Signals

New in version 0.6.

Starting with Flask 0.6, there is integrated support for signalling in Flask. This support is provided by the excellent blinker library and will gracefully fall back if it is not available.

What are signals? Signals help you decouple applications by sending notifications when actions occur elsewhere in the core framework or another Flask extensions. In short, signals allow certain senders to notify subscribers that something happened.

Flask comes with a couple of signals and other extensions might provide more. Also keep in mind that signals are intended to notify subscribers and should not encourage subscribers to modify data. You will notice that there are signals that appear to do the same thing like some of the builtin decorators do (eg: request_started is very similar to before_request()). There are however difference in how they work. The core before_request() handler for example is executed in a specific order and is able to abort the request early by returning a response. In contrast all signal handlers are executed in undefined order and do not modify any data.

The big advantage of signals over handlers is that you can safely subscribe to them for the split of a second. These temporary subscriptions are helpful for unittesting for example. Say you want to know what templates were rendered as part of a request: signals allow you to do exactly that.

Subscribing to Signals

To subscribe to a signal, you can use the connect() method of a signal. The first argument is the function that should be called when the signal is emitted, the optional second argument specifies a sender. To unsubscribe from a signal, you can use the disconnect() method.

For all core Flask signals, the sender is the application that issued the signal. When you subscribe to a signal, be sure to also provide a sender unless you really want to listen for signals of all applications. This is especially true if you are developing an extension.

Here for example a helper context manager that can be used to figure out in a unittest which templates were rendered and what variables were passed to the template:

from flask import template_rendered
from contextlib import contextmanager

@contextmanager
def captured_templates(app):
    recorded = []
    def record(sender, template, context, **extra):
        recorded.append((template, context))
    template_rendered.connect(record, app)
    try:
        yield recorded
    finally:
        template_rendered.disconnect(record, app)

This can now easily be paired with a test client:

with captured_templates(app) as templates:
    rv = app.test_client().get('/')
    assert rv.status_code == 200
    assert len(templates) == 1
    template, context = templates[0]
    assert template.name == 'index.html'
    assert len(context['items']) == 10

Make sure to subscribe with an extra **extra argument so that your calls don’t fail if Flask introduces new arguments to the signals.

All the template rendering in the code issued by the application app in the body of the with block will now be recorded in the templates variable. Whenever a template is rendered, the template object as well as context are appended to it.

Additionally there is a convenient helper method (connected_to()). that allows you to temporarily subscribe a function to a signal with a context manager on its own. Because the return value of the context manager cannot be specified that way one has to pass the list in as argument:

from flask import template_rendered

def captured_templates(app, recorded, **extra):
    def record(sender, template, context):
        recorded.append((template, context))
    return template_rendered.connected_to(record, app)

The example above would then look like this:

templates = []
with captured_templates(app, templates, **extra):
    ...
    template, context = templates[0]

Blinker API Changes

The connected_to() method arrived in Blinker with version 1.1.

Creating Signals

If you want to use signals in your own application, you can use the blinker library directly. The most common use case are named signals in a custom Namespace.. This is what is recommended most of the time:

from blinker import Namespace
my_signals = Namespace()

Now you can create new signals like this:

model_saved = my_signals.signal('model-saved')

The name for the signal here makes it unique and also simplifies debugging. You can access the name of the signal with the name attribute.

For Extension Developers

If you are writing a Flask extension and you want to gracefully degrade for missing blinker installations, you can do so by using the flask.signals.Namespace class.

Sending Signals

If you want to emit a signal, you can do so by calling the send() method. It accepts a sender as first argument and optionally some keyword arguments that are forwarded to the signal subscribers:

class Model(object):
    ...

    def save(self):
        model_saved.send(self)

Try to always pick a good sender. If you have a class that is emitting a signal, pass self as sender. If you emitting a signal from a random function, you can pass current_app._get_current_object() as sender.

Passing Proxies as Senders

Never pass current_app as sender to a signal. Use current_app._get_current_object() instead. The reason for this is that current_app is a proxy and not the real application object.

Signals and Flask’s Request Context

Signals fully support The Request Context when receiving signals. Context-local variables are consistently available between request_started and request_finished, so you can rely on flask.g and others as needed. Note the limitations described in Sending Signals and the request_tearing_down signal.

Decorator Based Signal Subscriptions

With Blinker 1.1 you can also easily subscribe to signals by using the new connect_via() decorator:

from flask import template_rendered

@template_rendered.connect_via(app)
def when_template_rendered(sender, template, context, **extra):
    print 'Template %s is rendered with %s' % (template.name, context)

Core Signals

The following signals exist in Flask:

flask.template_rendered

This signal is sent when a template was successfully rendered. The signal is invoked with the instance of the template as template and the context as dictionary (named context).

Example subscriber:

def log_template_renders(sender, template, context, **extra):
    sender.logger.debug('Rendering template "%s" with context %s',
                        template.name or 'string template',
                        context)

from flask import template_rendered
template_rendered.connect(log_template_renders, app)
flask.request_started

This signal is sent before any request processing started but when the request context was set up. Because the request context is already bound, the subscriber can access the request with the standard global proxies such as request.

Example subscriber:

def log_request(sender, **extra):
    sender.logger.debug('Request context is set up')

from flask import request_started
request_started.connect(log_request, app)
flask.request_finished

This signal is sent right before the response is sent to the client. It is passed the response to be sent named response.

Example subscriber:

def log_response(sender, response, **extra):
    sender.logger.debug('Request context is about to close down.  '
                        'Response: %s', response)

from flask import request_finished
request_finished.connect(log_response, app)
flask.got_request_exception

This signal is sent when an exception happens during request processing. It is sent before the standard exception handling kicks in and even in debug mode, where no exception handling happens. The exception itself is passed to the subscriber as exception.

Example subscriber:

def log_exception(sender, exception, **extra):
    sender.logger.debug('Got exception during processing: %s', exception)

from flask import got_request_exception
got_request_exception.connect(log_exception, app)
flask.request_tearing_down

This signal is sent when the request is tearing down. This is always called, even if an exception is caused. Currently functions listening to this signal are called after the regular teardown handlers, but this is not something you can rely on.

Example subscriber:

def close_db_connection(sender, **extra):
    session.close()

from flask import request_tearing_down
request_tearing_down.connect(close_db_connection, app)

As of Flask 0.9, this will also be passed an exc keyword argument that has a reference to the exception that caused the teardown if there was one.

flask.appcontext_tearing_down

This signal is sent when the app context is tearing down. This is always called, even if an exception is caused. Currently functions listening to this signal are called after the regular teardown handlers, but this is not something you can rely on.

Example subscriber:

def close_db_connection(sender, **extra):
    session.close()

from flask import request_tearing_down
appcontext_tearing_down.connect(close_db_connection, app)

This will also be passed an exc keyword argument that has a reference to the exception that caused the teardown if there was one.