map_gen.py

def mapper1(_, line):
    for word in line:
        yield ((len(word), word[0]), 1)

def reducer1(key, values):
    yield key, sum(values)

def mapper2(key, value):
    length, letter = key
    yield (length, (value, letter))

def reducer2(key, values):
    count, letter = max(values)
    yield (key, letter)