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‎05-pytest.html

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<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8">
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<meta name="generator" content="pandoc">
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<title>Software Carpentry: Testing</title>
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<body class="lesson">
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<div class="container card">
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<div class="banner">
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<a href="http://software-carpentry.org" title="Software Carpentry">
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<img alt="Software Carpentry banner" src="img/software-carpentry-banner.png" />
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</a>
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</div>
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<article>
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<div class="row">
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<div class="col-md-10 col-md-offset-1">
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<a href="index.html"><h1 class="title">Testing</h1></a>
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<h2 class="subtitle">Running Tests with pytest</h2>
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<section class="objectives panel panel-warning">
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<div class="panel-heading">
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<h2 id="learning-objectives"><span class="glyphicon glyphicon-certificate"></span>Learning Objectives</h2>
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</div>
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<div class="panel-body">
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<ul>
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<li>Understand how to run a test suite using the pytest framework</li>
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<li>Understand how to read the output of a pytest test suite</li>
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</ul>
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</div>
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</section>
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<p>We created a suite of tests for our mean function, but it was annoying to run them one at a time. It would be a lot better if there were some way to run them all at once, just reporting which tests fail and which succeed.</p>
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<p>Thankfully, that exists. Recall our tests:</p>
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<div class="sourceCode"><pre class="sourceCode python"><code class="sourceCode python"><span class="im">from</span> mean <span class="im">import</span> <span class="op">*</span>
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<span class="kw">def</span> test_ints():
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num_list <span class="op">=</span> [<span class="dv">1</span>,<span class="dv">2</span>,<span class="dv">3</span>,<span class="dv">4</span>,<span class="dv">5</span>]
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obs <span class="op">=</span> mean(num_list)
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exp <span class="op">=</span> <span class="dv">3</span>
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<span class="cf">assert</span> obs <span class="op">==</span> exp
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<span class="kw">def</span> test_zero():
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num_list<span class="op">=</span>[<span class="dv">0</span>,<span class="dv">2</span>,<span class="dv">4</span>,<span class="dv">6</span>]
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obs <span class="op">=</span> mean(num_list)
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exp <span class="op">=</span> <span class="dv">3</span>
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<span class="cf">assert</span> obs <span class="op">==</span> exp
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<span class="kw">def</span> test_double():
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<span class="co"># This one will fail in Python 2</span>
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num_list<span class="op">=</span>[<span class="dv">1</span>,<span class="dv">2</span>,<span class="dv">3</span>,<span class="dv">4</span>]
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obs <span class="op">=</span> mean(num_list)
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exp <span class="op">=</span> <span class="fl">2.5</span>
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<span class="cf">assert</span> obs <span class="op">==</span> exp
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<span class="kw">def</span> test_long():
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big <span class="op">=</span> <span class="dv">100000000</span>
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obs <span class="op">=</span> mean(<span class="bu">range</span>(<span class="dv">1</span>,big))
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exp <span class="op">=</span> big<span class="op">/</span><span class="fl">2.0</span>
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<span class="cf">assert</span> obs <span class="op">==</span> exp
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<span class="kw">def</span> test_complex():
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<span class="co"># given that complex numbers are an unordered field</span>
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<span class="co"># the arithmetic mean of complex numbers is meaningless</span>
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num_list <span class="op">=</span> [<span class="dv">2</span> <span class="op">+</span> 3j, <span class="dv">3</span> <span class="op">+</span> 4j, <span class="op">-</span><span class="dv">32</span> <span class="op">-</span> 2j]
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obs <span class="op">=</span> mean(num_list)
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exp <span class="op">=</span> <span class="va">NotImplemented</span>
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<span class="cf">assert</span> obs <span class="op">==</span> exp</code></pre></div>
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<p>Once these tests are written in a file called <code>test_mean.py</code>, the command <code>py.test</code> can be called from the directory containing the tests:</p>
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<div class="sourceCode"><pre class="sourceCode bash"><code class="sourceCode bash">$ <span class="kw">py.test</span></code></pre></div>
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<pre class="output"><code>collected 5 items
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test_mean.py ....F
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================================== FAILURES ===================================
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________________________________ test_complex _________________________________
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def test_complex():
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# given that complex numbers are an unordered field
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# the arithmetic mean of complex numbers is meaningless
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num_list = [2 + 3j, 3 + 4j, -32 - 2j]
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obs = mean(num_list)
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exp = NotImplemented
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&gt; assert obs == exp
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E assert (-9+1.6666666666666667j) == NotImplemented
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test_mean.py:34: AssertionError
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===================== 1 failed, 4 passed in 2.71 seconds ======================</code></pre>
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<p>In the above case, the pytest package ‘sniffed-out’ the tests in the directory and ran them together to produce a report of the sum of the files and functions matching the regular expression <code>[Tt]est[-_]*</code>.</p>
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<p>The major boon a testing framework provides is exactly that, a utility to find and run the tests automatically. With pytest, this is the command-line tool called <code>py.test</code>. When <code>py.test</code> is run, it will search all directories below where it was called, find all of the Python files in these directories whose names start or end with <code>test</code>, import them, and run all of the functions and classes whose names start with <code>test</code> or <code>Test</code>. This automatic registration of test code saves tons of human time and allows us to focus on what is important: writing more tests.</p>
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<p>When you run <code>py.test</code>, it will print a dot (<code>.</code>) on the screen for every test that passes, an <code>F</code> for every test that fails or where there was an unexpected error. In rarer situations you may also see an <code>s</code> indicating a skipped tests (because the test is not applicable on your system) or a <code>x</code> for a known failure (because the developers could not fix it promptly). After the dots, pytest will print summary information.</p>
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<section class="challenge panel panel-success">
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<div class="panel-heading">
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<h2 id="fix-the-failing-code"><span class="glyphicon glyphicon-pencil"></span>Fix The Failing Code</h2>
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</div>
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<div class="panel-body">
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<p>Without changing the tests, alter the mean.py file from the previous section until it passes. When it passes, <code>py.test</code> will produce results like the following:</p>
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<div class="sourceCode"><pre class="sourceCode bash"><code class="sourceCode bash">$ <span class="kw">py.test</span></code></pre></div>
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<p><sub>~</sub> {.output} collected 5 items</p>
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<p>test_mean.py …..</p>
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<p>========================== 5 passed in 2.68 seconds ===========================</p>
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</div>
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</section>
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<p>As we write more code, we would write more tests, and pytest would produce more dots. Each passing test is a small, satisfying reward for having written quality scientific software. Now that you know how to write tests, let’s go into what can go wrong.</p>
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