1、 permutations 排列（有序，不重复选取）

import itertools
x=['a','b','c']
perm=itertools.permutations(x)
for i in perm:
    print(i)

('a', 'b', 'c')
('a', 'c', 'b')
('b', 'a', 'c')
('b', 'c', 'a')
('c', 'a', 'b')
('c', 'b', 'a')

2、combinations 组合（无序，不重复选取）

import itertools
x=['a','b','c']
prem=itertools.combinations(x,2)
for i in prem:
    print(i)

('a', 'b')
('a', 'c')
('b', 'c')

3、combinations_with_replacement 可重复组合

import itertools
x=['a','b','c']
prem=itertools.combinations_with_replacement(x,2)
for i in prem:
    print(i)

('a', 'a')
('a', 'b')
('a', 'c')
('b', 'b')
('b', 'c')
('c', 'c')

4、product（允许重复选取，不传第二个参数时，默认次数1）

import itertools
x=['a','b','c']
prem=itertools.product(x,repeat=2)
for i in prem:
    print(i)

('a', 'a')
('a', 'b')
('a', 'c')
('b', 'a')
('b', 'b')
('b', 'c')
('c', 'a')
('c', 'b')
('c', 'c')