class Memoized(object):

    '''

    Descriptor for population statistical varibles caching.

    '''

    def __init__(self, func):

        self.func = func

        self.result = None

        self.fitness = None

    def __get__(self, instance, cls):

        self.instance = instance

        return self

    def __call__(self, fitness):

        if ((not self.instance._updated)         # population not changed

                and (self.result is not None)    # result already cached

                and (fitness == self.fitness)):  # fitness not changed

            # Return cached result directly.

            return self.result

        else:

            # Update fitness function.

            self.fitness = fitness

            # Update and memoize result.

            self.result = self.func(self.instance, fitness)

            # Recover flag.

            self.instance._updated = False

            return self.result