Владимир

from functools import partial

writeonly_prop=partial(property, None)

class Test:
    @writeonly_prop
    def secret(self, val):
        self.__value = val

    def print_secret(self):
        print(self.__value)

    def __init__(self):
        self.__value = None

t = Test()
t.secret = 12
t.print_secret()