python 2.7 - What does reduce() do without mapper() in MRJob? -

i new python , trying build recommendation system following instruction http://www.yekeren.com/blog/archives/1005,

what confuses me :

def reducer3_init(self):         self.pop = { }          file = open(self.options.item_pop, "r")          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]]) 

the reduce3 has no corresponding mapper,how execute ? , json.load() do? lot thanks!!

the documentation says:

class mrjob.step.mrstep(**kwargs)

used mrjob.steps. see multi-step jobs sample usage. 

accepts following keyword arguments. parameters:

mapper – function same function signature mapper(), or none identity mapper. 

it idiom map defaults identity , reduce flatten.