Why middleware extends functionality in Flask

Introduction

Middleware helps add extra features to your web app without changing its main code. It acts like a helper that works between the user and your app.

You want to check user login before letting them see a page.

You need to log every request a user makes to your app.

You want to add security checks like blocking bad users.

You want to modify requests or responses, like adding headers.

You want to handle errors in one place for all requests.

Syntax

Flask

from flask import Flask, request

app = Flask(__name__)

@app.before_request
def middleware_function():
    # Code here runs before each request
    pass

@app.after_request
def after_middleware(response):
    # Code here runs after each request
    return response

@app.before_request runs code before each request reaches your route.

@app.after_request runs code after your route sends a response.

Examples

This middleware checks if the request has an Authorization header. If not, it stops the request with a 401 error.

Flask

from flask import Flask, request

app = Flask(__name__)

@app.before_request
def check_auth():
    if not request.headers.get('Authorization'):
        return 'Unauthorized', 401

This middleware adds a custom header to every response sent by the app.

Flask

from flask import Flask

app = Flask(__name__)

@app.after_request
def add_header(response):
    response.headers['X-Custom-Header'] = 'Hello'
    return response

Sample Program

This simple Flask app uses middleware to print the path of every request before showing the homepage.

Flask

from flask import Flask, request

app = Flask(__name__)

@app.before_request
def log_request():
    print(f"Request to {request.path}")

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

if __name__ == '__main__':
    app.run(debug=True)

Important Notes

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

Use middleware to keep your main code clean and focused on core tasks.

Flask middleware can stop a request early by returning a response inside before_request.

Summary

Middleware adds extra steps before or after your main app code runs.

It helps with tasks like security, logging, and modifying requests or responses.

Using middleware keeps your app organized and easier to maintain.