几个有用的python函数 (笛卡尔积, 排列, 组合)

>>> import itertools
>>> for i in itertools.product(‘ABCD‘, repeat = 2):
...     print i,
... 
(‘A‘, ‘A‘) (‘A‘, ‘B‘) (‘A‘, ‘C‘) (‘A‘, ‘D‘) (‘B‘, ‘A‘) (‘B‘, ‘B‘) (‘B‘, ‘C‘) (‘B‘, ‘D‘) (‘C‘, ‘A‘) (‘C‘, ‘B‘) (‘C‘, ‘C‘) (‘C‘, ‘D‘) (‘D‘, ‘A‘) (‘D‘, ‘B‘) (‘D‘, ‘C‘) (‘D‘, ‘D‘)
>>> for i in itertools.permutations(‘ABCD‘, 2):
...     print i,
... 
(‘A‘, ‘B‘) (‘A‘, ‘C‘) (‘A‘, ‘D‘) (‘B‘, ‘A‘) (‘B‘, ‘C‘) (‘B‘, ‘D‘) (‘C‘, ‘A‘) (‘C‘, ‘B‘) (‘C‘, ‘D‘) (‘D‘, ‘A‘) (‘D‘, ‘B‘) (‘D‘, ‘C‘)
>>> for i in itertools.combinations(‘ABCD‘, 2):
...     print i,
... 
(‘A‘, ‘B‘) (‘A‘, ‘C‘) (‘A‘, ‘D‘) (‘B‘, ‘C‘) (‘B‘, ‘D‘) (‘C‘, ‘D‘)
>>> for i in itertools.combinations_with_replacement(‘ABCD‘, 2):
...     print i,
... 
(‘A‘, ‘A‘) (‘A‘, ‘B‘) (‘A‘, ‘C‘) (‘A‘, ‘D‘) (‘B‘, ‘B‘) (‘B‘, ‘C‘) (‘B‘, ‘D‘) (‘C‘, ‘C‘) (‘C‘, ‘D‘) (‘D‘, ‘D‘)