually searches through <em>all</em> Python files in the current directory whose name starts with <code>test_</code> or ends with <code>_test</code>, which can be a bit surprising. So our practice will be to explicitly pass in the name of the current test file to <code>pytest.main</code>, wrapped in a list:</p>
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<div class="sourceCode" id="cb3"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb3-1"><a href="#cb3-1"></a><span class="co"># If we're in a file test_my_file.py</span></span>
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<span id="cb3-2"><a href="#cb3-2"></a></span>
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<span id="cb3-3"><a href="#cb3-3"></a><span class="cf">if</span> <span class="va">__name__</span> <span class="op">==</span> <span class="st">'__main__'</span>:</span>
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<span id="cb3-4"><a href="#cb3-4"></a> <span class="im">import</span> pytest</span>
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<span id="cb3-5"><a href="#cb3-5"></a> pytest.main([<span class="st">'test_my_file.py'</span>])</span></code></pre></div>
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<h2 id="testing-for-an-exceptions">Testing for an exceptions</h2>
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<p>It is possible to write a <code>pytest</code> test that checks whether a function raises a specific error. To do so, use <code>pytest.raises</code>, which takes an error type as an argument, inside a <code>with</code> statement. Here is an example</p>
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<div class="sourceCode" id="cb4"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb4-1"><a href="#cb4-1"></a><span class="im">import</span> pytest</span>
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<span id="cb4-2"><a href="#cb4-2"></a></span>
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<span id="cb4-3"><a href="#cb4-3"></a></span>
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<span id="cb4-4"><a href="#cb4-4"></a><span class="kw">def</span> add_one(n):</span>
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<span id="cb4-5"><a href="#cb4-5"></a> <span class="cf">return</span> n <span class="op">+</span> <span class="dv">1</span></span>
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<span id="cb4-6"><a href="#cb4-6"></a></span>
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<span id="cb4-7"><a href="#cb4-7"></a></span>
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<span id="cb4-8"><a href="#cb4-8"></a><span class="kw">def</span> test_add_one_type_error():</span>
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<span id="cb4-9"><a href="#cb4-9"></a> <span class="co">"""Test add_one when given a non-numeric argument."""</span></span>
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<span id="cb4-10"><a href="#cb4-10"></a> <span class="cf">with</span> pytest.raises(<span class="pp">TypeError</span>):</span>
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<span id="cb4-11"><a href="#cb4-11"></a> add_one(<span class="st">'hello'</span>)</span></code></pre></div>
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<h2 id="options-for-pytest.main">Options for <code>pytest.main</code></h2>
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<p><code>pytest.main</code> takes a list of strings as an argument because users can add options (as strings) to modify <code>pytest</code>’s default behaviour when running tests. The format for this is <code>pytest.main([<option1>, <option2>, ...])</code>.</p>
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<p>Here are some useful options:</p>
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<ul>
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<li><code>'<filename>'</code>: as we saw above, adding a filename restricts <code>pytest</code> to only running the tests in that Python file.</li>
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<li><code>'<filename>::<test_name>'</code>: restrict <code>pytest</code> to run a specific test in the given file (e.g., <code>'test_my_file.py::test_1'</code>)</li>
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<li><code>'-x'</code>: stop running tests after the first failure (by default, <code>pytest</code> runs all tests, regardless of the number of failures)</li>
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<li><code>'--pdb'</code>: start the Python debugger when a test fails</li>
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</ul>
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<h2 id="references">References</h2>
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<p>For the full documentation for the <code>pytest</code> library, check out <a href="https://docs.pytest.org/en/latest/" class="uri">https://docs.pytest.org/en/latest/</a>.</p>
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</section>
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