# Layman’s guide to Signals in Django

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# Introduction & Analogy:

What is a `signal` in **Django** ?

> Let's say you and your friend have planned a robbery. You will enter the bank and your friend will be waiting outside.
> 
> As soon as police arrives, your friend will give you a `signal` through a `dispatch` device, and you will receive that signal using a `receiver`.
> 
> Now it's your decision to make, how you will respond to that `signal`.

Similarly, in **Django** `signals` are fired at some defined actions, and you can **receive** the `signals` using `receiver` **decorators**.

# Built-in Signals in Django:

**Django** provides a set of built-in `signals` that lets your `code` get notified from **Django** of certain actions. You can import these signals from here - `django.db.models.signals`.

* `pre_save` & `post_save` **signals**, respectively sent before and after a **model’s** `save()` method is called.
    
* `pre_delete` & `post_delete`, respectively sent before and after a model’s `delete()` method or queryset’s `delete()` method is called.
    
* `request_started` & `request_finished`, respectively sent when **Django** starts and finishes an `HTTP` request.
    
* `m2m_changed`, sent when a `ManyToManyField` on a **model** is changed.
    

# Using a `signal`:

In most of the cases, we use `signals` for **model** actions, but you can define your own custom `signal` on a custom action.

We can **receive** a `signal` in two ways, first is through `connect()` method and second is through `@receiver` **decorator**.

* Using `connect()` **method**:
    

In the below example, `function_to_execute` will run just before any **model** instance will be saved.

```python
from django.core.signals import pre_save

def function_to_execute(sender, **kwargs): 
	print("Received the signal!") 
pre_save.connect(function_to_execute)
```

* Using `@receiver` **decorator:**
    

```python
from django.core.sigoaLs import pre_save 
from django.dispatch import receiver

@receiver(pre_save)
def function_to_execute(sender, **kwargs):
	print("Received the signal!")
```

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