Connecting signal handlers in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a signal handler is connected incorrectly?

Consider this Django signal connection code:

from django.db.models.signals import post_save
from django.dispatch import receiver

@receiver(post_save, sender=MyModel)
def my_handler(sender, instance, **kwargs):
    print('Saved:', instance)

What will happen if you forget to import MyModel correctly and connect the signal?

A The signal handler will be called for all models, ignoring the sender filter.

B Django will raise an ImportError at runtime when connecting the signal.

C The signal handler will be called twice for each save event.

D The signal handler will never be called because the sender is not recognized.

💡 Hint

Think about how the sender argument filters which model triggers the signal.

If the sender is not the correct model class, the signal handler won't be triggered for saves of that model. The handler is connected but never called because the sender filter doesn't match.

📝 Syntax

intermediate

Which option correctly connects a signal handler using the connect method?

You want to connect a function my_handler to Django's post_delete signal for MyModel. Which code snippet is correct?

A post_delete.connect(my_handler, sender=MyModel)

B post_delete.connect(sender=MyModel, my_handler)

C post_delete.connect(my_handler)

D post_delete.connect(my_handler, MyModel)

💡 Hint

Remember the order of arguments for the connect method.

The connect method takes the receiver function as the first argument and the sender as a keyword argument. Option A follows this pattern correctly.

❓ state_output

advanced

What is printed when a signal handler modifies instance data before saving?

Given this code:

from django.db.models.signals import pre_save
from django.dispatch import receiver

@receiver(pre_save, sender=MyModel)
def modify_name(sender, instance, **kwargs):
    instance.name = instance.name.upper()

obj = MyModel(name='hello')
obj.save()
print(obj.name)

What will be printed?

A hello

B HELLO

C None

D An error is raised

💡 Hint

The pre_save signal runs before the model instance is saved.

The pre_save signal handler changes the instance's name to uppercase before saving. The saved instance and the variable obj reflect this change, so printing obj.name outputs 'HELLO'.

🔧 Debug

advanced

Why does this signal handler run multiple times unexpectedly?

This code connects a signal handler inside a Django view function:

def my_view(request):
    from django.db.models.signals import post_save

    def handler(sender, instance, **kwargs):
        print('Saved:', instance)

    post_save.connect(handler, sender=MyModel)
    # ... rest of view code ...
    return HttpResponse('Done')

Why might the handler print multiple times on multiple requests?

A Because the signal connection happens on every request, adding duplicate handlers.

B Because post_save signals are only sent once per model class.

C Because the handler function is defined inside the view, it is not recognized.

D Because Django caches signal handlers and reuses them.

💡 Hint

Think about where the connect call is placed and how often the view runs.

Connecting a signal handler inside a view means it runs every time the view is called, adding a new handler each time. This causes multiple prints for one save event.

🧠 Conceptual

expert

What is the effect of using dispatch_uid when connecting signal handlers?

In Django signals, what does providing a unique dispatch_uid argument to connect do?

A Filters signals to only those with matching dispatch_uid.

B Changes the order in which signal handlers are called.

C Prevents the same signal handler from being connected multiple times.

D Makes the signal handler run asynchronously.

💡 Hint

Think about how Django avoids duplicate signal connections.

The dispatch_uid argument uniquely identifies a signal handler connection. If a handler with the same dispatch_uid is connected again, Django ignores it, preventing duplicates.