Zope’s many hooks

Description

What hooks does Zope provide for application code?

Zope provides many different hooks that can be used to execute code at various times during its lifecycle. The most important ones are outlined below.

Process lifecycle

zope.processlifetime defines three events:

• IDatabaseOpened, notified when the main ZODB has been opened, but before the root application object is set.

• IDatabaseOpenedWithRoot, notified later in the startup cycle, when the application root has been set and initialised.

• IProcessStarting, notified when the Zope startup process has completed, but before the Zope server runs (and so can listen to requests).

ZODB connection lifecycle

The list App.ZApplication.connection_open_hooks can be used to hold functions that are called with a ZODB connection as their sole argument just after traversal over the ZApplicationWrapper as it opens a ZODB connection for the request.

The ZODB transaction provides two methods to register hooks - addBeforeCommitHook() and addAfterCommitHook(). These can be passed functions and a (static) set of arguments and will be called just before, and just after, a transaction is committed. The hook function must take at least one argument, a boolean indicating whether the transaction succeeded.

Use transaction.get() to get hold of the transaction object. See transaction.interfaces.ITransaction for more details.

Request lifecycle

Request-scoped items may be held from garbage collection using request._hold(). If applicable, the item held can implement __del__(), which will be called when the request is destroyed.

The event zope.publisher.events.EndRequestEvent is triggered at the end of an event, just before any held items are cleared.

Publication

The publisher notifies a number of events, which can be used to hook into various stages of the publication process. These are all defined in the module ZPublisher.pubevents.

When an exception is raised, a view registered for the exception type as context (and a generic request) named index.html will be rendered as an error message, if it exists.

Traversal

If an object has a method __bobo_traverse__(self, request, name), this will be used during traversal in lieu of attribute or item access. It is expected to return the next item to traverse to given the path segment name. A more modern approach is to register an adapter to IPublishTraverse although this only applies to publication (URL) traversal, not path traversal.

The method __before_publishing_traverse__(self, object, request) can be implemented to be notified when traversal first finds an object. Implemented on a class, the self and object parameters will be the same.

See also the SiteAccess package, which implements a through-the-web manageable, generic multi-hook to let any callable be invoked before access through an “AccessRule”.

The event zope.traversing.interfaces.IBeforeTraverseEvent is notified when traversing over something that is a local component site, e.g. the Plone site root.

The __browser_default__ method can be implemented to specify a “default page” (akin to an index.html in a folder). A more modern way to do this is to register an adapter to IBrowserPublisher.

An adapter to ITraversable can be used to implement namespace traversal (.../++<namespace>++name/...). See above for further details.