Why middleware extends functionality in Flask - See It in Action

Why middleware extends functionality in Flask

📖 Scenario: You are building a simple Flask web app that logs every request's path before processing it. Middleware helps add this logging without changing the main app code.

🎯 Goal: Create a Flask app and add middleware that logs the request path for every incoming request, showing how middleware extends app functionality.

📋 What You'll Learn

Create a Flask app instance named app

Define a middleware function named log_middleware that logs the request path

Apply the middleware to the Flask app using app.wsgi_app

Create a route /hello that returns 'Hello, World!'

💡 Why This Matters

🌍 Real World

Middleware is used in real web apps to add features like logging, authentication, or error handling without changing core app code.

💼 Career

Understanding middleware helps developers build scalable and maintainable web applications by cleanly extending functionality.

1

Create the Flask app instance

Create a Flask app instance called app by importing Flask from flask and initializing it with Flask(__name__).

Flask

from flask import Flask

# Create the Flask app instance called app
# Your code here

Need a hint?

Use app = Flask(__name__) to create the app instance.

2

Define the middleware function

Define a middleware function named log_middleware that takes environ and start_response as parameters. Inside, print the request path from environ['PATH_INFO'], then call the next app in the chain with app.wsgi_app(environ, start_response) and return its result.

Flask

from flask import Flask

app = Flask(__name__)

# Define the middleware function log_middleware
# Your code here

Need a hint?

Middleware functions take environ and start_response, print the path, then call app.wsgi_app.

3

Apply the middleware to the Flask app

Assign the middleware function log_middleware to app.wsgi_app so it wraps the original app. Use app.wsgi_app = log_middleware.

Flask

from flask import Flask

app = Flask(__name__)

def log_middleware(environ, start_response):
    print(f"Request path: {environ['PATH_INFO']}")
    return app.wsgi_app(environ, start_response)

# Apply the middleware by assigning log_middleware to app.wsgi_app
# Your code here

Need a hint?

Save the original app.wsgi_app before wrapping it, then assign log_middleware to app.wsgi_app.

4

Add a route to test the middleware

Create a route /hello using @app.route('/hello') that returns the string 'Hello, World!'.

Flask

from flask import Flask

app = Flask(__name__)

original_wsgi_app = app.wsgi_app

def log_middleware(environ, start_response):
    print(f"Request path: {environ['PATH_INFO']}")
    return original_wsgi_app(environ, start_response)

app.wsgi_app = log_middleware

# Add a route /hello that returns 'Hello, World!'
# Your code here

Need a hint?

Use @app.route('/hello') to define the route and a function that returns the greeting.