class OnePingOnlyPleaseVassily(object):

    def __init__(self):
        self.value = None

def set(self, value):
    if self.value is not None:
        raise Exception('Already set.')
self.value = value
someStateMemo = OnePingOnlyPleaseVassily()
someStateMemo.set(aValue)
someStateMemo.set(aValue)
Эквивалент ключевого слова `final` для переменных в Python?