sample descriptor

>>> class Descriptor:
...     def __get__(self, instance, owner):
...         print('__get__', instance, owner)
...         print('called on a', 'class' if instance is None else 'instance')
...
>>> class Class:
...     foo = Descriptor()
...
>>> Class.foo
__get__ None <class '__main__.Class'>
called on a class
>>> Class().foo
__get__ <__main__.Class object at 0x7f95551cf4e0> <class '__main__.Class'>
called on a instance