Exception middleware in Django

Introduction

Exception middleware helps catch errors in your Django app and handle them gracefully. It keeps your app from crashing and shows friendly error messages.

You want to log errors automatically when something goes wrong.

You want to show custom error pages instead of default ones.

You want to send notifications when exceptions happen.

You want to clean up resources or do special actions after an error.

You want to handle specific errors differently in one place.

Syntax

Django

class YourExceptionMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response

    def __call__(self, request):
        try:
            response = self.get_response(request)
            return response
        except Exception as e:
            # Handle the exception here
            return self.handle_exception(request, e)

    def handle_exception(self, request, exception):
        # Your custom error handling logic
        pass

The middleware class must accept get_response in __init__.

The __call__ method processes the request and can catch exceptions.

Examples

This middleware catches any exception and returns a simple error message.

Django

from django.http import HttpResponse

class SimpleExceptionMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response

    def __call__(self, request):
        try:
            return self.get_response(request)
        except Exception:
            return HttpResponse('Something went wrong!', status=500)

This middleware logs the error before returning a 500 response.

Django

from django.http import HttpResponse
import logging

class LoggingExceptionMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response

    def __call__(self, request):
        try:
            return self.get_response(request)
        except Exception as e:
            logging.error(f'Error: {e}')
            return HttpResponse('Internal server error', status=500)

Sample Program

This middleware catches any exception during request processing and returns a 500 response with the error message.

Django

from django.http import HttpResponse

class ExceptionMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response

    def __call__(self, request):
        try:
            response = self.get_response(request)
            return response
        except Exception as e:
            return self.handle_exception(request, e)

    def handle_exception(self, request, exception):
        return HttpResponse(f'Error caught: {exception}', status=500)

# Usage in settings.py:
# MIDDLEWARE = [
#     'yourapp.middleware.ExceptionMiddleware',
#     ...
# ]

Important Notes

Always add your middleware class to the MIDDLEWARE list in settings.py.

Be careful not to hide useful error details during development; use custom error pages mainly in production.

You can customize handle_exception to log errors or send alerts.

Summary

Exception middleware catches errors during request handling.

It helps show friendly error messages or log problems.

Write a class with __init__ and __call__ methods to create it.

Practice

1. What is the main purpose of exception middleware in Django?

easy

A. To catch errors during request processing and handle them gracefully

B. To speed up database queries

C. To serve static files like images and CSS

D. To manage user authentication sessions

Solution

1. Step 1: Understand middleware role

   Middleware processes requests and responses in Django, and exception middleware specifically handles errors.

2. Step 2: Identify exception middleware purpose

   Its job is to catch exceptions during request handling and provide friendly error messages or logging.

3. Final Answer:

   To catch errors during request processing and handle them gracefully -> Option A

4. Quick Check:

   Exception middleware = catch errors [OK]

Hint: Exception middleware catches errors in requests [OK]

Common Mistakes:

• Confusing middleware with static file serving
• Thinking it manages database queries
• Assuming it handles user sessions

2. Which method must be implemented in a Django exception middleware class to process requests?

easy

A. __init__

B. __call__

C. process_exception

D. handle_request

Solution

1. Step 1: Recall middleware structure

   Django middleware classes require an __init__ and a __call__ method to be callable.

2. Step 2: Identify request processing method

   The __call__ method is called for each request and is where exception handling happens.

3. Final Answer:

   __call__ -> Option B

4. Quick Check:

   Request processing method = __call__ [OK]

Hint: Middleware uses __call__ to handle requests [OK]

Common Mistakes:

• Choosing process_exception which is for old-style middleware
• Confusing __init__ as request handler
• Inventing non-existent handle_request method

3. Given this middleware snippet, what will be the output if a ZeroDivisionError occurs during request processing?

class ExceptionMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response

    def __call__(self, request):
        try:
            response = self.get_response(request)
        except ZeroDivisionError:
            return HttpResponse('Division error caught')
        return response

medium

A. The original response from get_response

B. A server error 500 page

C. No response, request hangs

D. HttpResponse with text 'Division error caught'

Solution

1. Step 1: Analyze try-except block

   The middleware calls get_response inside try; if ZeroDivisionError occurs, it returns a custom HttpResponse.

2. Step 2: Determine output on error

   When ZeroDivisionError happens, the except block returns HttpResponse('Division error caught').

3. Final Answer:

   HttpResponse with text 'Division error caught' -> Option D

4. Quick Check:

   Error caught returns custom response [OK]

Hint: Exception triggers except block response [OK]

Common Mistakes:

• Assuming original response is returned despite error
• Expecting default 500 error page
• Thinking request will hang without response

4. Identify the error in this exception middleware code:

class ExceptionMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response

    def __call__(self, request):
        try:
            response = self.get_response(request)
        except Exception as e:
            print('Error:', e)
        return response

medium

A. No __init__ method defined

B. Incorrect method name __call__

C. Missing return statement inside except block

D. Using print instead of logging

Solution

1. Step 1: Check exception handling flow

   If an exception occurs, except block prints error but does not return a response.

2. Step 2: Understand middleware response requirement

   Middleware must always return a response; missing return in except causes NameError or no response.

3. Final Answer:

   Missing return statement inside except block -> Option C

4. Quick Check:

   Exception block must return response [OK]

Hint: Always return response in except block [OK]

Common Mistakes:

• Ignoring missing return causes runtime error
• Thinking print is enough for error handling
• Confusing method names or missing __init__

5. You want to create exception middleware that logs errors and returns a JSON error response with status 500. Which code snippet correctly implements this behavior?

import json
from django.http import HttpResponse

class ExceptionMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response

    def __call__(self, request):
        try:
            response = self.get_response(request)
        except Exception as e:
            # Log the error
            print(f'Error: {e}')
            # Return JSON error response
            error_content = json.dumps({'error': 'Server error'})
            return HttpResponse(error_content, content_type='application/json', status=500)
        return response

hard

A. Correctly logs error and returns JSON response with status 500

B. Does not log error, only returns JSON response

C. Returns HTML response instead of JSON

D. Raises exception instead of handling it

Solution

1. Step 1: Check error logging

   The except block prints the error, which acts as logging here.

2. Step 2: Verify JSON response and status

   It returns HttpResponse with JSON content, correct content_type, and status 500.

3. Final Answer:

   Correctly logs error and returns JSON response with status 500 -> Option A

4. Quick Check:

   Logs error + JSON 500 response [OK]

Hint: Print error then return JSON with status 500 [OK]

Common Mistakes:

• Forgetting to set content_type to application/json
• Not returning response in except block
• Raising exception instead of handling