Custom middleware creation in Flask

Introduction

Middleware lets you add extra steps to handle requests and responses in your web app. It helps you run code before or after your app processes data.

You want to log every request and response for debugging.

You need to check user authentication before allowing access.

You want to modify request data before your app uses it.

You want to add custom headers to all responses.

You want to handle errors in a centralized way.

Syntax

Flask

from flask import Flask, request

class CustomMiddleware:
    def __init__(self, app):
        self.app = app

    def __call__(self, environ, start_response):
        # Code before request
        print(f"Before request: {environ['PATH_INFO']}")

        def custom_start_response(status, headers, exc_info=None):
            # Code before response
            print(f"Before response: {status}")
            return start_response(status, headers, exc_info)

        return self.app(environ, custom_start_response)

app = Flask(__name__)
app.wsgi_app = CustomMiddleware(app.wsgi_app)

@app.route('/')
def home():
    return "Hello from Flask!"

The middleware class wraps the Flask app's WSGI app.

The __call__ method handles each request and response.

Examples

This middleware just prints a message before passing the request on.

Flask

class SimpleMiddleware:
    def __init__(self, app):
        self.app = app

    def __call__(self, environ, start_response):
        print("Request received")
        return self.app(environ, start_response)

This middleware adds a custom header to every response.

Flask

class HeaderMiddleware:
    def __init__(self, app):
        self.app = app

    def __call__(self, environ, start_response):
        def custom_start_response(status, headers, exc_info=None):
            headers.append(("X-Custom-Header", "MyValue"))
            return start_response(status, headers, exc_info)
        return self.app(environ, custom_start_response)

Sample Program

This Flask app uses a custom middleware to print the request path before handling it and the response status before sending it back.

Flask

from flask import Flask

class LoggingMiddleware:
    def __init__(self, app):
        self.app = app

    def __call__(self, environ, start_response):
        path = environ.get('PATH_INFO', '')
        print(f"Request path: {path}")

        def custom_start_response(status, headers, exc_info=None):
            print(f"Response status: {status}")
            return start_response(status, headers, exc_info)

        return self.app(environ, custom_start_response)

app = Flask(__name__)
app.wsgi_app = LoggingMiddleware(app.wsgi_app)

@app.route('/')
def index():
    return "Welcome to the homepage!"

@app.route('/about')
def about():
    return "About page here."

Important Notes

Middleware runs for every request, so keep it fast and simple.

You can chain multiple middleware by wrapping app.wsgi_app multiple times.

Middleware works at the WSGI level, so it can handle requests before Flask routes them.

Summary

Middleware lets you add code before and after Flask handles requests.

Create a class with __call__ to wrap the Flask app.

Use middleware to log, modify, or check requests and responses easily.

Practice

1. What is the main purpose of creating custom middleware in a Flask application?

easy

A. To run code before and after each request is processed by Flask

B. To define new routes and views in the Flask app

C. To replace Flask's built-in request handling completely

D. To create database models for the application

Solution

1. Step 1: Understand middleware role

   Middleware runs code around request processing, before and after Flask handles requests.

2. Step 2: Compare options

   Only To run code before and after each request is processed by Flask describes this behavior. Other options describe unrelated tasks.

3. Final Answer:

   To run code before and after each request is processed by Flask -> Option A

4. Quick Check:

   Middleware purpose = run code around requests [OK]

Hint: Middleware wraps requests to add extra processing [OK]

Common Mistakes:

• Confusing middleware with route creation
• Thinking middleware replaces Flask core
• Mixing middleware with database models

2. Which of the following is the correct way to define a custom middleware class in Flask?

easy

A. class MyMiddleware:
def __init__(self, app):
self.app = app
def __call__(self, environ, start_response):
return self.app(environ, start_response)

B. def MyMiddleware(app):
return app

C. class MyMiddleware:
def handle_request(self):
pass

D. class MyMiddleware:
def __init__(self):
pass

Solution

1. Step 1: Check middleware class structure

   Middleware must accept the app in __init__ and implement __call__ with environ and start_response.

2. Step 2: Validate options

   Only class MyMiddleware: def __init__(self, app): self.app = app def __call__(self, environ, start_response): return self.app(environ, start_response) matches this pattern correctly. Others miss __call__ or parameters.

3. Final Answer:

   Correct middleware class with __init__ and __call__ methods -> Option A

4. Quick Check:

   Middleware class needs __call__ method [OK]

Hint: Middleware class must implement __call__(environ, start_response) [OK]

Common Mistakes:

• Omitting __call__ method
• Not passing app to __init__
• Using function instead of class

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

class LoggerMiddleware:
    def __init__(self, app):
        self.app = app
    def __call__(self, environ, start_response):
        print('Before request')
        response = self.app(environ, start_response)
        print('After request')
        return response

app.wsgi_app = LoggerMiddleware(app.wsgi_app)

medium

A. Only After request printed

B. Only Before request printed

C. No output printed

D. Before request printed, then After request printed

Solution

1. Step 1: Analyze __call__ method

   Prints 'Before request', calls app, then prints 'After request'.

2. Step 2: Understand call order

   Both prints happen on each request, before and after app call.

3. Final Answer:

   Before request printed, then After request printed -> Option D

4. Quick Check:

   Prints before and after app call [OK]

Hint: Middleware __call__ runs code before and after app call [OK]

Common Mistakes:

• Thinking only one print runs
• Confusing order of prints
• Assuming prints happen outside __call__

4. Identify the error in this middleware code snippet:

class BadMiddleware:
    def __init__(self, app):
        self.app = app
    def __call__(self, request):
        print('Middleware active')
        return self.app(request)

medium

A. Missing __init__ method to accept app

B. __call__ method has wrong parameters; should be (environ, start_response)

C. Middleware class should not print anything

D. The return statement should be outside __call__

Solution

1. Step 1: Check __call__ method signature

   WSGI middleware __call__ must accept (environ, start_response), not (request).

2. Step 2: Validate other parts

   __init__ is correct, printing is allowed, return inside __call__ is correct.

3. Final Answer:

   __call__ method has wrong parameters; should be (environ, start_response) -> Option B

4. Quick Check:

   Middleware __call__ signature = (environ, start_response) [OK]

Hint: Middleware __call__ needs environ and start_response params [OK]

Common Mistakes:

• Using Flask request object instead of WSGI params
• Omitting start_response parameter
• Misplacing return statement

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

hard

A. class HeaderMiddleware:
def __init__(self, app):
self.app = app
def __call__(self, environ, start_response):
response = self.app(environ, start_response)
response.headers['X-Hello'] = 'World'
return response

B. class HeaderMiddleware:
def __init__(self, app):
self.app = app
def __call__(self, environ, start_response):
start_response('200 OK', [('X-Hello', 'World')])
return self.app(environ, start_response)

C. class HeaderMiddleware:
def __init__(self, app):
self.app = app
def __call__(self, environ, start_response):
def custom_start_response(status, headers, exc_info=None):
headers.append(('X-Hello', 'World'))
return start_response(status, headers, exc_info)
return self.app(environ, custom_start_response)

D. class HeaderMiddleware:
def __init__(self, app):
self.app = app
def __call__(self, environ, start_response):
environ['X-Hello'] = 'World'
return self.app(environ, start_response)

Solution

1. Step 1: Understand WSGI header modification

   Headers are set by calling start_response with status and headers. To add headers, wrap start_response.

2. Step 2: Check options for correct header addition

   class HeaderMiddleware: def __init__(self, app): self.app = app def __call__(self, environ, start_response): def custom_start_response(status, headers, exc_info=None): headers.append(('X-Hello', 'World')) return start_response(status, headers, exc_info) return self.app(environ, custom_start_response) wraps start_response to append header before calling original start_response. Others misuse start_response or response object.

3. Final Answer:

   Wrap start_response to add header before calling original -> Option C

4. Quick Check:

   Add headers by wrapping start_response [OK]

Hint: Wrap start_response to add headers in middleware [OK]

Common Mistakes:

• Calling start_response twice incorrectly
• Trying to modify response object directly in WSGI
• Adding headers to environ instead of response