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index.html
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<!DOCTYPE html>
<html lang="en"><head>
<script src="slides_files/libs/clipboard/clipboard.min.js"></script>
<script src="slides_files/libs/quarto-html/tabby.min.js"></script>
<script src="slides_files/libs/quarto-html/popper.min.js"></script>
<script src="slides_files/libs/quarto-html/tippy.umd.min.js"></script>
<link href="slides_files/libs/quarto-html/tippy.css" rel="stylesheet">
<link href="slides_files/libs/quarto-html/light-border.css" rel="stylesheet">
<link href="slides_files/libs/quarto-html/quarto-html.min.css" rel="stylesheet" data-mode="light">
<link href="slides_files/libs/quarto-html/quarto-syntax-highlighting.css" rel="stylesheet" id="quarto-text-highlighting-styles">
<link href="slides_files/libs/quarto-contrib/fontawesome6-0.1.0/all.css" rel="stylesheet">
<link href="slides_files/libs/quarto-contrib/fontawesome6-0.1.0/latex-fontsize.css" rel="stylesheet"><meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<meta charset="utf-8">
<meta name="generator" content="quarto-1.5.45">
<title>slides</title>
<meta name="apple-mobile-web-app-capable" content="yes">
<meta name="apple-mobile-web-app-status-bar-style" content="black-translucent">
<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no, minimal-ui">
<link rel="stylesheet" href="slides_files/libs/revealjs/dist/reset.css">
<link rel="stylesheet" href="slides_files/libs/revealjs/dist/reveal.css">
<style>
code{white-space: pre-wrap;}
span.smallcaps{font-variant: small-caps;}
div.columns{display: flex; gap: min(4vw, 1.5em);}
div.column{flex: auto; overflow-x: auto;}
div.hanging-indent{margin-left: 1.5em; text-indent: -1.5em;}
ul.task-list{list-style: none;}
ul.task-list li input[type="checkbox"] {
width: 0.8em;
margin: 0 0.8em 0.2em -1em; /* quarto-specific, see https://github.com/quarto-dev/quarto-cli/issues/4556 */
vertical-align: middle;
}
/* CSS for syntax highlighting */
pre > code.sourceCode { white-space: pre; position: relative; }
pre > code.sourceCode > span { line-height: 1.25; }
pre > code.sourceCode > span:empty { height: 1.2em; }
.sourceCode { overflow: visible; }
code.sourceCode > span { color: inherit; text-decoration: inherit; }
div.sourceCode { margin: 1em 0; }
pre.sourceCode { margin: 0; }
@media screen {
div.sourceCode { overflow: auto; }
}
@media print {
pre > code.sourceCode { white-space: pre-wrap; }
pre > code.sourceCode > span { display: inline-block; text-indent: -5em; padding-left: 5em; }
}
pre.numberSource code
{ counter-reset: source-line 0; }
pre.numberSource code > span
{ position: relative; left: -4em; counter-increment: source-line; }
pre.numberSource code > span > a:first-child::before
{ content: counter(source-line);
position: relative; left: -1em; text-align: right; vertical-align: baseline;
border: none; display: inline-block;
-webkit-touch-callout: none; -webkit-user-select: none;
-khtml-user-select: none; -moz-user-select: none;
-ms-user-select: none; user-select: none;
padding: 0 4px; width: 4em;
color: #aaaaaa;
}
pre.numberSource { margin-left: 3em; border-left: 1px solid #aaaaaa; padding-left: 4px; }
div.sourceCode
{ color: #003b4f; background-color: #f1f3f5; }
@media screen {
pre > code.sourceCode > span > a:first-child::before { text-decoration: underline; }
}
code span { color: #003b4f; } /* Normal */
code span.al { color: #ad0000; } /* Alert */
code span.an { color: #5e5e5e; } /* Annotation */
code span.at { color: #657422; } /* Attribute */
code span.bn { color: #ad0000; } /* BaseN */
code span.bu { } /* BuiltIn */
code span.cf { color: #003b4f; font-weight: bold; } /* ControlFlow */
code span.ch { color: #20794d; } /* Char */
code span.cn { color: #8f5902; } /* Constant */
code span.co { color: #5e5e5e; } /* Comment */
code span.cv { color: #5e5e5e; font-style: italic; } /* CommentVar */
code span.do { color: #5e5e5e; font-style: italic; } /* Documentation */
code span.dt { color: #ad0000; } /* DataType */
code span.dv { color: #ad0000; } /* DecVal */
code span.er { color: #ad0000; } /* Error */
code span.ex { } /* Extension */
code span.fl { color: #ad0000; } /* Float */
code span.fu { color: #4758ab; } /* Function */
code span.im { color: #00769e; } /* Import */
code span.in { color: #5e5e5e; } /* Information */
code span.kw { color: #003b4f; font-weight: bold; } /* Keyword */
code span.op { color: #5e5e5e; } /* Operator */
code span.ot { color: #003b4f; } /* Other */
code span.pp { color: #ad0000; } /* Preprocessor */
code span.sc { color: #5e5e5e; } /* SpecialChar */
code span.ss { color: #20794d; } /* SpecialString */
code span.st { color: #20794d; } /* String */
code span.va { color: #111111; } /* Variable */
code span.vs { color: #20794d; } /* VerbatimString */
code span.wa { color: #5e5e5e; font-style: italic; } /* Warning */
</style>
<link rel="stylesheet" href="slides_files/libs/revealjs/dist/theme/quarto.css">
<link rel="stylesheet" href="style.css">
<link href="slides_files/libs/revealjs/plugin/quarto-line-highlight/line-highlight.css" rel="stylesheet">
<link href="slides_files/libs/revealjs/plugin/reveal-menu/menu.css" rel="stylesheet">
<link href="slides_files/libs/revealjs/plugin/reveal-menu/quarto-menu.css" rel="stylesheet">
<link href="slides_files/libs/revealjs/plugin/quarto-support/footer.css" rel="stylesheet">
<style type="text/css">
.callout {
margin-top: 1em;
margin-bottom: 1em;
border-radius: .25rem;
}
.callout.callout-style-simple {
padding: 0em 0.5em;
border-left: solid #acacac .3rem;
border-right: solid 1px silver;
border-top: solid 1px silver;
border-bottom: solid 1px silver;
display: flex;
}
.callout.callout-style-default {
border-left: solid #acacac .3rem;
border-right: solid 1px silver;
border-top: solid 1px silver;
border-bottom: solid 1px silver;
}
.callout .callout-body-container {
flex-grow: 1;
}
.callout.callout-style-simple .callout-body {
font-size: 1rem;
font-weight: 400;
}
.callout.callout-style-default .callout-body {
font-size: 0.9rem;
font-weight: 400;
}
.callout.callout-titled.callout-style-simple .callout-body {
margin-top: 0.2em;
}
.callout:not(.callout-titled) .callout-body {
display: flex;
}
.callout:not(.no-icon).callout-titled.callout-style-simple .callout-content {
padding-left: 1.6em;
}
.callout.callout-titled .callout-header {
padding-top: 0.2em;
margin-bottom: -0.2em;
}
.callout.callout-titled .callout-title p {
margin-top: 0.5em;
margin-bottom: 0.5em;
}
.callout.callout-titled.callout-style-simple .callout-content p {
margin-top: 0;
}
.callout.callout-titled.callout-style-default .callout-content p {
margin-top: 0.7em;
}
.callout.callout-style-simple div.callout-title {
border-bottom: none;
font-size: .9rem;
font-weight: 600;
opacity: 75%;
}
.callout.callout-style-default div.callout-title {
border-bottom: none;
font-weight: 600;
opacity: 85%;
font-size: 0.9rem;
padding-left: 0.5em;
padding-right: 0.5em;
}
.callout.callout-style-default div.callout-content {
padding-left: 0.5em;
padding-right: 0.5em;
}
.callout.callout-style-simple .callout-icon::before {
height: 1rem;
width: 1rem;
display: inline-block;
content: "";
background-repeat: no-repeat;
background-size: 1rem 1rem;
}
.callout.callout-style-default .callout-icon::before {
height: 0.9rem;
width: 0.9rem;
display: inline-block;
content: "";
background-repeat: no-repeat;
background-size: 0.9rem 0.9rem;
}
.callout-title {
display: flex
}
.callout-icon::before {
margin-top: 1rem;
padding-right: .5rem;
}
.callout.no-icon::before {
display: none !important;
}
.callout.callout-titled .callout-body > .callout-content > :last-child {
padding-bottom: 0.5rem;
margin-bottom: 0;
}
.callout.callout-titled .callout-icon::before {
margin-top: .5rem;
padding-right: .5rem;
}
.callout:not(.callout-titled) .callout-icon::before {
margin-top: 1rem;
padding-right: .5rem;
}
/* Callout Types */
div.callout-note {
border-left-color: #4582ec !important;
}
div.callout-note .callout-icon::before {
background-image: url('data:image/png;base64,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');
}
div.callout-note.callout-style-default .callout-title {
background-color: #dae6fb
}
div.callout-important {
border-left-color: #d9534f !important;
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<section id="introduction-to-snapshot-testing-in-r" class="slide level2" style="margin-top: 1em;"><h2>Introduction to Snapshot Testing in R</h2>
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<p>Indrajeet Patil</p>
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</section><section id="unit-testing" class="slide level2 smaller"><h2>Unit testing</h2>
<p>The goal of a unit test is to capture the <em>expected</em> output of a function using <em>code</em> and making sure that <em>actual</em> output after any changes matches the expected output.</p>
<p><a href="https://testthat.r-lib.org/"><code>{testthat}</code></a> is a popular framework for writing unit tests in R.</p>
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<p><strong>Benefits of unit testing</strong></p>
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<li>insures against unintentionally changing function behaviour</li>
<li>prevents re-introducing already fixed bugs</li>
<li>acts as the most basic form of developer-focused documentation</li>
<li>catches breaking changes coming from upstream dependencies</li>
<li>etc.</li>
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<p><strong>Test output</strong></p>
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<p>Test pass only when actual function behaviour matches expected.</p>
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</section><section id="unit-testing-with-testthat-a-recap" class="slide level2 smaller"><h2>Unit testing with <code>{testthat}</code>: A recap</h2>
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<p><strong>Test organization</strong></p>
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<p>Testing infrastructure for R package has the following hierarchy:</p>
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<br><i class="fa-regular fa-file-code fa-2xl" aria-label="file-code"></i> <strong>Test file</strong>
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<td style="text-align: left;">Tests for code situated in <code>R/foo.R</code> will typically be in the file <code>tests/testthat/test-foo.R</code>.</td>
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<td style="text-align: left;">A single test can have multiple expectations that formalize what you expect code to do.</td>
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<p><strong>Example test file</strong></p>
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<li><p>Every test is a call to <code><a href="https://testthat.r-lib.org/reference/test_that.html">testthat::test_that()</a></code> function.</p></li>
<li><p>Every expectation is represented by <code>testthat::expect_*()</code> function.</p></li>
<li><p>You can generate a test file using <code>usethis::use_test()</code> function.</p></li>
</ul>
<div class="sourceCode" id="cb1"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb1-1"><a href=""></a><span class="co"># File: tests/testthat/test-op.R</span></span>
<span id="cb1-2"><a href=""></a></span>
<span id="cb1-3"><a href=""></a><span class="co"># test-1</span></span>
<span id="cb1-4"><a href=""></a><span class="fu">test_that</span>(<span class="st">"multiplication works"</span>, {</span>
<span id="cb1-5"><a href=""></a> <span class="fu">expect_equal</span>(<span class="dv">2</span> <span class="sc">*</span> <span class="dv">2</span>, <span class="dv">4</span>) <span class="co"># expectation-1</span></span>
<span id="cb1-6"><a href=""></a> <span class="fu">expect_equal</span>(<span class="sc">-</span><span class="dv">2</span> <span class="sc">*</span> <span class="dv">2</span>, <span class="sc">-</span><span class="dv">4</span>) <span class="co"># expectation-2</span></span>
<span id="cb1-7"><a href=""></a>})</span>
<span id="cb1-8"><a href=""></a></span>
<span id="cb1-9"><a href=""></a><span class="co"># test-2</span></span>
<span id="cb1-10"><a href=""></a><span class="fu">test_that</span>(<span class="st">"addition works"</span>, {</span>
<span id="cb1-11"><a href=""></a> <span class="fu">expect_equal</span>(<span class="dv">2</span> <span class="sc">+</span> <span class="dv">2</span>, <span class="dv">4</span>) <span class="co"># expectation-1</span></span>
<span id="cb1-12"><a href=""></a> <span class="fu">expect_equal</span>(<span class="sc">-</span><span class="dv">2</span> <span class="sc">+</span> <span class="dv">2</span>, <span class="dv">0</span>) <span class="co"># expectation-2</span></span>
<span id="cb1-13"><a href=""></a>})</span>
<span id="cb1-14"><a href=""></a></span>
<span id="cb1-15"><a href=""></a>...</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
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</section><section id="what-is-different-about-snapshot-testing" class="slide level2"><h2>What is different about snapshot testing?</h2>
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<p>A <strong>unit test</strong> records the code to describe expected output.</p>
<p><br></p>
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<p>(actual) <i class="fa-regular fa-file-code fa-2xl" aria-label="file-code"></i> <i class="fa-solid fa-arrows-left-right fa-2xl" aria-label="arrows-left-right"></i> <i class="fa-solid fa-file-code fa-2xl" aria-label="file-code"></i> (expected)</p>
<p><br></p>
</div>
<div class="fragment">
<p>A <strong>snapshot test</strong> records expected output in a separate, human-readable file.</p>
<p><br></p>
<p>(actual) <i class="fa-regular fa-file-code fa-2xl" aria-label="file-code"></i> <i class="fa-solid fa-arrows-left-right fa-2xl" aria-label="arrows-left-right"></i> <i class="fa-solid fa-file-lines fa-2xl" aria-label="file-lines"></i> (expected)</p>
</div>
</section><section id="why-do-you-need-snapshot-testing" class="slide level2"><h2>Why do you need snapshot testing?</h2>
<p>If you develop R packages and have struggled to</p>
<div>
<ul>
<li class="fragment">test that text output <em>prints</em> as expected</li>
<li class="fragment">test that an entire file <em>is</em> as expected</li>
<li class="fragment">test that generated graphical output <em>looks</em> as expected</li>
<li class="fragment">update such tests <em>en masse</em>
</li>
</ul>
</div>
<div class="fragment">
<p>then you should be excited to know more about <em>snapshot tests</em> (aka <em>golden tests</em>)! 🤩</p>
</div>
</section><section><section id="prerequisites" class="title-slide slide level1 center"><h1>Prerequisites</h1>
<p>Familiarity with writing unit tests using <a href="https://testthat.r-lib.org/index.html" target="_blank"><code>{testthat}</code></a>.</p>
<p>If not, have a look at <a href="https://r-pkgs.org/testing-basics.html" target="_blank">this</a> chapter from <em>R Packages</em> book.</p>
</section><section id="section" class="slide level2"><h2></h2>
<p><br></p>
<div class="callout callout-important callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong><em>Nota bene</em></strong></p>
</div>
<div class="callout-content">
<p>In the following slides, in all snapshot tests, I include the following line of code:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb2-1"><a href=""></a><span class="fu">local_edition</span>(<span class="dv">3</span>)</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<p><strong>You don’t need to do this in your package tests!</strong></p>
<p>It’s sufficient to update the <code>DESCRIPTION</code> file to use <a href="https://testthat.r-lib.org">testthat</a> 3rd edition:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb3-1"><a href=""></a>Config<span class="sc">/</span>testthat<span class="sc">/</span>edition<span class="sc">:</span> <span class="dv">3</span></span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<p>For more, see <a href="https://testthat.r-lib.org/articles/third-edition.html" target="_blank">this</a> article.</p>
</div>
</div>
</div>
</section></section><section><section id="testing-text-outputs" class="title-slide slide level1 center"><h1>Testing text outputs</h1>
<p>Snapshot tests can be used to test that text output <em>prints</em> as expected.</p>
<p>Important for testing functions that pretty-print R objects to the console, create elegant and informative exceptions, etc.</p>
</section><section id="example-function" class="slide level2 smaller"><h2>Example function</h2>
<p>Let’s say you want to write a unit test for the following function:</p>
<div class="columns">
<div class="column" style="width:60%;">
<p><strong>Source code</strong></p>
<div class="cell">
<div class="sourceCode cell-code" id="cb4"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb4-1"><a href=""></a>print_movies <span class="ot"><-</span> <span class="cf">function</span>(keys, values) {</span>
<span id="cb4-2"><a href=""></a> <span class="fu">paste0</span>(</span>
<span id="cb4-3"><a href=""></a> <span class="st">"Movie: </span><span class="sc">\n</span><span class="st">"</span>,</span>
<span id="cb4-4"><a href=""></a> <span class="fu">paste0</span>(<span class="st">" "</span>, keys, <span class="st">": "</span>, values, <span class="at">collapse =</span> <span class="st">"</span><span class="sc">\n</span><span class="st">"</span>)</span>
<span id="cb4-5"><a href=""></a> )</span>
<span id="cb4-6"><a href=""></a>}</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</div>
</div><div class="column" style="width:40%;">
<p><strong>Output</strong></p>
<div class="cell">
<div class="sourceCode cell-code" id="cb5"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb5-1"><a href=""></a><span class="fu">cat</span>(<span class="fu">print_movies</span>(</span>
<span id="cb5-2"><a href=""></a> <span class="fu">c</span>(<span class="st">"Title"</span>, <span class="st">"Director"</span>),</span>
<span id="cb5-3"><a href=""></a> <span class="fu">c</span>(<span class="st">"Salaam Bombay!"</span>, <span class="st">"Mira Nair"</span>)</span>
<span id="cb5-4"><a href=""></a>))</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>Movie:
Title: Salaam Bombay!
Director: Mira Nair</code></pre>
</div>
</div>
</div>
</div>
<div class="fragment">
<p><br></p>
<p>Note that you want to test that the printed output <em>looks</em> as expected.</p>
<p>Therefore, you need to check for all the little bells and whistles in the printed output.</p>
</div>
</section><section id="example-test" class="slide level2 smaller"><h2>Example test</h2>
<p>Even testing this simple function is a bit painful because you need to keep track of every escape character, every space, etc.</p>
<div class="cell">
<div class="sourceCode cell-code" id="cb7"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb7-1"><a href=""></a><span class="fu">test_that</span>(<span class="st">"`print_movies()` prints as expected"</span>, {</span>
<span id="cb7-2"><a href=""></a> <span class="fu">expect_equal</span>(</span>
<span id="cb7-3"><a href=""></a> <span class="fu">print_movies</span>(</span>
<span id="cb7-4"><a href=""></a> <span class="fu">c</span>(<span class="st">"Title"</span>, <span class="st">"Director"</span>),</span>
<span id="cb7-5"><a href=""></a> <span class="fu">c</span>(<span class="st">"Salaam Bombay!"</span>, <span class="st">"Mira Nair"</span>)</span>
<span id="cb7-6"><a href=""></a> ),</span>
<span id="cb7-7"><a href=""></a> <span class="st">"Movie: </span><span class="sc">\n</span><span class="st"> Title: Salaam Bombay!</span><span class="sc">\n</span><span class="st"> Director: Mira Nair"</span></span>
<span id="cb7-8"><a href=""></a> )</span>
<span id="cb7-9"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>Test passed 🌈</code></pre>
</div>
</div>
<div class="fragment">
<p>With a more complex code, it’d be impossible for a human to reason about what the output is supposed to look like.</p>
</div>
<div class="fragment">
<div class="callout callout-important callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>Important</strong></p>
</div>
<div class="callout-content">
<p>If this is a utility function used by many other functions, changing its behaviour would entail <em>manually</em> changing expected outputs for many tests.</p>
<p>This is not maintainable! 😩</p>
</div>
</div>
</div>
</div>
</section><section id="alternative-snapshot-test" class="slide level2 smaller"><h2>Alternative: Snapshot test</h2>
<p>Instead, you can use <code>expect_snapshot()</code>, which, when run for the first time, generates a Markdown file with expected/reference output.</p>
<div class="cell">
<div class="sourceCode cell-code" id="cb9"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb9-1"><a href=""></a><span class="fu">test_that</span>(<span class="st">"`print_movies()` prints as expected"</span>, {</span>
<span id="cb9-2"><a href=""></a> <span class="fu">local_edition</span>(<span class="dv">3</span>)</span>
<span id="cb9-3"><a href=""></a> <span class="fu">expect_snapshot</span>(<span class="fu">cat</span>(<span class="fu">print_movies</span>(</span>
<span id="cb9-4"><a href=""></a> <span class="fu">c</span>(<span class="st">"Title"</span>, <span class="st">"Director"</span>),</span>
<span id="cb9-5"><a href=""></a> <span class="fu">c</span>(<span class="st">"Salaam Bombay!"</span>, <span class="st">"Mira Nair"</span>)</span>
<span id="cb9-6"><a href=""></a> )))</span>
<span id="cb9-7"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>── Warning: `print_movies()` prints as expected ────────────────────────────────
Adding new snapshot:
Code
cat(print_movies(c("Title", "Director"), c("Salaam Bombay!", "Mira Nair")))
Output
Movie:
Title: Salaam Bombay!
Director: Mira Nair</code></pre>
</div>
</div>
<div class="fragment">
<div class="callout callout-warning callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>Warning</strong></p>
</div>
<div class="callout-content">
<p>The first time a snapshot is created, it becomes <em>the truth</em> against which future function behaviour will be compared.</p>
<p>Thus, it is <strong>crucial</strong> that you carefully check that the output is indeed as expected. 🔎</p>
</div>
</div>
</div>
</div>
</section><section id="human-readable-markdown-file" class="slide level2 smaller"><h2>Human-readable Markdown file</h2>
<p>Compared to your unit test code representing the expected output</p>
<div class="sourceCode" id="cb11"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb11-1"><a href=""></a><span class="st">"Movie: </span><span class="sc">\n</span><span class="st"> Title: Salaam Bombay!</span><span class="sc">\n</span><span class="st"> Director: Mira Nair"</span></span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<p>notice how much more human-friendly the Markdown output is!</p>
<div class="sourceCode" id="cb12"><pre class="sourceCode numberSource md number-lines code-with-copy"><code class="sourceCode markdown"><span id="cb12-1"><a href=""></a>Code</span>
<span id="cb12-2"><a href=""></a> cat(print_movies(c("Title", "Director"), c("Salaam Bombay!", "Mira Nair")))</span>
<span id="cb12-3"><a href=""></a>Output</span>
<span id="cb12-4"><a href=""></a> Movie: </span>
<span id="cb12-5"><a href=""></a> Title: Salaam Bombay!</span>
<span id="cb12-6"><a href=""></a> Director: Mira Nair</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<p>It is easy to <em>see</em> what the printed text output is <em>supposed</em> to look like. In other words, snapshot tests are useful when the <em>intent</em> of the code can only be verified by a human.</p>
<div class="fragment">
<div class="callout callout-note callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>More about snapshot Markdown files</strong></p>
</div>
<div class="callout-content">
<ul>
<li><p>If test file is called <code>test-foo.R</code>, the snapshot will be saved to <code>test/testthat/_snaps/foo.md</code>.</p></li>
<li><p>If there are multiple snapshot tests in a single file, corresponding snapshots will also share the same <code>.md</code> file.</p></li>
<li><p>By default, <code>expect_snapshot()</code> will capture the code, the object values, and any side-effects.</p></li>
</ul>
</div>
</div>
</div>
</div>
</section><section id="what-test-success-looks-like" class="slide level2 smaller"><h2>What test success looks like</h2>
<p>If you run the test again, it’ll succeed:</p>
<div class="cell">
<div class="sourceCode cell-code" id="cb13"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb13-1"><a href=""></a><span class="fu">test_that</span>(<span class="st">"`print_movies()` prints as expected"</span>, {</span>
<span id="cb13-2"><a href=""></a> <span class="fu">local_edition</span>(<span class="dv">3</span>)</span>
<span id="cb13-3"><a href=""></a> <span class="fu">expect_snapshot</span>(<span class="fu">cat</span>(<span class="fu">print_movies</span>(</span>
<span id="cb13-4"><a href=""></a> <span class="fu">c</span>(<span class="st">"Title"</span>, <span class="st">"Director"</span>),</span>
<span id="cb13-5"><a href=""></a> <span class="fu">c</span>(<span class="st">"Salaam Bombay!"</span>, <span class="st">"Mira Nair"</span>)</span>
<span id="cb13-6"><a href=""></a> )))</span>
<span id="cb13-7"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>Test passed 🎉</code></pre>
</div>
</div>
<div class="fragment">
<p><br></p>
<div class="callout callout-note callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>Why does my test fail on a re-run?</strong></p>
</div>
<div class="callout-content">
<p>If testing a snapshot you just generated fails on re-running the test, this is most likely because your test is not deterministic. For example, if your function deals with random number generation.</p>
<p>In such cases, setting a seed (e.g. <code>set.seed(42)</code>) should help.</p>
</div>
</div>
</div>
</div>
</section><section id="what-test-failure-looks-like" class="slide level2 smaller"><h2>What test failure looks like</h2>
<p>When function changes, snapshot doesn’t match the reference, and the test fails:</p>
<div class="columns">
<div class="column" style="width:35%;">
<p><strong>Changes to function</strong></p>
<div class="sourceCode" id="cb15" data-code-line-numbers="5"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb15-1"><a href=""></a>print_movies <span class="ot"><-</span> <span class="cf">function</span>(keys, values) {</span>
<span id="cb15-2"><a href=""></a> <span class="fu">paste0</span>(</span>
<span id="cb15-3"><a href=""></a> <span class="st">"Movie: </span><span class="sc">\n</span><span class="st">"</span>,</span>
<span id="cb15-4"><a href=""></a> <span class="fu">paste0</span>(</span>
<span id="cb15-5"><a href=""></a> <span class="st">" "</span>, keys, <span class="st">"- "</span>, values,</span>
<span id="cb15-6"><a href=""></a> <span class="at">collapse =</span> <span class="st">"</span><span class="sc">\n</span><span class="st">"</span></span>
<span id="cb15-7"><a href=""></a> )</span>
<span id="cb15-8"><a href=""></a> )</span>
<span id="cb15-9"><a href=""></a>}</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<p><br></p>
<p>Failure message provides expected (<code>-</code>) vs observed (<code>+</code>) diff.</p>
</div><div class="column" style="width:65%;">
<p><strong>Test failure</strong></p>
<div class="sourceCode" id="cb16"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb16-1"><a href=""></a><span class="fu">test_that</span>(<span class="st">"`print_movies()` prints as expected"</span>, {</span>
<span id="cb16-2"><a href=""></a> <span class="fu">expect_snapshot</span>(<span class="fu">cat</span>(<span class="fu">print_movies</span>(</span>
<span id="cb16-3"><a href=""></a> <span class="fu">c</span>(<span class="st">"Title"</span>, <span class="st">"Director"</span>),</span>
<span id="cb16-4"><a href=""></a> <span class="fu">c</span>(<span class="st">"Salaam Bombay!"</span>, <span class="st">"Mira Nair"</span>)</span>
<span id="cb16-5"><a href=""></a> )))</span>
<span id="cb16-6"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell">
<div class="cell-output cell-output-stdout">
<pre><code>── Failure: `print_movies()` prints as expected ────────────────────────────────
Snapshot of code has changed:
old[2:6] vs new[2:6]
cat(print_movies(c("Title", "Director"), c("Salaam Bombay!", "Mira Nair")))
Output
Movie:
- Title: Salaam Bombay!
+ Title- Salaam Bombay!
- Director: Mira Nair
+ Director- Mira Nair
* Run `testthat::snapshot_accept('slides.qmd')` to accept the change.
* Run `testthat::snapshot_review('slides.qmd')` to interactively review the change.</code></pre>
</div>
<div class="cell-output cell-output-error">
<pre><code>Error:
! Test failed</code></pre>
</div>
</div>
</div>
</div>
</section><section id="fixing-tests" class="slide level2 smaller"><h2>Fixing tests</h2>
<p>Message accompanying failed tests make it explicit how to fix them.</p>
<div class="fragment">
<ul>
<li>If the change was <em>deliberate</em>, you can accept the new snapshot as the current <em>truth</em>.</li>
</ul>
<div class="sourceCode" id="cb19"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb19-1"><a href=""></a><span class="sc">*</span> Run <span class="st">`</span><span class="at">snapshot_accept('foo.md')</span><span class="st">`</span> to accept the change</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<ul>
<li>If this was <em>unexpected</em>, you can review the changes, and decide whether to change the snapshot or to correct the function behaviour instead.</li>
</ul>
<div class="sourceCode" id="cb20"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb20-1"><a href=""></a><span class="sc">*</span> Run <span class="st">`</span><span class="at">snapshot_review('foo.md')</span><span class="st">`</span> to interactively review the change</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</div>
<div class="fragment">
<p><br></p>
<div class="callout callout-tip callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>Fixing multiple snapshot tests</strong></p>
</div>
<div class="callout-content">
<p>If this is a utility function used by many other functions, changing its behaviour would lead to failure of many tests.</p>
<p>You can update <em>all</em> new snapshots with <code>snapshot_accept()</code>. And, of course, check the diffs to make sure that the changes are expected.</p>
</div>
</div>
</div>
</div>
</section><section id="capturing-messages-and-warnings" class="slide level2 smaller"><h2>Capturing messages and warnings</h2>
<p>So far you have tested text output printed to the console, but you can also use snapshots to capture messages, warnings, and errors.</p>
<div class="columns">
<div class="column" style="width:50%;">
<p><strong>message</strong></p>
<div class="cell">
<div class="sourceCode cell-code" id="cb21"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb21-1"><a href=""></a>f <span class="ot"><-</span> <span class="cf">function</span>() <span class="fu">message</span>(<span class="st">"Some info for you."</span>)</span>
<span id="cb21-2"><a href=""></a></span>
<span id="cb21-3"><a href=""></a><span class="fu">test_that</span>(<span class="st">"f() messages"</span>, {</span>
<span id="cb21-4"><a href=""></a> <span class="fu">local_edition</span>(<span class="dv">3</span>)</span>
<span id="cb21-5"><a href=""></a> <span class="fu">expect_snapshot</span>(<span class="fu">f</span>())</span>
<span id="cb21-6"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>── Warning: f() messages ───────────────────────────────────────────────────────
Adding new snapshot:
Code
f()
Message
Some info for you.</code></pre>
</div>
</div>
</div><div class="column" style="width:50%;">
<p><strong>warning</strong></p>
<div class="cell">
<div class="sourceCode cell-code" id="cb23"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb23-1"><a href=""></a>g <span class="ot"><-</span> <span class="cf">function</span>() <span class="fu">warning</span>(<span class="st">"Managed to recover."</span>)</span>
<span id="cb23-2"><a href=""></a></span>
<span id="cb23-3"><a href=""></a><span class="fu">test_that</span>(<span class="st">"g() warns"</span>, {</span>
<span id="cb23-4"><a href=""></a> <span class="fu">local_edition</span>(<span class="dv">3</span>)</span>
<span id="cb23-5"><a href=""></a> <span class="fu">expect_snapshot</span>(<span class="fu">g</span>())</span>
<span id="cb23-6"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>── Warning: g() warns ──────────────────────────────────────────────────────────
Adding new snapshot:
Code
g()
Condition
Warning in `g()`:
Managed to recover.</code></pre>
</div>
</div>
</div>
</div>
<div class="callout callout-tip callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>Tip</strong></p>
</div>
<div class="callout-content">
<p>Snapshot records both the <em>condition</em> and the corresponding <em>message</em>.</p>
<p>You can now rest assured that the users are getting informed the way you want! 😌</p>
</div>
</div>
</div>
</section><section id="capturing-errors" class="slide level2 smaller"><h2>Capturing errors</h2>
<p>In case of an error, the function <code>expect_snapshot()</code> itself will produce an error. You have two ways around this:</p>
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<p><strong>Option-1</strong> (recommended)</p>
<div class="sourceCode" id="cb25" data-code-line-numbers="3"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb25-1"><a href=""></a><span class="fu">test_that</span>(<span class="st">"`log()` errors"</span>, {</span>
<span id="cb25-2"><a href=""></a> <span class="fu">local_edition</span>(<span class="dv">3</span>)</span>
<span id="cb25-3"><a href=""></a> <span class="fu">expect_snapshot</span>(<span class="fu">log</span>(<span class="st">"x"</span>), <span class="at">error =</span> <span class="cn">TRUE</span>)</span>
<span id="cb25-4"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
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<pre><code>── Warning: `log()` errors ─────────────────────────────────────────────────────
Adding new snapshot:
Code
log("x")
Condition
Error in `log()`:
! non-numeric argument to mathematical function</code></pre>
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<p><strong>Option-2</strong></p>
<div class="sourceCode" id="cb27" data-code-line-numbers="3"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb27-1"><a href=""></a><span class="fu">test_that</span>(<span class="st">"`log()` errors"</span>, {</span>
<span id="cb27-2"><a href=""></a> <span class="fu">local_edition</span>(<span class="dv">3</span>)</span>
<span id="cb27-3"><a href=""></a> <span class="fu">expect_snapshot_error</span>(<span class="fu">log</span>(<span class="st">"x"</span>))</span>
<span id="cb27-4"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
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<pre><code>── Warning: `log()` errors ─────────────────────────────────────────────────────
Adding new snapshot:
non-numeric argument to mathematical function</code></pre>
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</div>
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<i class="callout-icon"></i>
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<p><strong>Which option should I use?</strong></p>
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<ul>
<li><p>If you want to capture both the code and the error message, use <code>expect_snapshot(..., error = TRUE)</code>.</p></li>
<li><p>If you want to capture only the error message, use <code>expect_snapshot_error()</code>.</p></li>
</ul>
</div>
</div>
</div>
</section><section id="further-reading" class="slide level2 smaller"><h2>Further reading</h2>
<ul>
<li><p><a href="https://testthat.r-lib.org">testthat</a> article on <a href="https://testthat.r-lib.org/articles/snapshotting.html" target="_blank">snapshot testing</a></p></li>
<li><p>Introduction to <a href="https://ro-che.info/articles/2017-12-04-golden-tests" target="_blank">golden testing</a></p></li>
<li><p>Docs for <a href="https://jestjs.io/docs/snapshot-testing" target="_blank">Jest</a> library in JavaScript, which inspired snapshot testing implementation in <a href="https://testthat.r-lib.org">testthat</a></p></li>
</ul></section></section><section><section id="testing-graphical-outputs" class="title-slide slide level1 center"><h1>Testing graphical outputs</h1>
<p>To create graphical expectations, you will use <a href="https://testthat.r-lib.org">testthat</a> extension package: <a href="https://vdiffr.r-lib.org/" target="_blank"><code>{vdiffr}</code></a>.</p>
</section><section id="how-does-vdiffr-work" class="slide level2 smaller"><h2>How does <code>{vdiffr}</code> work?</h2>
<p><a href="https://vdiffr.r-lib.org/">vdiffr</a> introduces <code>expect_doppelganger()</code> to generate <a href="https://testthat.r-lib.org">testthat</a> expectations for graphics. It does this by writing SVG snapshot files for outputs!</p>
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<p>The figure to test can be:</p>
<ul>
<li>a <code>ggplot</code> object (from <code><a href="https://ggplot2.tidyverse.org/reference/ggplot.html">ggplot2::ggplot()</a></code>)</li>
<li>a <code>recordedplot</code> object (from <code><a href="https://rdrr.io/r/grDevices/recordplot.html">grDevices::recordPlot()</a></code>)</li>
<li>any object with a <code><a href="https://rdrr.io/r/base/print.html">print()</a></code> method</li>
</ul>
</div>
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<p><strong>Note</strong></p>
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<ul>
<li><p>If test file is called <code>test-foo.R</code>, the snapshot will be saved to <code>test/testthat/_snaps/foo</code> folder.</p></li>
<li><p>In this folder, there will be one <code>.svg</code> file for every test in <code>test-foo.R</code>.</p></li>
<li><p>The name for the <code>.svg</code> file will be sanitized version of <code>title</code> argument to <code>expect_doppelganger()</code>.</p></li>
</ul>
</div>
</div>
</div>
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</section><section id="example-function-1" class="slide level2 smaller"><h2>Example function</h2>
<p>Let’s say you want to write a unit test for the following function:</p>
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<p><strong>Source code</strong></p>
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<div class="sourceCode cell-code" id="cb29"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb29-1"><a href=""></a><span class="fu">library</span>(ggplot2)</span>
<span id="cb29-2"><a href=""></a></span>
<span id="cb29-3"><a href=""></a>create_scatter <span class="ot"><-</span> <span class="cf">function</span>() {</span>
<span id="cb29-4"><a href=""></a> <span class="fu">ggplot</span>(mtcars, <span class="fu">aes</span>(wt, mpg)) <span class="sc">+</span></span>
<span id="cb29-5"><a href=""></a> <span class="fu">geom_point</span>(<span class="at">size =</span> <span class="dv">3</span>, <span class="at">alpha =</span> <span class="fl">0.75</span>) <span class="sc">+</span></span>
<span id="cb29-6"><a href=""></a> <span class="fu">geom_smooth</span>(<span class="at">method =</span> <span class="st">"lm"</span>)</span>