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} code > span.ot { color: #007020; } code > span.al { color: #ff0000; font-weight: bold; } code > span.fu { color: #900; font-weight: bold; } code > span.er { color: #a61717; background-color: #e3d2d2; } </style> </head> <body> <h1 class="title toc-ignore">Custom expectations</h1> <p>This vignette shows you how to create custom expectations that work identically to the built-in <code>expect_</code> functions. Since these functions will need to be loaded when your package is loaded for testing, it is recommended that <code>expect_</code> functions be defined in <code>test-helpers.R</code> in your packages <code>R/</code> directory.</p> <div id="creating-an-expectation" class="section level2"> <h2>Creating an expectation</h2> <p>There are three main parts to writing an expectation, as illustrated by <code>expect_length()</code>:</p> <div class="sourceCode" id="cb1"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a>expect_length <span class="ot"><-</span> <span class="cf">function</span>(object, n) {</span> <span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a> <span class="co"># 1. Capture object and label</span></span> <span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a> act <span class="ot"><-</span> <span class="fu">quasi_label</span>(rlang<span class="sc">::</span><span class="fu">enquo</span>(object), <span class="at">arg =</span> <span class="st">"object"</span>)</span> <span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a></span> <span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a> <span class="co"># 2. Call expect()</span></span> <span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a> act<span class="sc">$</span>n <span class="ot"><-</span> <span class="fu">length</span>(act<span class="sc">$</span>val)</span> <span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a> <span class="fu">expect</span>(</span> <span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a> act<span class="sc">$</span>n <span class="sc">==</span> n,</span> <span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a> <span class="fu">sprintf</span>(<span class="st">"%s has length %i, not length %i."</span>, act<span class="sc">$</span>lab, act<span class="sc">$</span>n, n)</span> <span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a> )</span> <span id="cb1-11"><a href="#cb1-11" aria-hidden="true" tabindex="-1"></a></span> <span id="cb1-12"><a href="#cb1-12" aria-hidden="true" tabindex="-1"></a> <span class="co"># 3. Invisibly return the value</span></span> <span id="cb1-13"><a href="#cb1-13" aria-hidden="true" tabindex="-1"></a> <span class="fu">invisible</span>(act<span class="sc">$</span>val)</span> <span id="cb1-14"><a href="#cb1-14" aria-hidden="true" tabindex="-1"></a>}</span></code></pre></div> </div> <div id="quasi-labelling" class="section level2"> <h2>Quasi-labelling</h2> <p>The first step in any expectation is to capture the actual object, and generate a label for it to use if a failure occur. All testthat expectations support quasiquotation so that you can unquote variables. This makes it easier to generate good labels when the expectation is called from a function or within a for loop.</p> <p>By convention, the first argument to every <code>expect_</code> function is called <code>object</code>, and you capture it’s value (<code>val</code>) and label (<code>lab</code>) with <code>act <- quasi_label(enquo(object))</code>, where <code>act</code> is short for actual.</p> <div id="verify-the-expectation" class="section level3"> <h3>Verify the expectation</h3> <p>Next, you should verify the expectation. This often involves a little computation (here just figuring out the <code>length</code>), and you should typically store the results back into the <code>act</code> object.</p> <p>Next you call <code>expect()</code>. This has two arguments:</p> <ol style="list-style-type: decimal"> <li><p><code>ok</code>: was the expectation successful? This is usually easy to write</p></li> <li><p><code>failure_message</code>: What informative error message should be reported to the user so that they can diagnose the problem. This is often hard to write!</p> <p>For historical reasons, most built-in expectations generate these with <code>sprintf()</code>, but today I’d recommend using the <a href="https://glue.tidyverse.org">glue</a> package</p></li> </ol> </div> <div id="invisibly-return-the-input" class="section level3"> <h3>Invisibly return the input</h3> <p>Expectation functions are called primarily for their side-effects (triggering a failure), so should invisibly return their input, <code>act$val</code>. This allows expectations to be chained:</p> <div class="sourceCode" id="cb2"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a>mtcars <span class="sc">%>%</span></span> <span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a> <span class="fu">expect_type</span>(<span class="st">"list"</span>) <span class="sc">%>%</span></span> <span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a> <span class="fu">expect_s3_class</span>(<span class="st">"data.frame"</span>) <span class="sc">%>%</span> </span> <span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a> <span class="fu">expect_length</span>(<span class="dv">11</span>)</span></code></pre></div> </div> </div> <div id="succeed-and-fail" class="section level2"> <h2><code>succeed()</code> and <code>fail()</code></h2> <p>For expectations with more complex logic governing when success or failure occurs, you can use <code>succeed()</code> and <code>fail()</code>. These are simple wrappers around <code>expect()</code> that allow you to write code that looks like this:</p> <div class="sourceCode" id="cb3"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a>expect_length <span class="ot"><-</span> <span class="cf">function</span>(object, n) {</span> <span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a> act <span class="ot"><-</span> <span class="fu">quasi_label</span>(rlang<span class="sc">::</span><span class="fu">enquo</span>(object), <span class="at">arg =</span> <span class="st">"object"</span>)</span> <span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a></span> <span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a> act<span class="sc">$</span>n <span class="ot"><-</span> <span class="fu">length</span>(act<span class="sc">$</span>val)</span> <span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a> <span class="cf">if</span> (act<span class="sc">$</span>n <span class="sc">==</span> n) {</span> <span id="cb3-6"><a href="#cb3-6" aria-hidden="true" tabindex="-1"></a> <span class="fu">succeed</span>()</span> <span id="cb3-7"><a href="#cb3-7" aria-hidden="true" tabindex="-1"></a> <span class="fu">return</span>(<span class="fu">invisible</span>(act<span class="sc">$</span>val))</span> <span id="cb3-8"><a href="#cb3-8" aria-hidden="true" tabindex="-1"></a> }</span> <span id="cb3-9"><a href="#cb3-9" aria-hidden="true" tabindex="-1"></a></span> <span id="cb3-10"><a href="#cb3-10" aria-hidden="true" tabindex="-1"></a> message <span class="ot"><-</span> <span class="fu">sprintf</span>(<span class="st">"%s has length %i, not length %i."</span>, act<span class="sc">$</span>lab, act<span class="sc">$</span>n, n)</span> <span id="cb3-11"><a href="#cb3-11" aria-hidden="true" tabindex="-1"></a> <span class="fu">fail</span>(message)</span> <span id="cb3-12"><a href="#cb3-12" aria-hidden="true" tabindex="-1"></a>}</span></code></pre></div> </div> <div id="testing-your-expectations" class="section level2"> <h2>Testing your expectations</h2> <p>Use the expectations <code>expect_success()</code> and <code>expect_failure()</code> to test your expectation.</p> <div class="sourceCode" id="cb4"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a><span class="fu">test_that</span>(<span class="st">"length computed correctly"</span>, {</span> <span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a> <span class="fu">expect_success</span>(<span class="fu">expect_length</span>(<span class="dv">1</span>, <span class="dv">1</span>))</span> <span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a> <span class="fu">expect_failure</span>(<span class="fu">expect_length</span>(<span class="dv">1</span>, <span class="dv">2</span>), <span class="st">"has length 1, not length 2."</span>)</span> <span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a> <span class="fu">expect_success</span>(<span class="fu">expect_length</span>(<span class="dv">1</span><span class="sc">:</span><span class="dv">10</span>, <span class="dv">10</span>))</span> <span id="cb4-5"><a href="#cb4-5" aria-hidden="true" tabindex="-1"></a> <span class="fu">expect_success</span>(<span class="fu">expect_length</span>(letters[<span class="dv">1</span><span class="sc">:</span><span class="dv">5</span>], <span class="dv">5</span>))</span> <span id="cb4-6"><a href="#cb4-6" aria-hidden="true" tabindex="-1"></a>})</span> <span id="cb4-7"><a href="#cb4-7" aria-hidden="true" tabindex="-1"></a><span class="co">#> Test passed 🥇</span></span></code></pre></div> </div> <!-- code folding --> <!-- dynamically load mathjax for compatibility with self-contained --> <script> (function () { var script = document.createElement("script"); script.type = "text/javascript"; script.src = "https://mathjax.rstudio.com/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"; document.getElementsByTagName("head")[0].appendChild(script); })(); </script> </body> </html>