Edgewall Software

Version 38 (modified by Carsten Klein <carsten.klein@…>, 10 years ago) ( diff )

made the link to Writing the plugin code more clear

Writing Plugins for Trac

Starting with version 0.9, you can develop plugins for Trac that extend the builtin functionality. The plugin functionality is based on the component architecture, so please read that document before continuing here.

Extension points

Trac offers an increasing number of extension points that allow you to plugin custom extensions for various functions. You can view a list of provided extension points on the page About Trac/Plugins of your Trac installation.

Declaring Custom Extension Points

Plugins can declare additional extension points. In order to do so, one must inherit from trac.core.Interface.

from trac.core import Interface

class ICustomExtensionPoint(Interface):
    """The extension point interface allows plugins to ...
    """

    def do_something():
        """Does something."""

Please note that all extension point interfaces need to be authored so that their declared methods are unbound, that is, you simply omit the 'self' as first parameter to the method. This is so, that future extensions or changes to the API can be introduced, by for example adding new or removing existing parameters without causing breakage of existing components implementing the interface.

Implementing Extension Points

In order to implement an extension point, your class must be derived from trac.core.Component and it must implement the extension point interface.

from trac.core import Component, implements

class CustomComponent(Component):

    implements(ICustomExtensionPoint)

    # ICustomExtensionPoint methods

    def do_something(self):
        self.log.debug("Hello World")

When implementing the methods of the interface, the methods have to be bound to the instance, so you will have to specify 'self' as the first parameter.

More detailed information on this can be found below under Writing the plugin code.

Using Extension Points

In order for your custom component making use of plugins that implement all or one custom extension point interface provided by you, you will have to again derive your component from trac.core.Component, and then use trac.core.ExtensionPoint to get the instances that implement your custom extension point interface.

from trac.core import Component, ExtensionPoint

class SampleModule(Component):

    custom_extensions = ExtensionPoint(ICustomExtensionPoint)

    def example():
        for ext in self.custom_extensions:
            ext.do_something()

Trac Extension Points

Extension Point InterfaceSourceDescription
trac.env.IEnvironmentSetupParticipanttrac.env.IEnvironmentSetupParticipantPlugins that provide their own data models must implement this interface to be able to create the required tables in the database on either the creation of a new environment, or during the upgrade of an existing environment.
trac.db.api.IDatabaseConnectortrac.db.api.IDatabaseConnectorBy implementing this interface you can add additional database backends to the system. Currently available are backends for MySQL, PostgreSQL and SQLITE2/3.
trac.admin.api.IAdminCommandProvidertrac.admin.api.IAdminCommandProviderAllows plugins to add additional commands to the trac-admin console command.
trac.admin.api.IAdminPanelProvidertrac.admin.api.IAdminPanelProviderAllows plugins to add additional admin panels to the web-based administration module.
trac.mimeview.api.IContentConvertertrac.mimeview.api.IContentConverterAllows plugins to implement conversion strategies for arbitrary content types. (NOTE: API is likely to change in the future)
trac.mimeview.api.IHTMLPreviewAnnotatortrac.mimeview.api.IHTMLPreviewAnnotatorAllows plugins to add additional information to an XHTML representation of a given file, for example meta data on the file and so on.
trac.mimeview.api.IHTMLPreviewRenderertrac.mimeview.api.IHTMLPreviewRendererAllows plugins to provide support for rendering specific content of a specific type as HTML (used for TracSyntaxColoring and image preview).
trac.perm.IPermissionRequestortrac.perm.IPermissionRequestorPlugins can use this extension point to define additional "actions", or permission roles, for the permission system.
trac.prefs.api.IPreferencePanelProvidertrac.prefs.api.IPreferencePanelProviderAllows plugins to add additional preferences panels to the user's preferences page.
trac.resource.IResourceManagertrac.resource.IResourceManagerAllows plugins to take over full control of the realms that they provide, incl. also introducing new resource types to the system.
trac.search.api.ISearchSourcetrac.search.api.ISearchSourceAllows plugins to provide additional searchable (re)sources to the system.
trac.ticket.api.ITicketActionControllertrac.ticket.api.ITicketActionControllerAllows plugins to participate in a ticket's workflow.
trac.ticket.api.ITicketChangeListenertrac.ticket.api.ITicketChangeListenerExtension point interface for components that require notification on when tickets are created, modified, or deleted.
trac.ticket.api.ITicketManipulatortrac.ticket.api.ITicketManipulatorAllows plugins to both prepare tickets on creation and also to validate them prior to that they get stored in the database.
trac.ticket.api.IMilestoneChangeListenertrac.ticket.api.IMilestoneChangeListenerAllows plugins to listen on changes to either existing or newly created milestones.
trac.ticket.roadmap.ITicketGroupStatsProvidertrac.ticket.roadmap.ITicketGroupStatsProviderAllows plugins to implement their own scheme of ticket stats, and provide that to the system via the RoadmapModule.
trac.timeline.ITimelineEventProvidertrac.timeline.ITimelineEventProviderAllows plugins to contribute events to the timeline.
trac.web.api.IAuthenticatortrac.web.api.IAuthenticatorAllows plugins to authenticate users and HTTP sessions thereof. The first authenticator able to authenticate a user is the authoritative authenticator, meaning that other authenticators available in the system will not be called. Users that cannot be authenticated by the request and an associated session thereof are called 'anonymous'.
trac.web.api.IRequestFiltertrac.web.api.IRequestFilterAllows plugins to both preprocess and postprocess HTTP requests.
trac.web.api.IRequestHandlertrac.web.api.IRequestHandlerAllows plugins to process HTTP requests.
trac.web.chrome.INavigationContributortrac.web.chrome.INavigationContributorAllows plugins to extend the navigation menus of the web interface.
trac.web.chrome.ITemplateProvidertrac.web.chrome.ITemplateProviderExtension point interface for components that provide their own templates and accompanying static resources.
trac.wiki.api.IWikiChangeListenertrac.wiki.api.IWikiChangeListenerAllows plugins to observe creation, modification and renaming (since trac-0.12), and deletion of wiki pages.
TracDev/PluginDevelopment/ExtensionPoints/trac.wiki.api.IWikiMacroProvidertrac.wiki.api.IWikiMacroProviderAllows plugins to contribute WikiMacros to Trac.
trac.wiki.api.IWikiPageManipulatortrac.wiki.api.IWikiPageManipulatorAllows plugins to validate wiki pages prior to that they get stored in the database.
trac.wiki.api.IWikiSyntaxProvidertrac.wiki.api.IWikiSyntaxProviderPlugins can extend this extension point to add custom syntax rules to the wiki formatting system. In particular, this allows registration of additional TracLinks types.

Note that plugins can themselves add new extension points, so the list above is incomplete by nature.

Writing the plugin code

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

from trac.core import *
from trac.util.html import html
from trac.web import IRequestHandler
from trac.web.chrome import INavigationContributor

class HelloWorldPlugin(Component):
    implements(INavigationContributor, IRequestHandler)

    # INavigationContributor methods
    def get_active_navigation_item(self, req):
        return 'helloworld'
    def get_navigation_items(self, req):
        yield ('mainnav', 'helloworld',
            html.A('Hello world', href= req.href.helloworld()))

    # IRequestHandler methods
    def match_request(self, req):
        return req.path_info == '/helloworld'
    def process_request(self, req):
        req.send_response(200)
        req.send_header('Content-Type', 'text/plain')
        req.end_headers()
        req.write('Hello world!')

Look at the API documentation for the extension point interfaces to see what you're expected to return. See the tutorial page for more plugin tutorials.

Component member variables

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

  • env: The environment, an instance of the trac.env.Environment class (see trac.env).
  • config: The configuration, an instance of the trac.config.Configuration class (see trac.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.

Packaging plugins

TracPlugins are packaged as Python Eggs. You can use setuptools to make a setup.py script that will produce a Python egg for your plugin.

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

from setuptools import find_packages, setup

setup(
    name='TracHelloWorld', version='1.0',
    packages=find_packages(exclude=['*.tests*']),
    entry_points = """
        [trac.plugins]
        helloworld = myplugs.helloworld
    """,
)

This assumes that the HelloWorldPlugin example above is defined in the module helloworld.py in the myplugs package. The entry point name (in this example “helloworld”) 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.

For backwards-compatibility reasons, Trac still supports an alternative to entry points: you can have a file named trac_plugin.txt in the EGG-INFO directory. This file should contain the names of all modules that need to be imported by Trac to register your components. Note that this method is deprecated and will be removed in a future version of Trac (probably as soon as 0.11).

Localization of plugin texts

relevant since trac-0.12 introduced l10n support for Trac utilizing Babel

See the plugin l10n cookbook page for details.

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/pluginsource
$ python setup.py install

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

$ cd /path/to/pluginsource
$ python setup.py bdist_egg
$ cp 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 setuptools develop command:

$ cd /path/to/pluginsource
$ python setup.py develop --multi-version --install-dir /path/to/projenv/plugins

or the short version:

$ python setup.py develop -md /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 on TracPlugins.

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 the default_components list in trac.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 special trac.ini section called [components]. This section contains the qualified name of the components to disable, along with disabled or off as the value.

For example, to disable the built-in Wiki macro RecentChanges, you'd include the following in trac.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 (although wiki formatting will still work in the remaining modules, of course):

[components]
trac.wiki.* = disabled

Debugging

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

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

where env is the Environment instance.

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

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

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

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, TracPlugins, TracHacks:wiki:EggCookingTutorialTrac0.11

Note: See TracWiki for help on using the wiki.