An event is an object broadcast by the Pyramid framework at interesting points during the lifetime of an application. You don’t need to use events in order to create most Pyramid applications, but they can be useful when you want to perform slightly advanced operations. For example, subscribing to an event can allow you to run some code as the result of every new request.
Events in Pyramid are always broadcast by the framework. However, they only become useful when you register a subscriber. A subscriber is a function that accepts a single argument named event:
def mysubscriber(event): print event
The above is a subscriber that simply prints the event to the console when it’s called.
The mere existence of a subscriber function, however, is not sufficient to
arrange for it to be called. To arrange for the subscriber to be called,
you’ll need to use the
pyramid.config.Configurator.add_subscriber() method or you’ll
need to use the
pyramid.events.subscriber() decorator to decorate a
function found via a scan.
Configuring an Event Listener Imperatively¶
You can imperatively configure a subscriber function to be called
for some event type via the
method (see also Configurator):
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from pyramid.events import NewRequest from subscribers import mysubscriber # "config" below is assumed to be an instance of a # pyramid.config.Configurator object config.add_subscriber(mysubscriber, NewRequest)
The first argument to
add_subscriber() is the
subscriber function (or a dotted Python name which refers
to a subscriber callable); the second argument is the event type.
Configuring an Event Listener Using a Decorator¶
You can configure a subscriber function to be called for some event
type via the
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from pyramid.events import NewRequest from pyramid.events import subscriber @subscriber(NewRequest) def mysubscriber(event): event.request.foo = 1
subscriber() decorator is used a
scan must be performed against the package containing the
decorated function for the decorator to have any effect.
Either of the above registration examples implies that every time the
Pyramid framework emits an event object that supplies an
pyramid.events.NewRequest interface, the
will be called with an event object.
As you can see, a subscription is made in terms of a class (such as
pyramid.events.NewResponse). The event object sent to a subscriber
will always be an object that possesses an interface. For
pyramid.events.NewResponse, that interface is
pyramid.interfaces.INewResponse. The interface documentation
provides information about available attributes and methods of the event
The return value of a subscriber function is ignored. Subscribers to the same event type are not guaranteed to be called in any particular order relative to each other.
All the concrete Pyramid event types are documented in the pyramid.events API documentation.
If you create event listener functions in a
subscribers.py file in
your application like so:
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def handle_new_request(event): print 'request', event.request def handle_new_response(event): print 'response', event.response
You may configure these functions to be called at the appropriate times by adding the following code to your application’s configuration startup:
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# config is an instance of pyramid.config.Configurator config.add_subscriber('myproject.subscribers.handle_new_request', 'pyramid.events.NewRequest') config.add_subscriber('myproject.subscribers.handle_new_response', 'pyramid.events.NewResponse')
Either mechanism causes the functions in
subscribers.py to be
registered as event subscribers. Under this configuration, when the
application is run, each time a new request or response is detected, a
message will be printed to the console.
Each of our subscriber functions accepts an
event object and
prints an attribute of the event object. This begs the question: how
can we know which attributes a particular event has?
We know that
pyramid.events.NewRequest event objects have a
request attribute, which is a request object, because the
interface defined at
pyramid.interfaces.INewRequest says it must.
Likewise, we know that
pyramid.interfaces.NewResponse events have a
response attribute, which is a response object constructed by your
application, because the interface defined at
pyramid.interfaces.INewResponse says it must
pyramid.events.NewResponse objects also have a