Edgewall Software

This page was a draft. See TracDev/PluginDevelopment/ExtensionPoints/trac.notification.api.INotificationFormatter page now.

The Open questions section might still contain unimplemented ideas.

Extension Point : INotificationFormatter


The INotificationFormatter formats notification events to messages in various formats ready to be distributed.


Trac provides an extendible and flexible notification system, that historically has sent plain text emails for ticket changes. Notifications of different event realms (e.g. wiki notifications), transports (e.g. SMS) and message formats (e.g. HTML messages) might all require different formatting logic though.


Implementing the interface follows the standard guidelines found in TracDev/ComponentArchitecture and of course TracDev/PluginDevelopment.

A simple get_supported_styles() method lists the supported MIME types and event realms for a given transport.

The format() method formats a notification event. It receives the following parameters:

  • transport: The name of the transports that should be used. (See INotificationDistributor)
  • style: The style that should be used. One of those returned by get_supported_styles().
  • event: A trac.notification.api.NotificationEvent instance describing the event about which the recipients should be notified.

It should return the formatted message. The exact return type depends on the transport.


The following example formats notifications by SMS. (The sms library does not exist. Several commercial SMS services provide real APIs.)

from trac.core import *
from trac.notification.api import INotificationFormatter

class ShortTicketNotificationFormatter(Component):


    # INotificationFormatter methods
    def get_supported_styles(self, transport):
        if transport == 'sms':
            yield ('text/plain', 'ticket')

    def format(self, transport, style, event):
        if transport == 'sms' and event.realm == 'ticket':
            return "#{0} {1} by {2}" % (event.target,

Available Implementations

At first only trac.ticket.notification.TicketFormatter is part of core Trac.

A subsequent part of this proposal would add trac.wiki.notification.WikiFormatter.

Various other formatters will be part of th:AnnouncerPlugin.

Additional Information and References

  • epydoc
  • API Reference
  • Related to the INotificationDistributor
  • This interface originated in th:AnnouncerPlugin as IAnnouncementFormatter.
    • DONE Dropped the alternative_style_for() method. (The distributor can select fallbacks without this.)
    • DONE Dropped the realm parameter to the format() method. (Use event.realm instead.)
    • DONE Switched from styles(transport, realm) to get_supported_styles(transport) (See below)

Open questions

Enumerate all styles?

Maybe styles() should be changed to enumerate the supported style / transport / realm combinations as tuples:

    def get_supported_styles(self):
        yield ('text/plain', 'sms', 'ticket')

Advantage: This would give access to the list of all supported styles, without guessing the supported realms. For example a preferences panel could then create a preferred style selection UI, listing all supported styles.

Disadvantage: Formatters could not implement catch-all fallbacks that e.g. claim to support all transports. But all current Announcer formatters do just this.

Possible solutions:

  • Do not allow formatters to support all transports. Is that actually even feasible? The formatter interface even states that it must be explicitly designed to work with a specific distributor.
  • Require all formatters to support all transports. Remove the transports parameter. This would prevent using different formatters for different transports. (At least the return type of format() could then probably be defined to some standardized type.)
  • Allow formatters to return None or '*' to support all transports.
  • Define the interface with two methods. One method with parameters as before; another method without parameters must list all supported styles for any transport or realm.
  • Define the interface with one method, with optional parameters. If the parameters are None the method must list all supported styles for any transport or realm.
  • Introduce a mechanism similar to IAnnouncementProducer.realms() that enumerates all supported realms. Iterating over all possible realm / transport combinations then gets a complete list of supported styles.

Similary, could some formatters want to support all realms? If not, another possible solution could be to keep the transport parameter, but not the realm parameter:

    def get_supported_styles(self, transport):
        if transport == 'sms':
            yield ('text/plain', 'ticket')

This solves the immediate problem for listing the supported styles per transport. (But does not help if in the future we want to allow the user to selected his preferred style per realm-specific rule.)

The chosen solution should be simple and easy to implement correctly by plugin authors.

DONE This was implemented in changeset:ac2132adf2c0/psuter

Quality level?

Other extension points that enumerate supported cases (e.g. IRepositoryConnector, IHTMLPreviewRenderer, IContentConverter and IDatabaseConnector) also return some kind of quality level. Should something like this be added:

    def get_supported_styles(self):
        yield ('text/plain', 'sms', 'ticket', 8)
Last modified 9 years ago Last modified on Dec 6, 2014, 1:10:40 PM
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