python-2.7 - MRJob 中没有 mapper() 的 reduce() 会做什么？

Question

我是 python 的新手，正在尝试按照说明 http://www.yekeren.com/blog/archives/1005 构建推荐系统,

让我困惑的是:

def reducer3_init(self):
        self.pop = { }

        file = open(self.options.item_pop, "r")

        for line in file.readlines():
            movieid_jstr, pop_jstr = line.strip().split("\t")
            movieid = json.loads(movieid_jstr)
            pop     = json.loads(pop_jstr)

            self.pop[movieid] = pop
        file.close()

    def reducer3(self, key, values):
        yield key, sum(values) / math.sqrt(self.pop[key[0]] * self.pop[key[1]])

reduce3没有对应的mapper，它是如何执行的？ json.load() 是做什么的？ 非常感谢!!

Answer 1

documentation说:

class mrjob.step.MRStep(**kwargs)

Used by MRJob.steps. See Multi-step jobs for sample usage.

Accepts the following keyword arguments. Parameters:

mapper – function with same function signature as mapper(), or None for an identity mapper.

这是一个成语，将默认值映射到身份并减少到扁平化。