Using Events

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 add_subscriber() method:

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.

See also

See also Configurator.

Configuring an Event Listener Using a Decorator

You can configure a subscriber function to be called for some event type via the pyramid.events.subscriber() function.

from pyramid.events import NewRequest
from pyramid.events import subscriber

@subscriber(NewRequest)
def mysubscriber(event):
    event.request.foo = 1

When the 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 mysubscriber function 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 objects.

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.

An Example

If you create event listener functions in a subscribers.py file in your application like so:

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:

# 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 request).

Creating Your Own Events

In addition to using the events that the Pyramid framework creates, you can create your own events for use in your application. This can be useful to decouple parts of your application.

For example, suppose your application has to do many things when a new document is created. Rather than putting all this logic in the view that creates the document, you can create the document in your view and then fire a custom event. Subscribers to the custom event can take other actions, such as indexing the document, sending email, or sending a message to a remote system.

An event is simply an object. There are no required attributes or method for your custom events. In general, your events should keep track of the information that subscribers will need. Here are some example custom event classes:

class DocCreated(object):
    def __init__(self, doc, request):
        self.doc = doc
        self.request = request

class UserEvent(object):
    def __init__(self, user):
        self.user = user

class UserLoggedIn(UserEvent):
    pass

Some Pyramid applications choose to define custom events classes in an events module.

You can subscribe to custom events in the same way that you subscribe to Pyramid events—either imperatively or with a decorator. You can also use custom events with subscriber predicates. Here's an example of subscribing to a custom event with a decorator:

from pyramid.events import subscriber
from .events import DocCreated
from .index import index_doc

@subscriber(DocCreated)
def index_doc(event):
    # index the document using our application's index_doc function
    index_doc(event.doc, event.request)

The above example assumes that the application defines a DocCreated event class and an index_doc function.

To fire your custom events use the pyramid.registry.Registry.notify() method, which is most often accessed as request.registry.notify. For example:

from .events import DocCreated

def new_doc_view(request):
    doc = MyDoc()
    event = DocCreated(doc, request)
    request.registry.notify(event)
    return {'document': doc}

This example view will notify all subscribers to the custom DocCreated event.

Note that when you fire an event, all subscribers are run synchronously so it's generally not a good idea to create event handlers that may take a long time to run. Although event handlers could be used as a central place to spawn tasks on your own message queues.