class Descriptor:

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

    def __get__(self, instance, owner=None):
       print('Acessando o __get__')
       return self.obj


class Grok:

    attr = Descriptor('value')

# Output
>>> g = Grok()
>>> g.attr
Acessando o __get__
value

class Grok:

    def __init__(self, attr):
        self.attr = Descriptor(attr)


# Output
>>> g = Grok('value')
>>> g.attr
<__main__.Descriptor at 0x7fe5bca77550>