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product 用于求多个可迭代对象的笛卡尔积(Cartesian Product)，它跟嵌套的 for 循环等价.即:
product(A, B) 和 ((x,y) for x in A for y in B)的效果是一样的。
使用形式如下：
itertools.product(*iterables, repeat=1)
iterables 是可迭代对象, repeat指定 iterable 重复几次,即:
product(A,repeat=3)等价于product(A,A,A)
Coding Time:
>>> from itertools import product as product
>>> A = [1, 2, 3]
>>> B = [100, 200, 300]
>>> for item in product(A, B):
... print(item)
...
(1, 100)
(1, 200)
(1, 300)
(2, 100)
(2, 200)
(2, 300)
(3, 100)
(3, 200)
(3, 300)
>>> for item in product(A, repeat=3):
... print(item)
...
(1, 1, 1)
(1, 1, 2)
(1, 1, 3)
(1, 2, 1)
(1, 2, 2)
(1, 2, 3)
(1, 3, 1)
(1, 3, 2)
(1, 3, 3)
(2, 1, 1)
(2, 1, 2)
(2, 1, 3)
(2, 2, 1)
(2, 2, 2)
(2, 2, 3)
(2, 3, 1)
(2, 3, 2)
(2, 3, 3)
(3, 1, 1)
(3, 1, 2)
(3, 1, 3)
(3, 2, 1)
(3, 2, 2)
(3, 2, 3)
(3, 3, 1)
(3, 3, 2)
(3, 3, 3)