What is the "endpoint" in flask's .add_url_rule()?

Consider the following code

import flask

class API:
    def hello(self):
        return flask.Response('hello', 200)

api = API()
app = flask.Flask(__name__)
app.add_url_rule('/', 'hello', api.hello)
app.run()

It returns "hello" upon a GET call to /.

The documentation for add_url_rule states that

The route() decorator is a shortcut to call this with the view_func argument.

It requires however at least three parameters. The first and third one are understandable and mimic @route(). What is the second one (hello in my case)?

The documentation further states that this is

endpoint – the endpoint for the registered URL rule. Flask itself assumes the name of the view function as endpoint

What does this mean? Why isn't the URL (/) and the method to call (api.hello) sufficient? What is the role of the "endpoint"? How is it exactly used?

Solution

It's the name for the route; the one you'd use in the url_for() function, for example. The endpoint name is the registration key for views, a symbolic name by which you can reference the route from other parts of your application.

@route() takes the same parameter; the default is the name of the decorated function. This is documented both in the add_url_rule() documentation as well as the documentation for the underlying Rule() object, which is what these calls create:

endpoint – The endpoint for this rule. This can be anything. A reference to a function, a string, a number etc. The preferred way is using a string because the endpoint is used for URL generation.

The @route() decorator documentation itself states this:

The endpoint name for the route defaults to the name of the view function if the endpoint parameter isn’t passed.

(bold italic emphasis mine).

So for the specific example in the @route documentation:

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

the endpoint name is index.

Since you asked your question, the Flask documentation has been updated and improved a lot, and the documentation for add_url_rule() has been updated to make this point clear too; the text Defaults to view_func.__name__ has been added, and these arguments have been made keyword arguments.