Python中的分组函数（groupby、itertools）

转载自：https://www.cnblogs.com/dreamer-fish/p/5522687.html

from operator import itemgetter #itemgetter用来去dict中的key，省去了使用lambda函数from itertools import groupby #itertool还包含有其他很多函数，比如将多个list联合起来。。d1={'name':'zhangsan','age':20,'country':'China'}d2={'name':'wangwu','age':19,'country':'USA'}d3={'name':'lisi','age':22,'country':'JP'}d4={'name':'zhaoliu','age':22,'country':'USA'}d5={'name':'pengqi','age':22,'country':'USA'}d6={'name':'lijiu','age':22,'country':'China'}lst=[d1,d2,d3,d4,d5,d6]#通过country进行分组：lst.sort(key=itemgetter('country')) #需要先排序，然后才能groupby。lst排序后自身被改变lstg = groupby(lst,itemgetter('country')) #lstg = groupby(lst,key=lambda x:x['country']) 等同于使用itemgetter()for key,group in lstg:    for g in group: #group是一个迭代器，包含了所有的分组列表        print key,g返回：China {'country': 'China', 'age': 20, 'name': 'zhangsan'}China {'country': 'China', 'age': 22, 'name': 'lijiu'}JP {'country': 'JP', 'age': 22, 'name': 'lisi'}USA {'country': 'USA', 'age': 19, 'name': 'wangwu'}USA {'country': 'USA', 'age': 22, 'name': 'zhaoliu'}USA {'country': 'USA', 'age': 22, 'name': 'pengqi'}print [key for key,group in lstg] #返回：['China', 'JP', 'USA']print [(key,list(group)) for key,group in lstg]

返回的list中包含着三个元组：
[(‘China’, [{‘country’: ‘China’, ‘age’: 20, ‘name’: ‘zhangsan’}, {‘country’: ‘China’, ‘age’: 22, ‘name’: ‘lijiu’}]), (‘JP’, [{‘country’: ‘JP’, ‘age’: 22, ‘name’: ‘lisi’}]), (‘USA’, [{‘country’: ‘USA’, ‘age’: 19, ‘name’: ‘wangwu’}, {‘country’: ‘USA’, ‘age’: 22, ‘name’: ‘zhaoliu’}, {‘country’: ‘USA’, ‘age’: 22, ‘name’: ‘pengqi’}])]

print dict([(key,list(group)) for key,group in lstg])

返回每个分组的个数：{'JP': 1, 'China': 2, 'USA': 3}lstg = groupby(sorted(lst,key=itemgetter('country')),key=itemgetter('country')) lstgall=[(key,list(group)) for key,group in lstg ]print dict(filter(lambda x:len(x[1])>2,lstgall)) 

过滤出分组后的元素个数大于2个的分组，返回：
{‘USA’: [{‘country’: ‘USA’, ‘age’: 19, ‘name’: ‘wangwu’}, {‘country’: ‘USA’, ‘age’: 22, ‘name’: ‘zhaoliu’}, {‘country’: ‘USA’, ‘age’: 22, ‘name’: ‘pengqi’}]}

自定义分组

from itertools import groupbylst=[2,8,11,25,43,6,9,29,51,66]def gb(num):    if num <= 10:        return 'less'    elif num >=30:        return 'great'    else:        return 'middle'print [(k,list(g))for k,g in groupby(sorted(lst),key=gb)]

返回：
[(‘less’, [2, 6, 8, 9]), (‘middle’, [11, 25, 29]), (‘great’, [43, 51, 66])]

lst = [1,1,0,0,0,0,1,1,0,1]print max(len(list(g)) for k, g in groupby(s) if k==1)