avg_by_key.py

import sys

d = {}
for line in sys.stdin:
    key,value = line.strip().split('\t')
    count, running_value = d.get(key, [0, 0])
    d[key] = (count + 1, running_value + float(value))

for k,v in d.iteritems():
    print '%s\t%.2f' % (k, v[1] / v[0])