Is this a decorator of some type in Python?

Question

I have been reading through some code and came across this below.

I do not understand whether the @SdServer.appId(APP_ID) is a decorator. It has the @ from a decorator, but the class method appId does not look like a decorator syntax that I am used to. I do not understand what this is doing.

The print statements that I include at the end looking for the appID in the SdApp class returns this:

SdApp class instance ['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'request'] 

SdApp instance request ['__call__', '__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__func__', '__ge__', '__get__', '__getattribute__', '__gt__', '__hash__', '__init__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__self__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__'] 

method list ['request']

The code

APP_ID = 'oA'

class SdServer(object):
    APP_ID_HANDLERS = {}

    def __init__(self, originator):
        self.originator = originator

    @classmethod
    def appId(cls, appId):
        def _handler(f):
            cls.APP_ID_HANDLERS[appId] = f
            return f

        return _handler


@SdServer.appId(APP_ID)
class SdApp(object):
    @classmethod
    def request(cls, originator, body=None):
        try:
            print(cls)
        except OException as e:
            log.error('Cannot process request: %s', e)

# me trying to figure out what it is doing below

first = SdApp()

print('SdApp class instance', dir(first), '\n')
print('SdApp instance request', dir(first.request), '\n')

method_list = [func for func in dir(SdApp) if callable(getattr(SdApp, func)) and not func.startswith("__")]

print('method list', method_list)

Answer 1

The classmethod itself is not the decorator but rather its return value. In your example the @SdServer.appId(APP_ID) will call the classmethod and use the result as a decorator. Following your example further this would be the _handler function which seems to register the decorated class with the SdServer class. This returned decorator contains closures over the cls and the appId variables hence the somewhat convoluted implementation.