Qiitaで検索すると解答例がみつかりますよ。
模範にならないべた書き例:
import sys
scores = {}
for line in open(sys.argv[1]).readlines()[1:]:
timestamp, player_id, score = line.split(',')
scores.setdefault(player_id, []).append(int(score))
ranking = sorted([(player_id, int(sum(score) / len(score) + 0.5))
for player_id, score in scores.items()], key=lambda r: -r[1])
print('rank,player_id,mean_score')
last_score = -1
for i, (player_id, score) in enumerate(ranking, 1):
if last_score != score:
last_score = score
rank = i
if rank > 10:
break
print(rank, player_id, score, sep=',')
Like!