# Τεκμηρίωση του Django - 1.10.7

# Signals

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.

## Listening to signals

To receive a signal, register a receiver function using the `Signal.connect()` method. The receiver function is called when the signal is sent.

`Signal.``connect`(receiver, sender=None, weak=True, dispatch_uid=None)

  Παράμετροι: | receiver – The callback function which will be connected to this signal. See Receiver functions for more information. sender – Specifies a particular sender to receive signals from. See Connecting to signals sent by specific senders for more information. weak – Django stores signal handlers as weak references by default. Thus, if your receiver is a local function, it may be garbage collected. To prevent this, pass `weak=False` when you call the signal’s `connect()` method. dispatch_uid – A unique identifier for a signal receiver in cases where duplicate signals may be sent. See Preventing duplicate signals for more information.

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.

### 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:

`receiver`(signal)

  Παράμετροι: | 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()`.

Σημείωση

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.

### 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 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")

## Defining and sending signals

Your applications can take advantage of the signal infrastructure and provide its own signals.

### 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()` (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(object):
        ...

        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()`.

## Disconnecting signals

`Signal.``disconnect`(receiver=None, sender=None, dispatch_uid=None)

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.

Αποσύρθηκε στην έκδοση 1.9: The `weak` argument is deprecated as it has no effect. It will be removed in Django 2.0.