Is it possible to use methods on decorators?

Question

I was looking around the aiortc examples when I notice a decorator that has a method on it:

@pc.on("datachannel")
    def on_datachannel(channel):
    ...

I don't really understand how this work or what does this code do. I've been searching about decorators and I know it's possible to have class decorators but none about using methods. Can anyone elaborate on this?

Answer 1

@foo
def bar(): ...

This syntax is merely sugar for this:

def bar(): ...
bar = foo(bar)

So, this:

@pc.on('datachannel')
def on_datachannel(channel): ...

is the same as:

def on_datachannel(channel): ...
on_datachannel = pc.on('datachannel')(on_datachannel)

pc is some object, pc.on is a method on it, pc.on('datachannel') calls it and it returns a function, pc.on('datachannel')(on_datachannel) calls that returned function passing it the on_datachannel function.

The implementation of pc.on is something like this:

class PC:
    def on(self, event):
        ...
        def wrapper(fn):
            ...
            def inner_wrapper(*args, **kwargs):
                ...
                fn(*args, **kwargs)

            return inner_wrapper

        return wrapper

pc = PC()

All that inner part is entirely a regular decorator accepting arguments. That it's defined on a class makes no difference to it.