Exception middleware in Django - Mini Project: Build & Apply

Create Custom Exception Middleware in Django

📖 Scenario: You are building a Django web application that needs to handle errors gracefully. Instead of showing default error pages, you want to catch exceptions globally and return a simple JSON response with an error message.

🎯 Goal: Build a custom exception middleware in Django that catches all exceptions and returns a JSON response with a message and status code.

📋 What You'll Learn

Create a middleware class named ExceptionMiddleware

Add a __init__ method that accepts get_response

Add a __call__ method that calls get_response and catches exceptions

Return a JSON response with {"error": "An error occurred"} and status code 500 when an exception happens

💡 Why This Matters

🌍 Real World

Web applications often need to handle errors gracefully and provide clear feedback to users or clients, especially APIs that expect JSON responses.

💼 Career

Understanding how to write custom middleware and handle exceptions globally is a valuable skill for backend developers working with Django or similar web frameworks.

1

Create the ExceptionMiddleware class

Create a class called ExceptionMiddleware with an __init__ method that takes get_response as a parameter and stores it in self.get_response.

Django

class ExceptionMiddleware:
    def __init__(self, get_response):
        # Your code here

Hint

The __init__ method is called once when the middleware is created. Store get_response so you can call it later.

2

Add the __call__ method to process requests

Add a __call__ method to ExceptionMiddleware that takes request as a parameter and calls self.get_response(request). Return the response from this call.

Django

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

    def __call__(self, request):
        # Your code here

Hint

The __call__ method handles each request. Call the next middleware or view by calling self.get_response(request).

3

Catch exceptions and return JSON error response

Modify the __call__ method to catch any exception raised by self.get_response(request). If an exception occurs, import JsonResponse from django.http and return JsonResponse({"error": "An error occurred"}, status=500).

Django

from django.http import JsonResponse

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

    def __call__(self, request):
        # Your code here

Hint

Use a try-except block to catch errors. Return a JSON response with status 500 inside the except block.

4

Add middleware to Django settings

Add the string "yourapp.middleware.ExceptionMiddleware" to the MIDDLEWARE list in your Django settings.py file to activate the middleware.

Django

# In settings.py
MIDDLEWARE = [
    # other middleware entries
    # Your code here
]

Hint

Insert the middleware path as a string in the MIDDLEWARE list to enable 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