It's actually encapsulating old wine , Put in a new bottle , Add some more stuff yourself . Retention pit , It's not used yet .

Flask Your request extension is Django Middleware .Django The middleware is not Flask Middleware

from flask import Flask

app = Flask(__name__)

@app.route('/')
def index():
    return 'Hello World!'

class Md(object):
    def __init__(self,old_wsgi_app):
        self.old_wsgi_app = old_wsgi_app

    def __call__(self,  environ, start_response):
        print(' Before the start ')
        # ... Some custom actions 
        ret = self.old_wsgi_app(environ, start_response)
        # ... Some custom actions 
        print(' After that ')
        return ret

if __name__ == '__main__':
    #  Encapsulate the old app.wsgi_app, This is a app.run() Principle , Look at the source code . Would call __call__ Method 
    app.wsgi_app = Md(app.wsgi_app)
    app.run()

from flask import Flask, flash, redirect, render_template, request
 
app = Flask(__name__)
app.secret_key = 'some_secret'
 
@app.route('/')
def index1():
    return render_template('index.html')
 
@app.route('/set')
def index2():
    v = request.args.get('p')
    flash(v)
    return 'ok'
 
class MiddleWare:
    def __init__(self,wsgi_app):
        self.wsgi_app = wsgi_app
 
    def __call__(self, *args, **kwargs):
 
        return self.wsgi_app(*args, **kwargs)
 
if __name__ == "__main__":
    app.wsgi_app = MiddleWare(app.wsgi_app)
    app.run(port=9999)

problem ： Is this Flask-extensions Implementation principle of ？ See that it is extended by a third party . Future research