RB34 RB34 - 1 month ago 12
Python Question

how do decorated functions work in flask/python? (app.route)

Edit 2: I misunderstood how decorators work. The decorator is run even if the decorated function is not called (even though you might not see it's effects). function = dec(function) would be an equivalent way of showing what a decorator does and obviously in that scenario the dec() function runs without any calls to function().

Edit: Why is my post being down voted with no explanation? How can I fix it? There are multiple answers, one which clearly answers the question. What is the issue?

I've been learning about decorators in python and I think I have a good grasp on them. However I am still slightly confused about how the app.route decorator works in flask. By my understanding the decorator changes the behavior of a function but does not run unless the function is called. So if I have:

def hello():
return "Hello world"


The hello function will get passed to app.route and whatever behavior the decorator dictates will execute. However in flask apps the function itself never appears to be run (in my above example it is). How is the route function/decorator executed if the function it decorates is never called? I know that app.route essentially stores the "/" along with its corresponding function in a dictionary but I don't understand how this code is ever executed without any calls of the decorated function. I assume it is somehow connected to the app.run at the end of flask apps but I am unclear on how app.run can call the functions you defined.

Edit: to add to what I've shown here. There is an example from this explanation: https://ains.co/blog/things-which-arent-magic-flask-part-1.html That raises the same questions. I would think that hello() needs to be called in order for the route function to do anything.

class NotFlask():
def __init__(self):
self.routes = {}

def route(self, route_str):
def decorator(f):
self.routes[route_str] = f
return f

return decorator

def serve(self, path):
view_function = self.routes.get(path)
if view_function:
return view_function()
raise ValueError('Route "{}"" has not been

app = NotFlask()

def hello():
return "Hello World!"

Answer Source

Python decorators like this:

def func():

Can be changed to look like this instead:

def func():


Or in other words, they're functions that take functions. In some circumstances, you might not see the effects of the decorator immediately, so it may seem like the decorator itself isn't used until the function it decorates is called, but that's not an actual restriction of Python decorators.

In your code, @app.route("/") is a decorator which adds an endpoint to the app object. It doesn't actually modify any behavior of the function, and is instead sugar to simplify the process. Without the route() decorator, this is how you'd do the equivalent route registration in Flask.

from flask import Flask
app = Flask(_name_)

def hello():
    return "Hello world"

app.add_url_rule("/", "hello", hello)

And if you look at the implementation of the route decorator in Flask, you'll see that this is the equivalent.

def route(self, rule, **options):
        """A decorator that is used to register a view function for a
        given URL rule.  This does the same thing as :meth:`add_url_rule`
        but is intended for decorator usage::
            def index():
                return 'Hello World'
        For more information refer to :ref:`url-route-registrations`.
        :param rule: the URL rule as string
        :param endpoint: the endpoint for the registered URL rule.  Flask
                         itself assumes the name of the view function as
        :param options: the options to be forwarded to the underlying
                        :class:`~werkzeug.routing.Rule` object.  A change
                        to Werkzeug is handling of method options.  methods
                        is a list of methods this rule should be limited
                        to (``GET``, ``POST`` etc.).  By default a rule
                        just listens for ``GET`` (and implicitly ``HEAD``).
                        Starting with Flask 0.6, ``OPTIONS`` is implicitly
                        added and handled by the standard request handling.
        def decorator(f):
            endpoint = options.pop('endpoint', None)
            self.add_url_rule(rule, endpoint, f, **options)
            return f
        return decorator

So you can see that route adds it to the app's router, so once the Flask app gets a request, it decides how to execute code/view for the endpoint that's been requested.