Introduction

In this tutorial, we will explore how to use middleware in Flask to add custom functionality to your application. Middleware acts as a bridge between a web application and various functionalities like request processing and response handling. By the end of this tutorial, you will learn how to create middleware, register it with your Flask application, and use it to process requests and responses.

Prerequisites:
- Basic understanding of Python
- Basic knowledge of Flask framework
- Python and Flask installed on your system

Step-by-Step Guide

Middleware in Flask is implemented by creating classes or functions that wrap around the Flask application instance. These classes or functions are called before and after the actual request is processed.

Creating Middleware:
- Middleware in Flask can be created by defining a class with methods that accept a Flask application instance, a wsgi_app, and a response as parameters.
- The __call__ method is a special method in Python classes which makes an object callable. We'll use it to define the actions that should take place before and after the request in our middleware.

Registering Middleware:
- After creating the middleware, it has to be registered to the application. This is done by wrapping the Flask application instance with the middleware class.

Processing Requests and Responses:
- The middleware can be used to preprocess requests before they reach the actual route handlers, or to post-process responses before they are sent back to the client.

Code Examples

Let's create a simple middleware that prints a message before and after processing a request.

class SimpleMiddleware(object):
    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

Here, the __init__ method takes the Flask app instance and saves it. The __call__ method prints a message before and after the request is processed.

To register this middleware, you would wrap your Flask app instance like this:

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

Now, whenever a request is made to the server, "Before Request" will be printed before the request is processed and "After Request" will be printed after.

Summary

In this tutorial, we have learnt how to create, register and use middleware in Flask to add custom functionalities like pre-processing requests and post-processing responses. As a next step, you can try creating middleware for more complex functionalities like logging, authentication, and more.

Additional resources:
- Flask Documentation
- WSGI Middleware

Practice Exercises

1. Create a middleware that logs the start time and end time of each request.

2. Create a middleware that checks if the request method is 'GET' and only then allows it to proceed.

3. Create a middleware that adds a custom header to all responses.

Remember, the real learning happens when you start applying what you've learnt, so make sure to practice these exercises. Happy coding!