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<!--
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<h1 id="developing-a-pants-plugin">Developing a Pants Plugin</h1>
<p>This page documents how to develop a Pants plugin, a set of code that
defines new Pants functionality. If you
<a href="dev_tasks.html">develop a new task</a>
or target to add to Pants (or to
override an existing part of Pants), a plugin gives you a way to
register your code with Pants.</p>
<p>Much of Pants' own functionality is organized in plugins; see them in
<a href="https://github.com/pantsbuild/pants/tree/master/src/python/pants/backend"><code>src/python/pants/backend/*</code></a>.</p>
<p>A plugin registers its functionality with Pants by defining some
functions in a <code>register.py</code> file in its top directory. For example,
Pants' <code>jvm</code> code registers in
<a href="https://github.com/pantsbuild/pants/blob/master/src/python/pants/backend/jvm/register.py">src/python/pants/backend/jvm/register.py</a>
Pants' backend-loader code assumes your plugin has a <code>register.py</code> file
there.</p>
<h2 id="simple-configuration">Simple Configuration</h2>
<p>If you want to extend Pants without adding any 3rd-party libraries that aren't already referenced by
Pants, you can use the following technique using sources stored directly in your repo.
All you need to do is name the package where the plugin is defined, and the pythonpath entry to
load it from.</p>
<p>In the example below, the stock <code>JvmBinary</code> target will subclassed so that a custom task (not shown)
can consume it specifically but disregard regular <code>JvmBinary</code> instances (using <code>isinstance()</code>).</p>
<ul>
<li>
<p>Define a home for your plugins. In this example we'll use a top-level <code>plugins/</code> directory but
this is not special.</p>
</li>
<li>
<p>Create an empty <code>plugins/hadoop/targets/__init__.py</code> to define a python package structure
for your custom hadoop target type plugin. Also create empty <code>__init__.py</code> files in each
directory up to but not including the root directory of your python package layout; in this case,
just <code>plugins/hadoop/__init__.py</code>.</p>
</li>
<li>
<p>Create a python module for your custom target type in <code>plugins/hadoop/targets/hadoop_binary.py</code>:</p>
<div class="codehilite"><pre><span class="c"># plugins/hadoop/targets/hadoop_binary.py</span>
<span class="kn">from</span> <span class="nn">pants.backend.jvm.targets.jvm_binary</span> <span class="kn">import</span> <span class="n">JvmBinary</span>
<span class="k">class</span> <span class="nc">HadoopBinary</span><span class="p">(</span><span class="n">JvmBinary</span><span class="p">):</span>
<span class="k">pass</span>
</pre></div>
</li>
<li>
<p>Create <code>plugins/hadoop/register.py</code> to register the elements exposed by your plugin to BUILD file
authors:</p>
<div class="codehilite"><pre><span class="c"># plugins/hadoop/register.py</span>
<span class="kn">from</span> <span class="nn">pants.build_graph.build_file_aliases</span> <span class="kn">import</span> <span class="n">BuildFileAliases</span>
<span class="kn">from</span> <span class="nn">hadoop.targets.hadoop_binary</span> <span class="kn">import</span> <span class="n">HadoopBinary</span>
<span class="k">def</span> <span class="nf">build_file_aliases</span><span class="p">():</span>
<span class="k">return</span> <span class="n">BuildFileAliases</span><span class="p">(</span>
<span class="n">targets</span><span class="o">=</span><span class="p">{</span>
<span class="c"># NB: This allows a HadoopBinary target to be created in a BUILD file using the</span>
<span class="c"># hadoop_binary alias.</span>
<span class="s">'hadoop_binary'</span><span class="p">:</span> <span class="n">HadoopBinary</span><span class="p">,</span>
<span class="p">},</span>
<span class="p">)</span>
</pre></div>
</li>
<li>
<p>In <code>pants.ini</code>, add your new plugin package to the list of backends to load when pants starts.
This instructs pants to load a module named <code>hadoop.register</code>.</p>
<div class="codehilite"><pre><span class="k">[GLOBAL]</span>
<span class="err">pythonpath:</span> <span class="err">[</span>
<span class="err">'%(buildroot)s/plugins',</span>
<span class="err">]</span>
<span class="err">backend_packages:</span> <span class="err">[</span>
<span class="err">'hadoop',</span>
<span class="err">]</span>
</pre></div>
</li>
</ul>
<p>Note that you can also set the PYTHONPATH in your <code>./pants</code> wrapper script, instead of in
<code>pants.ini</code>, if you have other reasons to do so. Either way, pants will look for a <code>register.py</code>
file for each backend package you list by prefixing that package with the python path; ie roughly
<code>pythonpath + backend package + register.py</code> is tried for each pythonpath prefix until a
<code>register.py</code> is found, in this example <code>plugins/hadoop/register.py</code>.</p>
<h2 id="examples-from-twittercommons">Examples from <code>twitter/commons</code></h2>
<p>For an example of a code repo with plugins to add features to Pants when building in that repo,
take a look at <a href="https://github.com/twitter/commons"><code>twitter/commons</code></a>, especially its
<a href="https://github.com/twitter/commons/tree/32011ab5351fea23e8c70e24e752540b06d1389f/pants-plugins"><code>pants-plugins</code> directory</a>.</p>
<p>The repo's <a href="https://github.com/twitter/commons/blob/32011ab5351fea23e8c70e24e752540b06d1389f/pants.ini"><code>pants.ini</code> file</a> has a
<code>backend_packages</code> entry listing the plugin packages (packages with <code>register.py</code> files):</p>
<div class="codehilite"><pre><span class="p">[</span><span class="n">GLOBAL</span><span class="p">]</span>
<span class="n">pythonpath</span><span class="p">:</span> <span class="p">[</span>
<span class="s">"</span><span class="si">%(buildroot)s</span><span class="s">/pants-plugins/src/python"</span><span class="p">,</span>
<span class="p">]</span>
<span class="n">backend_packages</span><span class="p">:</span> <span class="p">[</span>
<span class="s">'twitter.common.pants.jvm.extras'</span><span class="p">,</span>
<span class="s">'twitter.common.pants.python.commons'</span><span class="p">,</span>
<span class="s">'pants.contrib.python.checks'</span><span class="p">,</span>
<span class="p">]</span>
</pre></div>
<p>The <a href="https://github.com/twitter/commons/blob/master/pants-plugins/src/python/twitter/common/pants/jvm/extras/register.py"><code>...jvm/extras/register.py</code></a>
file registers a <code>checkstyle</code> goal. To find the code for this task, come back to the
<code>pantsbuild/pants</code> repo: Pants defines the
<a href="https://github.com/pantsbuild/pants/blob/master/src/python/pants/backend/jvm/tasks/checkstyle.py"><code>Checkstyle</code> task class</a> but doesn't register it.
But other Pants workspaces can register it, as <code>twitter/commons</code> illustrates.</p>
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