Custom signals in Django - Step-by-Step Execution

Concept Flow - Custom signals

Define signal

↓

Connect receiver function

↓

Trigger signal.send()

↓

Django calls receiver

↓

Receiver executes custom code

↓

Signal handling complete

This flow shows how a custom signal is defined, connected to a receiver, triggered, and handled in Django.

Execution Sample

Django

from django.dispatch import Signal, receiver

# Define signal
my_signal = Signal()

# Connect receiver
def my_receiver(sender, **kwargs):
    print(f"Received: {kwargs['data']}")

my_signal.connect(my_receiver)

# Trigger signal
my_signal.send(sender=None, data="Hello")

This code defines a custom signal, connects a receiver function, and triggers the signal with data.

Execution Table

Step	Action	Evaluation	Result
1	Define my_signal	Signal object created	my_signal ready to use
2	Define my_receiver	Function defined	my_receiver ready
3	Connect my_receiver to my_signal	my_signal.connect(my_receiver)	Receiver registered
4	Call my_signal.send(sender=None, data='Hello')	Signal triggers	my_receiver called
5	Inside my_receiver: print(f"Received: {kwargs['data']}")	Print statement executed	Output: Received: Hello

💡 Signal handling complete after receiver executes

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	Final
my_signal	undefined	Signal object	Signal with receiver connected	Signal triggered	Signal ready for next use
my_receiver	undefined	undefined	Connected to signal	Called by signal	Function execution done

Key Moments - 3 Insights

Why do we connect the receiver function to the signal before sending it?

The receiver must be connected first so Django knows which function to call when the signal is sent, as shown in step 3 and 4 of the execution_table.

What happens if no receiver is connected when the signal is sent?

If no receiver is connected, sending the signal does nothing because no function is called, so no output appears (no step 5 execution).

Why do we pass 'sender' when sending the signal?

The 'sender' identifies who sent the signal; it can be None or any object. Receivers can use it to filter signals, as shown in step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is printed at step 5?

A Received: Hello

B Hello

C Signal sent

D No output

💡 Hint

Check the 'Result' column in step 5 of the execution_table.

At which step is the receiver function connected to the signal?

A Step 1

B Step 3

C Step 4

D Step 5

💡 Hint

Look for the action 'Connect my_receiver to my_signal' in the execution_table.

If we do not call my_signal.send(), what happens?

A Receiver is called anyway

B Signal is connected twice

C Receiver is never called

D Error occurs

💡 Hint

Refer to key_moments about what happens if no signal is sent.

Concept Snapshot

Custom signals in Django:
- Define with Signal()
- Connect receiver with signal.connect(func)
- Send signal with signal.send(sender, **kwargs)
- Receiver runs when signal sent
- Use for decoupled event handling

Full Transcript

This visual execution shows how to create and use custom signals in Django. First, a signal object is defined. Then, a receiver function is created and connected to the signal. When the signal is sent with data, Django calls the receiver function, which executes custom code such as printing a message. The variable tracker shows the signal and receiver states at each step. Key moments clarify why connection must happen before sending and what happens if no receiver is connected. The quiz tests understanding of the printed output, connection step, and behavior when the signal is not sent.