from weakref import WeakKeyDictionary as store

class WriteOnce:
  def __init__(self):
    self.__values = store()
  def __get__(self, obj, obj_type=None):
    return self.__values(obj) if obj_type is None else self

  def __set__(self, obj, value):
    if not obj in self.__values:
      self.__values[obj] = value
    else:
      return AttributeError