原始数据是每个用户对于所看过电影的评分：
我们的用户有'Lisa Rose'，'Gene Seymour'等等，原始数据cirtics是一个字典，key为用户名，value为一个电影评分字典，该字典的key为电影名，value为评分
如何找到所有的电影的集合呢？
1 critics = {'Lisa Rose': {'Lady in the Water': 2.5, 'Snakes on a Plane': 3.5, 2 'Just My Luck': 3.0, 'Superman Returns': 3.5, 'You, Me and Dupree': 2.5, 3 'The Night Listener': 3.0}, 4 'Gene Seymour': {'Lady in the Water': 3.0, 'Snakes on a Plane': 3.5, 5 'Just My Luck': 1.5, 'Superman Returns': 5.0, 'The Night Listener': 3.0, 6 'You, Me and Dupree': 3.5}, 7 'Michael Phillips': {'Lady in the Water': 2.5, 'Snakes on a Plane': 3.0, 8 'Superman Returns': 3.5, 'The Night Listener': 4.0}, 9 'Claudia Puig': {'Snakes on a Plane': 3.5, 'Just My Luck': 3.0, 10 'The Night Listener': 4.5, 'Superman Returns': 4.0, 11 'You, Me and Dupree': 2.5}, 12 'Mick LaSalle': {'Lady in the Water': 3.0, 'Snakes on a Plane': 4.0, 13 'Just My Luck': 2.0, 'Superman Returns': 3.0, 'The Night Listener': 3.0, 14 'You, Me and Dupree': 2.0}, 15 'Jack Matthews': {'Lady in the Water': 3.0, 'Snakes on a Plane': 4.0, 16 'The Night Listener': 3.0, 'Superman Returns': 5.0, 'You, Me and Dupree': 3.5}, 17 'Toby': {'Snakes on a Plane': 4.5, 'You, Me and Dupree': 1.0, 'Superman Returns': 4.0}} 18 aa = [critics[i].keys() for i in critics] 19 print aa 20 bb = list(set(item for sublist in aa for item in sublist)) 21 print bb