test_descriptor.py

class Foo(object):
  def __init__(self, key):
    self.key = key

  def __get__(self, obj, type):
    if int(obj.__dict__.get(self.key, 0)) > 90:
      return 'Great'
    else:
      return 'Shit'

  def __set__(self, obj, value):
    if value > 0:
      # here shows that __dict__ was an `None` so that can not be assigned.
      obj.__dict__[self.key] = value 
    else:
      raise ValueError('Cannot be negative.')

class Bar(object):
  score = Foo('score')

  def __init__(self):
    self.score = 70

  def __getattribute__(self, key):
    if key is 'score':
      return 'from __getattribute__ hi jack with score'
    else:
      super(Bar, self).__getattribute__(key)

b = Bar()
b.score # here comes an error