http://base-art.net/Articles/64/ <p>Eggs are being more and more used in Python land. One thing Python developers can do easily with them is plugins cooking. They are used in Paste*, TurboGears and Trac for instance, so i wanted to see how to design a simple mini plugins based system.</p> <p>The main requirement here is <a class="reference" href="http://cheeseshop.python.org/pypi/setuptools">setuptools</a> (i link directly to PyPI because Peak's wiki lags quite a bit currently). Setuptools supply a mechanism to plug systems together called <strong>entry points</strong>, so in your framework you can search plugins by entrypoint, and each egg supplying the entry point would be found.</p> <p>For the lazy people i made a little <a class="reference" href="http://base-art.net/static/sample_plugin_system.tgz">archive</a>, holding the few lines of code showed in this article. We'll use an entry point called &quot;my.plugins&quot;. Create a new tree structure which will be our first plugin base:</p> <pre class="literal-block"> foo |-- foo_plugin | |-- __init__.py | `-- foo.py `-- setup.py </pre> <p>In the <tt class="docutils literal"><span class="pre">setup.py</span></tt>, declare the setup() and don't forget the entry points!</p> <pre class="literal-block"> from setuptools import setup, find_packages setup( name=&quot;Foo&quot;, version=&quot;0.0&quot;, description=&quot;&quot;&quot;Foo plugin&quot;&quot;&quot;, author=&quot;Phil&quot;, packages=['foo_plugin'], entry_points=&quot;&quot;&quot; [my.plugins] myFoo = foo_plugin.foo:Foo &quot;&quot;&quot;) </pre> <p>Basically we have one entry point labelled <tt class="docutils literal"><span class="pre">myFoo</span></tt> in the &quot;category&quot; called &quot;my.plugins&quot;. The entry point points to the Foo class of the module foo_plugin.foo. That's quite simple, no ? Next step, in the <tt class="docutils literal"><span class="pre">foo_plugin/foo.py</span></tt>, declare your Foo class:</p> <pre class="literal-block"> print 'Foo loading!' class Foo: def echo(self, message): &quot;&quot;&quot; sample method, returning its argument &quot;&quot;&quot; return message </pre> <p>The next step is to package your foo plugin to a full fledged egg and to install it using easy_install:</p> <pre class="literal-block"> $ cd foo $ python setup.py bdist_egg $ sudo easy_install dist/Foo-0.0-py2.3.egg </pre> <p>One alternate, and simpler way to go when you are developing your plugin:</p> <pre class="literal-block"> $ cd foo $ sudo python setup.py develop </pre> <p>Thus, you don't need to re-compile the egg each time you modify your code, setuptools directly access to it via a link (as i understood it, i may be wrong here). Now the code to search and load our plugins, create a file called <tt class="docutils literal"><span class="pre">load_plugins.py</span></tt> with following code included:</p> <pre class="literal-block"> import pkg_resources for entrypoint in pkg_resources.iter_entry_points(&quot;my.plugins&quot;): plugin_class = entrypoint.load() print entrypoint.name, plugin_class </pre> <p>Execute it and here we go, the foo plugin magically pops up:</p> <pre class="literal-block"> $ python load_plugins.py Foo loading! foo_plugin.foo.Foo myFoo </pre> <p>After that, the developer can instantiate the class, and play with the plugin. That's all for now, i could have used interfaces to infer the plugin API, but i'm too lazy for now ;-) The interested user should read the <a class="reference" href="http://trac-hacks.org/wiki/EggCookingTutorial">Trac Egg Cooking tutorial</a> which explains how to design trac plugins using the trac components architecture based on interfaces. This document is really worth looking at, Trac is a good piece of code, really well-designed IMHO.</p> en The contents of this blog are available for non-commercial use only. Alinea http://pythonfr.org/alinea/ http://base-art.net/Comments/182/ http://base-art.net/Comments/182/ phil 2006-02-02T22:09:22Z