Python write once field

write_once.py

from weakref import WeakKeyDictionary
from collections.abc import Hashable

#Only works with hashable objects
class WriteOnce:
  def __init__(self):
    self.__values = WeakKeyDictionary()
    
  def __get__(self, obj, obj_type=None):
    if obj_type is not None:
      return self
    return self.__values.get(obj) if isinstance(obj, Hashable) else AttributeError
  
  def __set__(self, obj, value):
    if isinstance(obj, Hashable) and not obj in self.__values :
      self.__values[obj] = value
    else:
      return AttributeError