Lazily Evaluated Variable

lazy.py

def lazy(func):
    def get(self):
        val = func(self)
        setattr(self, func.func_name, val)
        return val
    def set(self, val):
        setattr(self, func.func_name, val)
    return property(get, set)

Decorating a method with @lazy will turn it into a property that will replace itself with its return value when evaluated.