Custom signals in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a custom signal is sent without any receivers?

Consider a Django custom signal defined and sent, but no receiver functions are connected to it. What is the behavior when the signal is sent?

Django

from django.dispatch import Signal

my_signal = Signal()

my_signal.send(sender=None)

A The signal send method returns an empty list, and no error occurs.

B A RuntimeError is raised because no receivers are connected.

C The signal send method returns None and logs a warning.

D The signal send method raises a TypeError due to missing sender argument.

💡 Hint

Think about what happens if no one is listening when you shout.

When a Django signal is sent but no receivers are connected, the send method returns an empty list indicating no receivers handled the signal. No error is raised.

📝 Syntax

intermediate

Which option correctly defines a custom Django signal with providing arguments?

You want to create a custom Django signal that sends two arguments: 'user' and 'action'. Which code snippet correctly defines this signal?

A my_signal = Signal(provides=['user', 'action'])

B my_signal = Signal(providing=['user', 'action'])

C my_signal = Signal(args=['user', 'action'])

D my_signal = Signal(providing_args=['user', 'action'])

💡 Hint

Check the exact keyword argument name for declaring arguments in Django signals.

The correct keyword argument to specify the arguments a signal provides is 'providing_args'. Other options are invalid and cause errors.

🔧 Debug

advanced

Why does this custom signal receiver not get called?

Given the code below, why does the receiver function never execute when the signal is sent?

Django

from django.dispatch import Signal, receiver

my_signal = Signal(providing_args=['data'])

@receiver(my_signal)
def my_receiver(sender, **kwargs):
    print('Received:', kwargs.get('data'))

# Somewhere else in code
my_signal.send(sender=None, data='Hello')

A The signal must be sent with a sender class, not None.

B The receiver is not connected because the module defining it was never imported.

C The signal send call is missing the required sender argument.

D The receiver function signature is incorrect; it must accept 'data' explicitly.

💡 Hint

Think about how Django connects signals and when decorators run.

Django connects signal receivers when the module containing them is imported. If the module is not imported, the receiver is never registered and won't be called.

❓ state_output

advanced

What is the output of this custom signal with multiple receivers?

Given the code below, what will be printed when the signal is sent?

Django

from django.dispatch import Signal, receiver

my_signal = Signal(providing_args=['value'])

@receiver(my_signal)
def receiver_one(sender, **kwargs):
    print('One:', kwargs.get('value'))
    return 'one'

@receiver(my_signal)
def receiver_two(sender, **kwargs):
    print('Two:', kwargs.get('value'))
    return 'two'

results = my_signal.send(sender=None, value=42)
print(results)

A [(<function receiver_one at ...>, 'one'), (<function receiver_two at ...>, 'two')]

B [('receiver_one', 'one'), ('receiver_two', 'two')]

C [(receiver_one, 'one'), (receiver_two, 'two')]

D [(<function receiver_one>, 'one'), (<function receiver_two>, 'two')]

💡 Hint

Recall what the send method returns: a list of tuples with receiver references and their return values.

The send method returns a list of tuples. Each tuple contains the receiver function object (with memory address) and its return value. The printed output shows function references with memory addresses.

🧠 Conceptual

expert

Which statement about Django custom signals is TRUE?

Select the only true statement about Django custom signals.

A Custom signals must always specify a sender class; None is not allowed.

B Signal receivers can modify the sender argument before other receivers are called.

C The send method returns a list of tuples with each receiver and its return value.

D Django automatically connects custom signals to model save events.

💡 Hint

Think about what the send method returns after notifying receivers.

The send method returns a list of tuples, each containing a receiver function and its return value. Other options are false: sender can be None, receivers cannot modify sender for others, and Django does not auto-connect custom signals to model events.