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161 lines
11 KiB
161 lines
11 KiB
11 months ago
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<a href="https://blog.juliobiason.me"><h1>Julio Biason .Me 4.3</h1></a>
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<p class="lead">Old school dev living in a 2.0 dev world</p>
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<div class="post">
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<h1 class="post-title">Mocking A Mock</h1>
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<span class="post-date">
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2016-07-21
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<a href="https://blog.juliobiason.me/tags/python/">#python</a>
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<a href="https://blog.juliobiason.me/tags/mock/">#mock</a>
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<a href="https://blog.juliobiason.me/tags/mongodb/">#mongodb</a>
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<a href="https://blog.juliobiason.me/tags/find/">#find</a>
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<a href="https://blog.juliobiason.me/tags/count/">#count</a>
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</span>
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<p>Mocks are an important part of testing, but learn how to properly mock stuff.</p>
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<span id="continue-reading"></span>
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<p>A few weeks ago we had a test failing. Now, tests failing is not something
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worth a blog post, but the solution -- and the reason it was failing -- is.</p>
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<p>A few background information first: The test is part of our Django project;
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this project stores part of the information on MongoDB, because the data is
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schemaless -- it comes from different sources and each source has its own
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format. Because MongoDB is external to our project, it had to be mocked
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(sidenote: mocks are there exactly to do this: the avoid having to manage
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something external to your project).</p>
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<p>PyMongo, the MongoDB driver for Python, has a <code>find()</code> function, pretty much
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like the MongoDB API; this function returns a list (or iterator, I guess) with
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all the result records in the collection. Because it is a list (iterator,
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whatever), it has a <code>count()</code> function that returns the number of records. So
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you have something like this:</p>
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<pre data-lang="javascript" style="background-color:#2b303b;color:#c0c5ce;" class="language-javascript "><code class="language-javascript" data-lang="javascript"><span style="color:#bf616a;">connector</span><span>.</span><span style="color:#bf616a;">collection</span><span>.</span><span style="color:#96b5b4;">find</span><span>({'</span><span style="color:#a3be8c;">field</span><span>': '</span><span style="color:#a3be8c;">value</span><span>'}).</span><span style="color:#8fa1b3;">count</span><span>()
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</span></code></pre>
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<p>(Find everything which has a field named "field" that has a value of "value"
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and count the results. Pretty simple, right?)</p>
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<p>The second hand of information you need is about the <code>mock</code> module. Python 3
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has a module for mocking external resources, which is also available to Python 2.
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The interface is the same, so you can
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<a href="https://docs.python.org/dev/library/unittest.mock.html">refer to the Python 3 documentation</a>
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for both versions.</p>
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<p>An usage example would be something like this: If I had a function like:</p>
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<pre data-lang="python" style="background-color:#2b303b;color:#c0c5ce;" class="language-python "><code class="language-python" data-lang="python"><span style="color:#b48ead;">def </span><span style="color:#8fa1b3;">request</span><span>():
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</span><span> </span><span style="color:#b48ead;">return </span><span>connector.collection.</span><span style="color:#bf616a;">find</span><span>({'</span><span style="color:#a3be8c;">field</span><span>': '</span><span style="color:#a3be8c;">value</span><span>'})
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</span></code></pre>
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<p>and I want to test it, I could this:</p>
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<pre data-lang="python" style="background-color:#2b303b;color:#c0c5ce;" class="language-python "><code class="language-python" data-lang="python"><span style="color:#b48ead;">class </span><span style="color:#ebcb8b;">TestRequest</span><span style="color:#eff1f5;">(</span><span style="color:#a3be8c;">unittest.TestCase</span><span style="color:#eff1f5;">):
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</span><span> @</span><span style="color:#bf616a;">patch</span><span>("</span><span style="color:#a3be8c;">MyModule.connector.collection.find</span><span>")
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</span><span> </span><span style="color:#b48ead;">def </span><span style="color:#8fa1b3;">test_request</span><span>(</span><span style="color:#bf616a;">self</span><span>, </span><span style="color:#bf616a;">mocked_find</span><span>):
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</span><span> mocked_find.return_value = [{'</span><span style="color:#a3be8c;">field</span><span>': '</span><span style="color:#a3be8c;">value</span><span>', '</span><span style="color:#a3be8c;">record</span><span>': </span><span style="color:#d08770;">1</span><span>},
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</span><span> {'</span><span style="color:#a3be8c;">field</span><span>': '</span><span style="color:#a3be8c;">value</span><span>', '</span><span style="color:#a3be8c;">record</span><span>': </span><span style="color:#d08770;">2</span><span>}]
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</span><span> result = </span><span style="color:#bf616a;">request</span><span>()
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</span><span> </span><span style="color:#bf616a;">self</span><span>.</span><span style="color:#bf616a;">assertDictEqual</span><span>(result, mocked_find.return_value)
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</span></code></pre>
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<p>Kinda sketchy for a test, but I just want to use to explain what is going on:
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the <code>@patch</code> decorator is creating a stub for any call for
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<code>MyModule.connector.collection.find</code>; inside the test itself, the stub is
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being converted to a mock by setting a <code>return_value</code>; when the test is run,
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the mock library will intercept a call to the <code>collection.find</code> inside
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<code>MyModule.connector</code> (because that module imported PyMongo driver to its
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namespace as <code>connector</code>) and return the <code>return_value</code> instead.</p>
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<p>Simple when someone explains like this, right? Well, at least I hope you got
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the basics of this mocked stuff.</p>
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<p>Now, what if you had to count the number of results? It's pretty damn easy to
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realize how to do so: just call <code>count()</code> on the resulting list, or make it
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return an object that has a <code>count()</code> property. </p>
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<p>The whole problem we had was that the result of <code>find()</code> was irrelevant and
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all we wanted was the count. Something like</p>
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<pre data-lang="python" style="background-color:#2b303b;color:#c0c5ce;" class="language-python "><code class="language-python" data-lang="python"><span style="color:#b48ead;">def </span><span style="color:#8fa1b3;">has_values</span><span>():
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</span><span> elements = connector.collection.</span><span style="color:#bf616a;">find</span><span>({'</span><span style="color:#a3be8c;">field</span><span>': '</span><span style="color:#a3be8c;">value</span><span>'}).</span><span style="color:#bf616a;">count</span><span>()
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</span><span> </span><span style="color:#b48ead;">return </span><span>elements > </span><span style="color:#d08770;">1
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</span></code></pre>
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<p>First of all, you can't patch <code>MyModule.connector.collection.find.count</code>
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because you'll only stub the <code>count</code> call, not <code>find</code>, which will actually try
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to connect on MongoDB; so the original patch is required. And you can't patch
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both <code>find</code> and <code>count</code> because the first patch will return a new <code>MagicMock</code>
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object, which will not be patched (after all, it is <em>another</em> object). The
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original developer tried to fix it this way:</p>
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<pre data-lang="python" style="background-color:#2b303b;color:#c0c5ce;" class="language-python "><code class="language-python" data-lang="python"><span>mocked_find.count.return_value = </span><span style="color:#d08770;">0
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</span></code></pre>
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<p>... which, again, doesn't work because the call to <code>find()</code> will return a
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<code>MagicMock</code> that doesn't have its <code>count</code> patched. But the developer never
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realized that because <code>MagicMock</code> tries its best to <em>not</em> blow up your tests,
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including having return values to conversions like... int. And it will always
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return 1.</p>
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<p>Is your head spinning yet? Mine sure did when I realized the whole mess it was
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being made. And let me repeat this: The problem was <em>not</em> that MongoDB was
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being mocked, but that it was being <em>mocked in the wrong way</em>.</p>
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<p>The solution? As pointed above, make <code>find</code> return an object with a <code>count</code>
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method.</p>
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<pre data-lang="python" style="background-color:#2b303b;color:#c0c5ce;" class="language-python "><code class="language-python" data-lang="python"><span>count_mock = </span><span style="color:#bf616a;">MagicMock</span><span>(</span><span style="color:#bf616a;">return_value</span><span>=</span><span style="color:#d08770;">0</span><span>)
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</span><span>mocked_find.return_value = </span><span style="color:#bf616a;">MagicMock</span><span>(
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</span><span> **{'</span><span style="color:#a3be8c;">count</span><span>': count_mock})
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</span></code></pre>
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