Python Decorator Self-firing

I am fairly new to Python and have been learning about decorators. After messing around with Flask, I am trying to write some code that simulates their route handler/decorators, ju

Solution 1:

@app.route("/", methods = ["GET","PUT"]) is an executable statement: it calls the route() method of the app object. Since it's at module level, it will be executed when the script is imported.

Now, the result of calling app.route(...) is a function, and because you've used the @ to mark it as a decorator, that function will wrap index. Note that the syntax is just a shortcut for this:

index = app.route(...)(index)

in other words, Python will call the function returned by app.route() with index as a parameter, and store the result as the new index function.

However, you're missing a level here. A normal decorator, without params, is written like this:

@foodefbar()
   pass

and when the module is imported, foo() is run and returns a function that wraps bar. But you're calling your route() function within the decorator call! So actually your function needs to return a decorator function that itself returns a function that wraps the original function... headscratching, to be sure.

Your route method should look more like this:

defroute(self, *rargs, **kargs):
    args = list(rargs)
    if kargs:
        print(kargs['methods'])
    defdecorator(f):
        defwrapped(index_args):
            f(args[0])
        return wrapped
    return decorator

Solution 2:

Basically... app.route(index, "/", ["GET", "PUT"]) is a function. And this is the function which is going to be called instead of index.

In your code, when you call index(), it calls app.route(index, "/", ["GET", "PUT"]). This starts by printing kargs['methods'], then creates the decorator function:

defdecorator(f):
    f(args[0])

This decorator will call the decorated function (f) with one argument, args[0], which here is "/". This prints route: /.

The best explanation of decorators I've found is here: How to make a chain of function decorators?

If you dont want the self-firing, you can define your decorator this way:

defroute(*rargs, **kargs):
    args = list(rargs)
    if kargs:
        print(kargs['methods'])
    defdecorator(f):
        f(args[0])
    return decorator

@app.route("/", methods = ["GET","PUT"])defindex(rt):
    print('route: ' + rt)

However, the rt argument of index will never be used, because route always calls index with args[0] which is always \...