Signal dispatch process in Django - Step-by-Step Execution

Concept Flow - Signal dispatch process

Signal sent

↓

Dispatcher receives signal

↓

Find connected receivers

↓

For each receiver

↓

Call receiver function

↓

Receiver processes signal

↓

Signal dispatch complete

This flow shows how Django sends a signal, finds all connected receivers, calls each one, and completes the dispatch.

Execution Sample

Django

from django.dispatch import Signal

my_signal = Signal()

receiver_called = False

def receiver(sender, **kwargs):
    global receiver_called
    receiver_called = True
    print('Signal received')

my_signal.connect(receiver)
my_signal.send(sender=None)

This code creates a signal, connects a receiver function, and sends the signal to trigger the receiver.

Execution Table

Step	Action	Evaluation	Result
1	Create signal object	my_signal = Signal()	Signal object created
2	Define receiver function	def receiver(...)	Function ready
3	Connect receiver	my_signal.connect(receiver)	Receiver added to signal
4	Send signal	my_signal.send(sender=None)	Dispatcher starts
5	Find receivers	Receivers list contains 'receiver'	One receiver found
6	Call receiver	receiver(sender=None)	Prints 'Signal received'
7	Finish dispatch	No more receivers	Signal dispatch complete

💡 All connected receivers called, signal dispatch ends

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 6	Final
my_signal.receivers	[]	[receiver]	[receiver]	[receiver]	[receiver]
receiver_called	False	False	False	True	True

Key Moments - 2 Insights

Why does the receiver function get called when we send the signal?

Because in step 5 the dispatcher finds the connected receiver, and in step 6 it calls the receiver function as shown in the execution_table.

What happens if no receivers are connected when the signal is sent?

The dispatcher finds an empty receivers list at step 5, so no functions are called and the dispatch ends immediately as per the exit_note.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of my_signal.receivers after step 3?

A None

B Empty list

C Contains the receiver function

D Contains multiple receivers

💡 Hint

Check variable_tracker column 'After Step 3' for my_signal.receivers

At which step does the receiver function actually get called?

A Step 4

B Step 6

C Step 5

D Step 7

💡 Hint

Look at execution_table row with action 'Call receiver'

If we connect two receivers before sending, how would the execution_table change?

A Step 5 would find two receivers, and step 6 would call each one

B Step 5 would find one receiver, step 6 calls only one

C Step 4 would not send the signal

D No change in execution_table

💡 Hint

Consider how the dispatcher finds and calls all connected receivers

Concept Snapshot

Django signals let parts of your app talk to each other.
Create a Signal object.
Connect receiver functions to it.
Send the signal to call all receivers.
Receivers run their code when signal is sent.
Signal dispatch calls each connected receiver in order.

Full Transcript

In Django, the signal dispatch process starts when a signal is sent. The dispatcher looks up all functions connected as receivers to that signal. It then calls each receiver function one by one, passing sender and any extra info. After all receivers run, the dispatch ends. This lets different parts of your app respond to events without tight connections. The example code shows creating a signal, connecting a receiver, and sending the signal to trigger the receiver. The execution table traces each step from signal creation to calling the receiver. Variables track the list of receivers and if the receiver was called. Key moments clarify why receivers get called and what happens if none are connected. The quiz tests understanding of when receivers are found and called, and how multiple receivers affect dispatch. The snapshot summarizes the process simply for quick recall.