1. Understanding Middleware in Flask

Middleware is a function that runs before or after every request in a Flask app. It is useful for logging, security checks, or modifying responses globally.

Step 1: Implementing a Simple Middleware

Create a middleware function that logs request details before processing:

from flask import Flask, request

app = Flask(__name__)

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

@app.after_request
def modify_response(response):
    response.headers["X-Custom-Header"] = "Middleware Applied"
    return response

@app.route("/")
def home():
    return "Welcome to Flask Middleware Example!"

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

📌 Explanation:

• @app.before_request: Runs before every request, logs request details.
• @app.after_request: Runs after every request, modifies response headers.

2. Using Flask Hooks for Request Handling

Flask provides request hooks that allow developers to execute code at specific points in the request lifecycle:

Hook	Purpose
@app.before_request	Executes before handling a request
@app.after_request	Executes after handling a request
@app.teardown_request	Executes after the request, even if an exception occurs
@app.before_first_request	Runs only once, before the first request

Step 2: Using before_first_request Hook

This hook runs only once, before the first request is processed:

@app.before_first_request
def initialize():
    print("Flask App Started - Running Before First Request!")

Step 3: Using teardown_request Hook

This hook executes after each request, even if an error occurs:

@app.teardown_request
def cleanup(error=None):
    if error:
        print(f"An error occurred: {error}")
    print("Request cleanup executed.")

reating a Custom Middleware Class

For more advanced use cases, middleware can be implemented as a custom class:

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

    def __call__(self, environ, start_response):
        print(f"Custom Middleware: {environ['REQUEST_METHOD']} {environ['PATH_INFO']}")
        return self.app(environ, start_response)

app.wsgi_app = CustomMiddleware(app.wsgi_app)

4. Running and Testing Flask Middleware

Step 4: Running the Flask App

python app.py

Step 5: Testing in Browser or Postman

• Open http://127.0.0.1:5000/
• Check the console/logs for request details and middleware output.

Conclusion

In this guide, we covered:

✅ Middleware in Flask for request logging and response modification.
✅ Flask request hooks (before_request, after_request, teardown_request).
✅ A custom middleware class to process requests globally.

Using middleware and hooks, we can enhance security, logging, and request handling efficiency in Flask applications. 🚀