Django includes a “signal dispatcher” which helps allow decoupled applications get notified when actions occur elsewhere in the framework. In a nutshell, signals allow certain senders to notify a set of receivers that some action has taken place. They’re especially useful when many pieces of code may be interested in the same events.
Django provides a set of built-in signals that let user code get notified by Django itself of certain actions. These include some useful notifications:
django.db.models.signals.pre_save &
django.db.models.signals.post_save
Sent before or after a model’s save() method
is called.
django.db.models.signals.pre_delete &
django.db.models.signals.post_delete
Sent before or after a model’s delete()
method or queryset’s delete()
method is called.
django.db.models.signals.m2m_changed
Sent when a ManyToManyField on a model is changed.
django.core.signals.request_started &
django.core.signals.request_finished
Sent when Django starts or finishes an HTTP request.
See the built-in signal documentation for a complete list, and a complete explanation of each signal.
You can also define and send your own custom signals; see below.
To receive a signal, register a receiver function using the
Signal.connect() method. The receiver function is called when the signal
is sent. All of the signal’s receiver functions are called one at a time, in
the order they were registered.
Signal.connect(receiver, sender=None, weak=True, dispatch_uid=None)[source]¶| Parameters: |
|
|---|
Let’s see how this works by registering a signal that
gets called after each HTTP request is finished. We’ll be connecting to the
request_finished signal.
First, we need to define a receiver function. A receiver can be any Python function or method:
def my_callback(sender, **kwargs):
print("Request finished!")
Notice that the function takes a sender argument, along with wildcard
keyword arguments (**kwargs); all signal handlers must take these arguments.
We’ll look at senders a bit later, but right now look at the **kwargs
argument. All signals send keyword arguments, and may change those keyword
arguments at any time. In the case of
request_finished, it’s documented as sending no
arguments, which means we might be tempted to write our signal handling as
my_callback(sender).
This would be wrong – in fact, Django will throw an error if you do so. That’s because at any point arguments could get added to the signal and your receiver must be able to handle those new arguments.
There are two ways you can connect a receiver to a signal. You can take the manual connect route:
from django.core.signals import request_finished
request_finished.connect(my_callback)
Alternatively, you can use a receiver() decorator:
receiver(signal)[source]¶| Parameters: | signal – A signal or a list of signals to connect a function to. |
|---|
Here’s how you connect with the decorator:
from django.core.signals import request_finished
from django.dispatch import receiver
@receiver(request_finished)
def my_callback(sender, **kwargs):
print("Request finished!")
Now, our my_callback function will be called each time a request finishes.
Where should this code live?
Strictly speaking, signal handling and registration code can live anywhere
you like, although it’s recommended to avoid the application’s root module
and its models module to minimize side-effects of importing code.
In practice, signal handlers are usually defined in a signals
submodule of the application they relate to. Signal receivers are
connected in the ready() method of your
application configuration class. If you’re using the receiver()
decorator, simply import the signals submodule inside
ready().
Note
The ready() method may be executed more than
once during testing, so you may want to guard your signals from
duplication, especially if you’re planning
to send them within tests.
Some signals get sent many times, but you’ll only be interested in receiving a
certain subset of those signals. For example, consider the
django.db.models.signals.pre_save signal sent before a model gets saved.
Most of the time, you don’t need to know when any model gets saved – just
when one specific model is saved.
In these cases, you can register to receive signals sent only by particular
senders. In the case of django.db.models.signals.pre_save, the sender
will be the model class being saved, so you can indicate that you only want
signals sent by some model:
from django.db.models.signals import pre_save
from django.dispatch import receiver
from myapp.models import MyModel
@receiver(pre_save, sender=MyModel)
def my_handler(sender, **kwargs):
...
The my_handler function will only be called when an instance of MyModel
is saved.
Different signals use different objects as their senders; you’ll need to consult the built-in signal documentation for details of each particular signal.
In some circumstances, the code connecting receivers to signals may run multiple times. This can cause your receiver function to be registered more than once, and thus called multiple times for a single signal event.
If this behavior is problematic (such as when using signals to
send an email whenever a model is saved), pass a unique identifier as
the dispatch_uid argument to identify your receiver function. This
identifier will usually be a string, although any hashable object will
suffice. The end result is that your receiver function will only be
bound to the signal once for each unique dispatch_uid value:
from django.core.signals import request_finished
request_finished.connect(my_callback, dispatch_uid="my_unique_identifier")
Your applications can take advantage of the signal infrastructure and provide its own signals.
When to use custom signals
Signals are implicit function calls which make debugging harder. If the sender and receiver of your custom signal are both within your project, you’re better off using an explicit function call.
All signals are django.dispatch.Signal instances. The
providing_args is a list of the names of arguments the signal will provide
to listeners. This is purely documentational, however, as there is nothing that
checks that the signal actually provides these arguments to its listeners.
For example:
import django.dispatch
pizza_done = django.dispatch.Signal(providing_args=["toppings", "size"])
This declares a pizza_done signal that will provide receivers with
toppings and size arguments.
Remember that you’re allowed to change this list of arguments at any time, so getting the API right on the first try isn’t necessary.
There are two ways to send signals in Django.
To send a signal, call either Signal.send() (all built-in signals use
this) or Signal.send_robust(). You must provide the sender argument
(which is a class most of the time) and may provide as many other keyword
arguments as you like.
For example, here’s how sending our pizza_done signal might look:
class PizzaStore:
...
def send_pizza(self, toppings, size):
pizza_done.send(sender=self.__class__, toppings=toppings, size=size)
...
Both send() and send_robust() return a list of tuple pairs
[(receiver, response), ... ], representing the list of called receiver
functions and their response values.
send() differs from send_robust() in how exceptions raised by receiver
functions are handled. send() does not catch any exceptions raised by
receivers; it simply allows errors to propagate. Thus not all receivers may
be notified of a signal in the face of an error.
send_robust() catches all errors derived from Python’s Exception class,
and ensures all receivers are notified of the signal. If an error occurs, the
error instance is returned in the tuple pair for the receiver that raised the error.
The tracebacks are present on the __traceback__ attribute of the errors
returned when calling send_robust().
To disconnect a receiver from a signal, call Signal.disconnect(). The
arguments are as described in Signal.connect(). The method returns
True if a receiver was disconnected and False if not.
The receiver argument indicates the registered receiver to disconnect. It
may be None if dispatch_uid is used to identify the receiver.
Nov 02, 2020