Python3 impossible to pass @property as decorator argument

Question

I've implemented decorator that can receive extra arguments and want to use it with class methods. I want to pass @property as decorator argument, but instead of @property result I got this:

<property object at 0x7f50f5195230>

This is my decorator:

class Decorator(object):
    def __init__(self, some_arg):
        self.func = None
        self.some_arg = some_arg

    def __get__(self, instance, owner):
        import functools
        return functools.partial(self.__call__, instance)

    def __call__(self, func):
        self.func = func
        def wrapper(*args, **kwargs):
            return self._process_sync(*args, **kwargs)
        return wrapper

    def _process_sync(self, *args, **kwargs):
        try:
            print(self.some_arg)
            return self.func(*args, **kwargs)
        except Exception as e:
            print(e)
            return None

My test class:

class Test(object):
    @property
    def some_data(self):
        return {'key': 'value'}

    @Decorator(some_data)
    def some_method(self):
        print('method output')
        return None

Usage:

test = Test()
test.some_method()

Two questions:

1. How to correctly pass property to receive @property result instead of <property object at 0x7f50f5195230>
2. Does it possible to pass class properties/methods to the decorator if they are below in code?

Answer 1

A property object is a descriptor. To get a value out of it, you need to call its __get__ method with an appropriate instance. Figuring out when to do that in your current code is not easy, since your Decorator object has a bunch of different roles. It's both a decorator factory (getting initialized with an argument in the @Decorator(x) line), and the decorator itself (getting called with the function to be decorated). You've given it a __get__ method, but I don't expect that to ever get used, since the instance of Decorator never gets assigned to a class variable (only the wrapper function that gets returned from __call__ ).

Anyway, here's a modified version where the Decorator handles almost all parts of the descriptor protocol itself:

class Decorator:
    def __init__(self, arg):
        self.arg = arg      # this might be a descriptor, like a property or unbound method

    def __call__(self, func):
        self.func = func
        return self         # we still want to be the descriptor in the class

    def __get__(self, instance, owner):
        try:
            arg = self.arg.__get__(instance, owner)   # try to bind the arg to the instance
        except AttributeError: # if it doesn't work, self.arg is not a descriptor, that's OK
            arg = self.arg

        def wrapper(*args, **kwargs):   # this is our version of a bound method object
            print(arg) # do something with the bound arg here
            return self.func.__get__(instance, owner)(*args, **kwargs)

        return wrapper