Question

%%file word_count.py
# From http://mrjob.readthedocs.org/en/latest/guides/quickstart.html#writing-your-first-job
from mrjob.job import MRJob

class MRWordFrequencyCount(MRJob):

    def mapper(self, _, line):
        yield "chars", len(line)
        yield "words", len(line.split())
        yield "lines", 1

    def reducer(self, key, values):
        yield key, sum(values)


if __name__ == '__main__':
    MRWordFrequencyCount.run()

Overwriting word_count.py