How Decorator-Based Routing Works in Python

We have seen usage of @route() decorator in Flask and they really make routing easy and backend development fast. But how do they really work? How do we implement one?

We wrote a decorator and added print("hello", f.__name__) to see if it really gets printed. If you are still not familiar how to write decorators, you can find a separate blog here.

def route(path):
    def _route(f):
        print("hello", f.__name__)
        return f
    return _route


@route("/")
def index():
    pass


if __name__ == '__main__':
    pass

As you can see, we did not call any function. Not even index() function. And you can find that there is a console output.

hello index

This proves that decorators are called during object initialization.

Let's add another decorator and check the output.

@route("/")
@route("/index")
def index():
    pass

After running the code, we will see the same output.

hello index
hello index

From here, we can easily register the paths.

ROUTE_MAP = {}


def route(path):
    def _route(f):
        ROUTE_MAP[path] = f
        return f
    return _route


@route("/")
@route("/index")
def index():
    print("index is called")


if __name__ == '__main__':
    print(ROUTE_MAP)  # {'/index': <function index at 0x103f89cb0>, '/': <function index at 0x103f89cb0>}

And calling the index function from the route is simply done using ROUTE_MAP.

if __name__ == '__main__':
    ROUTE_MAP["/"]()  # index is called
    ROUTE_MAP["/index"]()  # index is called

Conclusion

Decorators are always called during object initialization.

Decorators are alway called during object initialization and that's a fact. It makes decorators very versatile and very dangerous. 3rd-party decorators might do something bad during initialization so be careful on using one.