A simple Flask server

from flask import Flask
app = Flask(__name__)
 
@app.route('/') # -------------------- 2
def hello_world():
    return 'Hello World!'
 
if __name__ == '__main__':
    app.run() # ---------------------- 1

1 Flask Request Entry

This is a simple werkzeug server

from werkzeug.wrappers import Request, Response

@Request.application
def hello(request):
    return Response('Hello World!')

if __name__ == '__main__':
    from werkzeug.serving import run_simple
    # listen to `http://localhost:4000/` and do a hello() once we visit it.
    run_simple('localhost', 4000, hello)

app.run(): bases on werkzeug.serving.run_simple(host, port, self, **options). So once we visit its route, the entry of Flask application is app(), which is the Flask.__call__.
Flask.__call__: use wsgi_app
- return self.wsgi_app(environ, start_response)
In the end, the real entry is wsgi_app(environ, start_response)

2 Route Register

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

This Python Decorator is equivalant to

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def hello_world():
    return "Hello World"
app.route("/")(hello_world)

app.route(): base on self.add_url_rule(rule, endpoint, f, **options). It binds a function with a url. So these two are equivalant
- app.add_url_rule("/", hello_world.__name__, hello_world)
- app.route("/")(hello_world)
add_url_rule(rule, endpoint, view_func):
- rule = self.url_rule_class(rule, methods=methods, **options): combine url, methods(post, get…), endpoint together
- self.url_map.add(rule): this url_map is werkzeug.routing.Map
- self.view_function[endpoint] = view_func, it binds endpoint with function and is used when dispatching request

3 Dispatching Request

From Flask Request Entry, the real request entry is wsgi_app(environ, start_response), What does it do with the Routing system ?

push request_context into context
Flask.dispatch_request
- return self.view_functions[rule.endpoint](**req.view_args)

Finally, it is equal to return hello_world()