python排列组合之itertools模块

1. 参考

几个有用的python函数 (笛卡尔积, 排列, 组合)

9.7. itertools — Functions creating iterators for efficient looping

2. 代码

 # 有序排列permutations A。
 # 不放回抽球两次，r参数默认为len('abc')
 >>> for i in itertools.permutations('abc',2):
 ...  print(i)
 ...
 ('a', 'b')
 ('a', 'c')
 ('b', 'a')
 ('b', 'c')
 ('c', 'a')
 ('c', 'b')
 # 无序组合combinations C。
 # 不放回抽球两次，r必选
 >>> for i in itertools.combinations('abc',2):
 ...  print(i)
 ...
 ('a', 'b')
 ('a', 'c')
 ('b', 'c')

 # 笛卡尔积
 # 放回抽球，默认repeat=1
 # product(A, B) returns the same as:  ((x,y) for x in A for y in B).
 # repeat=2相当于for i in itertools.product('abc','abc')
 >>> for i in itertools.product('abc',repeat=2):
 ...  print(i)
 ...
 ('a', 'a')
 ('a', 'b')
 ('a', 'c')
 ('b', 'a')
 ('b', 'b')
 ('b', 'c')
 ('c', 'a')
 ('c', 'b')
 ('c', 'c')
 # 放回抽球，r必选，相当于product再去掉非自定义字典序'CBA'顺序的
 >>> for i in itertools.combinations_with_replacement('CBA', 2):
 ...  print(i)
 ...
 ('C', 'C')
 ('C', 'B')
 ('C', 'A')
 ('B', 'B')
 ('B', 'A')
 ('A', 'A')