class_init_val.py

#!/usr/bin/env python
from time import sleep

def do_heavy_process():
    sleep(3)
    return 10

class myclass():
    def __init__(self):
        self.__val = None
    @property
    def val(self):
        self.__val = self.__val or do_heavy_process()
        return self.__val


obj = myclass()
print obj.val
print obj.val