Receiver decorator in Django - Step-by-Step Execution

Concept Flow - Receiver decorator

Define signal handler function

↓

Apply @receiver decorator with signal

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Connect handler to signal

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Signal sent somewhere in app

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Django calls connected handler

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Handler executes with signal data

Shows how a function is decorated to listen for a signal, then called automatically when that signal is sent.

Execution Sample

Django

from django.dispatch import receiver, Signal

my_signal = Signal()

@receiver(my_signal)
def my_handler(sender, **kwargs):
    print('Signal received')

Defines a signal and a handler function that prints a message when the signal is received.

Execution Table

Step	Action	Evaluation	Result
1	Define my_signal as a new Signal	my_signal created	Signal object ready
2	Define my_handler function	Function created	Function exists but not connected
3	Apply @receiver(my_signal) decorator	Connect my_handler to my_signal	my_handler registered as listener
4	Send my_signal	Django finds connected handlers	my_handler called
5	my_handler runs	Print 'Signal received'	Message output to console
6	No more handlers	Signal processing ends	Execution complete

💡 All connected handlers called; signal processing finished

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	Final
my_signal	undefined	Signal object	Signal object	Signal object	Signal object
my_handler	undefined	Function object	Connected function	Connected function	Connected function

Key Moments - 2 Insights

Why does the handler function run only after the signal is sent?

Because the @receiver decorator only connects the function to the signal; the function runs when Django sends the signal (see execution_table step 4 and 5).

Can the handler function be called without sending the signal?

No, the handler runs only when the signal is sent. Defining and connecting it does not execute it immediately (see execution_table steps 2 and 3).

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 3?

A The handler function prints output

B The handler function is connected to the signal

C The signal is sent

D The signal object is created

💡 Hint

Check the 'Action' and 'Result' columns at step 3 in the execution_table

At which step does the handler function actually run?

A Step 5

B Step 4

C Step 2

D Step 1

💡 Hint

Look for when the handler prints output in the execution_table

If the signal is never sent, what happens to the handler function?

A It runs immediately after definition

B It runs when the app starts

C It never runs

D It runs twice

💡 Hint

Refer to the variable_tracker and execution_table steps 4 and 5

Concept Snapshot

Receiver decorator in Django:
- Use @receiver(signal) above a function
- Connects function to listen for that signal
- Function runs only when signal is sent
- Handler receives sender and kwargs
- Useful for decoupled event handling

Full Transcript

The receiver decorator in Django is used to connect a function to a signal. First, you define a signal object. Then, you write a handler function that will respond to that signal. By applying the @receiver decorator with the signal as argument, Django registers the function as a listener. When the signal is sent somewhere in the app, Django automatically calls the connected handler function, passing the sender and any extra data. The handler then executes its code, such as printing a message. This allows different parts of the app to communicate without tight coupling. The handler only runs when the signal is sent, not before.