Signals¶

Receiver functions¶

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.

Connecting receiver functions¶

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 when you define your receiver:

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.

Note that receiver can also take a list of signals to connect a function to.

The ability to pass a list of signals was added.

Connecting to signals sent by specific senders¶

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.

Preventing duplicate signals¶

In some circumstances, the module in which you are connecting signals may be imported multiple times. This can cause your receiver function to be registered more than once, and thus called multiples 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")

Defining signals¶

class Signal([providing_args=list])¶

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.

Sending signals¶

There are two ways to send signals in Django.

Signal.send(sender, **kwargs)¶

Signal.send_robust(sender, **kwargs)¶

To send a signal, call either Signal.send() or Signal.send_robust(). You must provide the sender argument, 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(object):
    ...

    def send_pizza(self, toppings, size):
        pizza_done.send(sender=self, 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.