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<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 in order for your tests to work, we
recommend putting them in an appropriately named helper file,
i.e. <code>tests/testthat/helper-expectations.R</code>.</p>
<div id="expectation-basics" class="section level2">
<h2>Expectation basics</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" tabindex="-1"></a>expect_length <span class="ot">&lt;-</span> <span class="cf">function</span>(object, n) {</span>
<span id="cb1-2"><a href="#cb1-2" tabindex="-1"></a>  <span class="co"># 1. Capture object and label</span></span>
<span id="cb1-3"><a href="#cb1-3" tabindex="-1"></a>  act <span class="ot">&lt;-</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">&quot;object&quot;</span>)</span>
<span id="cb1-4"><a href="#cb1-4" tabindex="-1"></a></span>
<span id="cb1-5"><a href="#cb1-5" tabindex="-1"></a>  <span class="co"># 2. Call expect()</span></span>
<span id="cb1-6"><a href="#cb1-6" tabindex="-1"></a>  act<span class="sc">$</span>n <span class="ot">&lt;-</span> <span class="fu">length</span>(act<span class="sc">$</span>val)</span>
<span id="cb1-7"><a href="#cb1-7" tabindex="-1"></a>  <span class="fu">expect</span>(</span>
<span id="cb1-8"><a href="#cb1-8" tabindex="-1"></a>    act<span class="sc">$</span>n <span class="sc">==</span> n,</span>
<span id="cb1-9"><a href="#cb1-9" tabindex="-1"></a>    <span class="fu">sprintf</span>(<span class="st">&quot;%s has length %i, not length %i.&quot;</span>, act<span class="sc">$</span>lab, act<span class="sc">$</span>n, n)</span>
<span id="cb1-10"><a href="#cb1-10" tabindex="-1"></a>  )</span>
<span id="cb1-11"><a href="#cb1-11" tabindex="-1"></a></span>
<span id="cb1-12"><a href="#cb1-12" tabindex="-1"></a>  <span class="co"># 3. Invisibly return the value</span></span>
<span id="cb1-13"><a href="#cb1-13" tabindex="-1"></a>  <span class="fu">invisible</span>(act<span class="sc">$</span>val)</span>
<span id="cb1-14"><a href="#cb1-14" tabindex="-1"></a>}</span></code></pre></div>
<div id="quasi-labelling" class="section level3">
<h3>Quasi-labelling</h3>
<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 &lt;- quasi_label(enquo(object))</code>, where
<code>act</code> is short for actual.</p>
</div>
<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$val</code>. This allows expectations to be chained:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb2-1"><a href="#cb2-1" tabindex="-1"></a>mtcars <span class="sc">%&gt;%</span></span>
<span id="cb2-2"><a href="#cb2-2" tabindex="-1"></a>  <span class="fu">expect_type</span>(<span class="st">&quot;list&quot;</span>) <span class="sc">%&gt;%</span></span>
<span id="cb2-3"><a href="#cb2-3" tabindex="-1"></a>  <span class="fu">expect_s3_class</span>(<span class="st">&quot;data.frame&quot;</span>) <span class="sc">%&gt;%</span> </span>
<span id="cb2-4"><a href="#cb2-4" tabindex="-1"></a>  <span class="fu">expect_length</span>(<span class="dv">11</span>)</span></code></pre></div>
</div>
</div>
<div id="succeed-and-fail" class="section level2">
<h2><code>succeed()</code> and <code>fail()</code></h2>
<p>For expectations with more complex logic governing when success or
failure occurs, you can use <code>succeed()</code> and
<code>fail()</code>. These are simple wrappers around
<code>expect()</code> that allow you to write code that looks like
this:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb3-1"><a href="#cb3-1" tabindex="-1"></a>expect_length <span class="ot">&lt;-</span> <span class="cf">function</span>(object, n) {</span>
<span id="cb3-2"><a href="#cb3-2" tabindex="-1"></a>  act <span class="ot">&lt;-</span> <span class="fu">quasi_label</span>(rlang<span class="sc">::</span><span class="fu">enquo</span>(object), <span class="at">arg =</span> <span class="st">&quot;object&quot;</span>)</span>
<span id="cb3-3"><a href="#cb3-3" tabindex="-1"></a></span>
<span id="cb3-4"><a href="#cb3-4" tabindex="-1"></a>  act<span class="sc">$</span>n <span class="ot">&lt;-</span> <span class="fu">length</span>(act<span class="sc">$</span>val)</span>
<span id="cb3-5"><a href="#cb3-5" tabindex="-1"></a>  <span class="cf">if</span> (act<span class="sc">$</span>n <span class="sc">==</span> n) {</span>
<span id="cb3-6"><a href="#cb3-6" tabindex="-1"></a>    <span class="fu">succeed</span>()</span>
<span id="cb3-7"><a href="#cb3-7" tabindex="-1"></a>    <span class="fu">return</span>(<span class="fu">invisible</span>(act<span class="sc">$</span>val))</span>
<span id="cb3-8"><a href="#cb3-8" tabindex="-1"></a>  }</span>
<span id="cb3-9"><a href="#cb3-9" tabindex="-1"></a></span>
<span id="cb3-10"><a href="#cb3-10" tabindex="-1"></a>  message <span class="ot">&lt;-</span> <span class="fu">sprintf</span>(<span class="st">&quot;%s has length %i, not length %i.&quot;</span>, act<span class="sc">$</span>lab, act<span class="sc">$</span>n, n)</span>
<span id="cb3-11"><a href="#cb3-11" tabindex="-1"></a>  <span class="fu">fail</span>(message)</span>
<span id="cb3-12"><a href="#cb3-12" tabindex="-1"></a>}</span></code></pre></div>
</div>



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