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panic
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panic
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<!DOCTYPE html>
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<title>Go by Example: Panic</title>
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<div class="example" id="panic">
<h2><a href="./">Go by Example</a>: Panic</h2>
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<p>A <code>panic</code> typically means something went unexpectedly
wrong. Mostly we use it to fail fast on errors that
shouldn’t occur during normal operation, or that we
aren’t prepared to handle gracefully.</p>
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<a href="https://go.dev/play/p/9-2vCvRuhmE"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<pre class="chroma"><code><span class="line"><span class="cl"><span class="kn">package</span> <span class="nx">main</span></span></span></code></pre>
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kn">import</span> <span class="s">"os"</span></span></span></code></pre>
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span></span></span></code></pre>
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<p>We’ll use panic throughout this site to check for
unexpected errors. This is the only program on the
site designed to panic.</p>
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<pre class="chroma"><code><span class="line"><span class="cl"> <span class="nb">panic</span><span class="p">(</span><span class="s">"a problem"</span><span class="p">)</span></span></span></code></pre>
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<p>A common use of panic is to abort if a function
returns an error value that we don’t know how to
(or want to) handle. Here’s an example of
<code>panic</code>king if we get an unexpected error when creating a new file.</p>
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<pre class="chroma"><code><span class="line"><span class="cl"> <span class="nx">_</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">os</span><span class="p">.</span><span class="nf">Create</span><span class="p">(</span><span class="s">"/tmp/file"</span><span class="p">)</span>
</span></span><span class="line"><span class="cl"> <span class="k">if</span> <span class="nx">err</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl"> <span class="nb">panic</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
</span></span><span class="line"><span class="cl"> <span class="p">}</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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<p>Running this program will cause it to panic, print
an error message and goroutine traces, and exit with
a non-zero status.</p>
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<p>When first panic in <code>main</code> fires, the program exits
without reaching the rest of the code. If you’d like
to see the program try to create a temp file, comment
the first panic out.</p>
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="gp">$</span> go run panic.go
</span></span><span class="line"><span class="cl"><span class="go">panic: a problem</span></span></span></code></pre>
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="go">goroutine 1 [running]:
</span></span></span><span class="line"><span class="cl"><span class="go">main.main()
</span></span></span><span class="line"><span class="cl"><span class="go"> /.../panic.go:12 +0x47
</span></span></span><span class="line"><span class="cl"><span class="go">...
</span></span></span><span class="line"><span class="cl"><span class="go">exit status 2</span></span></span></code></pre>
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<p>Note that unlike some languages which use exceptions
for handling of many errors, in Go it is idiomatic
to use error-indicating return values wherever possible.</p>
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<p class="next">
Next example: <a href="defer">Defer</a>.
</p>
<p class="footer">
by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> and <a href="https://eli.thegreenplace.net">Eli Bendersky</a> | <a href="https://github.com/mmcgrana/gobyexample">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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