Creating custom middleware in Django - Why You Should Know This

The Big Idea

What if you could control every web request with just one simple piece of code?

The Scenario

Imagine you want to check every web request to your Django app for a special token or log details before the page loads.

You try adding this check inside every view function manually.

The Problem

Manually adding checks in every view is tiring and easy to forget.

It leads to repeated code and bugs if you miss a spot.

Updating the logic means changing many places, which wastes time and causes errors.

The Solution

Custom middleware lets you write one piece of code that runs automatically on every request or response.

This keeps your code clean, consistent, and easy to update.

Before vs After

✗ Before

def my_view(request):
    if not check_token(request):
        return error_response()
    # rest of view code

✓ After

class TokenCheckMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response
    def __call__(self, request):
        if not check_token(request):
            return error_response()
        response = self.get_response(request)
        return response

What It Enables

You can control and modify all requests and responses in one place, making your app more secure and maintainable.

Real Life Example

Many websites use middleware to log user activity or block bad requests before they reach the main app logic.

Key Takeaways

Manual checks in every view cause repeated code and errors.

Custom middleware runs code automatically on all requests.

This makes your app easier to maintain and more reliable.

Practice

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

easy

A. To run code before and after a view processes a request

B. To define database models

C. To create HTML templates

D. To handle user authentication only

Solution

1. Step 1: Understand middleware role

   Middleware runs code before and after views handle requests.

2. Step 2: Compare options

   Only To run code before and after a view processes a request describes middleware's purpose correctly; others describe unrelated tasks.

3. Final Answer:

   To run code before and after a view processes a request -> Option A

4. Quick Check:

   Middleware = pre/post view code [OK]

Hint: Middleware runs code around views, not models or templates [OK]

Common Mistakes:

• Confusing middleware with models or templates
• Thinking middleware only handles authentication
• Believing middleware runs only after views

2. Which method must a Django custom middleware class implement to process requests and responses?

easy

A. __call__

B. __init__

C. process_request

D. handle_request

Solution

1. Step 1: Recall middleware class structure

   Custom middleware uses __init__ for setup and __call__ to handle requests and responses.

2. Step 2: Identify correct method for processing

   __call__ is the method that processes requests and responses; others are incorrect or deprecated.

3. Final Answer:

   __call__ -> Option A

4. Quick Check:

   Middleware processing method = __call__ [OK]

Hint: Use __call__ to process requests in custom middleware [OK]

Common Mistakes:

• Using process_request which is old style
• Confusing __init__ with request processing
• Inventing non-existent methods like handle_request

3. Given this middleware code, what will be printed when a request is processed?

class SimpleMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response
        print('Middleware initialized')

    def __call__(self, request):
        print('Before view')
        response = self.get_response(request)
        print('After view')
        return response

medium

A. After view\nBefore view\nMiddleware initialized

B. Middleware initialized\nBefore view\nAfter view

C. Before view\nAfter view\nMiddleware initialized

D. Middleware initialized only

Solution

1. Step 1: Understand when __init__ runs

   __init__ runs once when middleware is created, printing 'Middleware initialized'.

2. Step 2: Trace __call__ execution order

   __call__ prints 'Before view', calls the view, then prints 'After view'.

3. Final Answer:

   Middleware initialized\nBefore view\nAfter view -> Option B

4. Quick Check:

   Init then before and after view prints [OK]

Hint: Init prints once; __call__ prints before and after view [OK]

Common Mistakes:

• Thinking __init__ runs on every request
• Mixing order of print statements
• Ignoring that __call__ wraps the view call

4. What is wrong with this custom middleware code?

class MyMiddleware:
    def __init__(self):
        pass

    def __call__(self, request):
        response = self.get_response(request)
        return response

medium

A. No error, code is correct

B. __call__ should not return a response

C. Missing get_response parameter in __init__

D. Middleware classes cannot have __call__ method

Solution

1. Step 1: Check __init__ signature

   Middleware __init__ must accept get_response parameter to store it for later use.

2. Step 2: Identify missing attribute

   get_response is used in __call__, but not saved in __init__, causing an error.

3. Final Answer:

   Missing get_response parameter in __init__ -> Option C

4. Quick Check:

   __init__ needs get_response [OK]

Hint: Always accept get_response in __init__ for middleware [OK]

Common Mistakes:

• Omitting get_response parameter
• Not storing get_response as instance variable
• Thinking __call__ cannot return response

5. You want to create a custom middleware that adds a header 'X-Hello: World' to every response. Which code snippet correctly implements this?

hard

A. class HelloMiddleware:
def __init__(self, get_response):
self.get_response = get_response
def process_response(self, request, response):
response['X-Hello'] = 'World'
return response

B. class HelloMiddleware:
def __init__(self):
pass
def __call__(self, request):
response = self.get_response(request)
response.headers.add('X-Hello', 'World')
return response

C. class HelloMiddleware:
def __call__(self, request):
response = self.get_response(request)
response['X-Hello'] = 'World'
return response

D. class HelloMiddleware:
def __init__(self, get_response):
self.get_response = get_response
def __call__(self, request):
response = self.get_response(request)
response['X-Hello'] = 'World'
return response

Solution

1. Step 1: Confirm __init__ accepts get_response

   class HelloMiddleware: def __init__(self, get_response): self.get_response = get_response def __call__(self, request): response = self.get_response(request) response['X-Hello'] = 'World' return response correctly accepts and stores get_response in __init__.

2. Step 2: Check __call__ modifies response

   class HelloMiddleware: def __init__(self, get_response): self.get_response = get_response def __call__(self, request): response = self.get_response(request) response['X-Hello'] = 'World' return response calls get_response, adds header, and returns response properly.

3. Step 3: Identify errors in other options

   class HelloMiddleware: def __init__(self): pass def __call__(self, request): response = self.get_response(request) response.headers.add('X-Hello', 'World') return response misses get_response in __init__; class HelloMiddleware: def __call__(self, request): response = self.get_response(request) response['X-Hello'] = 'World' return response misses __init__; class HelloMiddleware: def __init__(self, get_response): self.get_response = get_response def process_response(self, request, response): response['X-Hello'] = 'World' return response uses old process_response method not supported in new style middleware.

4. Final Answer:

   class HelloMiddleware: def __init__(self, get_response): self.get_response = get_response def __call__(self, request): response = self.get_response(request) response['X-Hello'] = 'World' return response -> Option D

5. Quick Check:

   New middleware = __init__ + __call__ + modify response [OK]

Hint: New middleware needs get_response in __init__ and modifies response in __call__ [OK]

Common Mistakes:

• Omitting get_response in __init__
• Using old process_response method
• Trying to add headers before calling get_response