django builtin signals

1what are django signals?
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3The Django Signals is a strategy to allow decoupled applications to get notified when certain events occur
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5There are two key elements in the signals machinary: the senders and the receivers. As the name suggests, the sender is the one responsible to dispatch a signal, and the receiver is the one who will receive this signal and then do something.
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7A receiver must be a function or an instance method which is to receive signals.
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9A sender must either be a Python object, or None to receive events from any sender.
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11The connection between the senders and the receivers is done through “signal dispatchers”, which are instances of Signal, via the connect method.
12So to receive a signal, you need to register a receiver function that gets called when the signal is sent by using the Signal.connect() method.
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1django post_save signals
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3Let’s have a look on the post_save built-in signal. Its code lives in the django.db.models.signals module. This particular signal fires right after a model finish executing its save method.
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5from django.contrib.auth.models import User
6from django.db.models.signals import post_save
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8def save_profile(sender, instance, **kwargs):
9    instance.profile.save()
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11post_save.connect(save_profile, sender=User)
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13Another way to register a signal, is by using the @receiver decorator:
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15from django.contrib.auth.models import User
16from django.db.models.signals import post_save
17from django.dispatch import receiver
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19@receiver(post_save, sender=User)
20def save_profile(sender, instance, **kwargs):
21    instance.profile.save()
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1django built-in signals 
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3Django's built-in Signals:
4Django provides a set of built-in signals that let user code get notified by Django itself of certain actions. These include some useful notifications:
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6django.db.models.signals.pre_save & django.db.models.signals.post_save : Sent before or after a model's save() method is called
7django.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
8django.db.models.signals.m2m_changed : Sent when a ManyToManyField on a model is changed
9django.core.signals.request_started & django.core.signals.request_finished : Sent when Django starts or finishes an HTTP request