population.py 文件源码

def survive(self, fraction=None, n=None, luck=False) -> 'Population':
        """Let part of the population survive.

        Remove part of the population. If both fraction and n are specified,
        the minimum resulting population size is taken.

        :param fraction: Fraction of the original population that survives.
            Defaults to None.
        :type fraction: float/None
        :param n: Number of individuals of the population that survive.
            Defaults to None.
        :type n: int/None
        :param luck: If True, individuals randomly survive (with replacement!)
            with chances proportional to their fitness. Defaults to False.
        :type luck: bool
        :return: self
        """
        if fraction is None:
            if n is None:
                raise ValueError('everyone survives! must provide either "fraction" and/or "n".')
            resulting_size = n
        elif n is None:
            resulting_size = round(fraction*len(self.individuals))
        else:
            resulting_size = min(round(fraction*len(self.individuals)), n)
        self.evaluate(lazy=True)
        if resulting_size == 0:
            raise RuntimeError('no one survived!')
        if resulting_size > len(self.individuals):
            raise ValueError('everyone survives! must provide "fraction" and/or "n" < population size')
        if luck:
            self.individuals = choices(self.individuals, k=resulting_size,
                                       weights=[individual.fitness for individual in self.individuals])
        else:
            sorted_individuals = sorted(self.individuals, key=lambda x: x.fitness, reverse=self.maximize)
            self.individuals = sorted_individuals[:resulting_size]
        return self