Writing Plugins for Trac

Starting with version0.9, you can develop plugins for Trac that extend the builtin functionality. The plugin functionality is based on thecomponent architecture, so please read that document before continuing here. For more information, not covered here, seeTracDev.

Writing the plugin code

To extend Trac with custom functionality, you need to implement acomponent. For example, to add a new web module to Trac (i.e. a component that handles HTTP requests and extends the navigation bar), you would start with something like the following:

fromtrac.coreimportComponent,implementsfromtrac.util.htmlimporthtmlfromtrac.webimportIRequestHandlerfromtrac.web.chromeimportINavigationContributorclassHelloWorldPlugin(Component):implements(INavigationContributor,IRequestHandler)# INavigationContributor methodsdefget_active_navigation_item(self,req):return'helloworld'defget_navigation_items(self,req):yield('mainnav','helloworld',html.a('Hello world',href=req.href.helloworld()))# IRequestHandler methodsdefmatch_request(self,req):returnreq.path_info=='/helloworld'defprocess_request(self,req):content='Hello World!'req.send(content,'text/plain')

Extension points

The above example implements two of Trac's manyextension point interfaces. Look at the extension point specific pages (likeTracDev/PluginDevelopment/ExtensionPoints/trac.web.chrome.INavigationContributor) for an overview, or the API documentation to see what exactly you're expected to return.

Component member variables

Everycomponent that gets instantiated through the Trac environment gets three extra member variables for convenience:

• env: The environment, an instance of thetrac.env.Environment class (seetrac.env​).
• config: The configuration, an instance of thetrac.config.Configuration class (seetrac.config​).
• log: The configured logger, see the Python​logging API for more information.

These variables can also be accessed from the initializer (__init__) of a component.

Storing any other objects as instance variables of your component is probably a bad idea: remember that a component is only instantiated once for a given environment. Unless your plugin is used in a CGI deployment of Trac, that means that the same component instance will get invoked for multiple HTTP requests; if the server is multi-threaded, this will even happen concurrently.

Single file plugins

Plugins that consist of a single.py file can be dropped directly into either the project's or the sharedplugins directory. More complex plugins require some packaging.

Packaging plugins

TracPlugins are packaged using​setuptools, to create an egg or wheel package.

The plugin needs to export an​entry points group namedtrac.plugins, listing the names of the modules that Trac should import for the plugin-provided components to get registered. For example:

fromsetuptoolsimportfind_packages,setupsetup(name='TracHelloWorld',version='1.0',packages=find_packages(exclude=['*.tests*']),entry_points={'trac.plugins':['helloworld = myplugs.helloworld',],},)

This assumes that theHelloWorldPlugin example above is defined in the modulehelloworld.py in themyplugs package. The entry pointname (in this examplehelloworld) is required by the Python egg runtime, but not currently used by Trac. In most cases, you can simply use the qualified module name there. Themyplugs directory must contain an__init__.py to make it a regular package, but the file can be empty.

The structure of files and directories is:

setup.pymyplugin/myplugin/__init__.pymyplugin/helloworld.py

Internationalization/Localization of plugins

If you plan on supporting your plugin for i18n/l10n, see theplugin i18n/l10n cookbook page for details. Support for i18n/l10n has been introduced since Trac 0.12 and uses Babel.

Plugin deployment

A plugin can either be deployed globally, or only for a specific environment. Global deployment is done by installing the plugin:

cd /path/to/pluginsourcepython setup.py install

To deploy a plugin only to a specific Trac environment, copy the egg file into theplugins directory of that environment:

cd /path/to/pluginsourcepython setup.py bdist_eggcp dist/*.egg /path/to/projenv/plugins

During development of a plugin, it is inconvenient to have to install it in either of the ways described above. Instead, you should use the setuptoolsdevelop command:

cd /path/to/pluginsourcepython setup.py develop --multi-version --exclude-scripts --install-dir /path/to/projenv/plugins

or the short version:

python setup.py develop -mxd /path/to/projenv/plugins

You can omit the--install-dir and--multi-version arguments to make the development version of your plugin available globally.

This will install an.egg-link file instead of the actual egg. That file is basically a link to the source directory of your plugin, so that Trac will always see the latest version of your code. In this case you will have to explicitly enable your plugin in the Trac configuration as explained onTracPlugins.

A tutorial to build your own plugins is available​here.

Disabling built-in components

Sometimes you might want to write a plugin that completely replaces a built-in component, for example to develop an advanced variant of an existing module. Trac uses a list of default component to load, as specified in thedefault_components list intrac.db_default​. These built-in components are always loaded, and might therefore conflict with your replacement plugin.

You can however disable built-in components using a specialtrac.ini section called[components]. This section contains the qualified name of the components to disable, along withdisabled oroff as the value.

For example, to disable the built-in Wiki macroRecentChanges, you'd include the following intrac.ini:

[components]trac.wiki.macros.RecentChangesMacro=disabled

You can also use a wildcard at the end of a name, so you could even disable the complete Wiki module:

[components]trac.wiki.*=disabled

Wiki formatting will still work in the remaining modules, of course.

Debugging

The logging API is a very good debugging tool. For example, use this code when you want to view the value of a variable:

env.log.debug("*** Hey, varname is%r ***",varname)

whereenv is theEnvironment instance.

If you are inside the methods of aComponent subclass, better use:

self.log.debug("Hey, varname is%r",varname)

This will implicitly use theself.env Environment, but your component name will now be used for the $module (seeTracLogging#LogFormat). This makes it easier to identify the relevant debug lines.

During development it might become inconvenient to follow the log file. Consider using​TracDeveloperPlugin to access the log directly in the web browser.

Note that there's no way to log something at the global level, outside the scope of a Trac environment, as the configuration of logging is done at that level and usually the log file is located in$tracenv/log/trac.log.

See also:TracDev,TracDev/ComponentArchitecture,TracDev/PluginDevelopment/ExtensionPoints,TracPlugins