python的itertools库

import itertools
1. 生成的列表累加，在生成新的列表
    x = itertools.accumulate(range(10))
    print(list(x))
    结果：[0, 1, 3, 6, 10, 15, 21, 28, 36, 45]
2. 连接多个列表或者迭代器
    x = itertools.chain(range(3), range(4), [3,2,1])
    print(list(x))
    结果：[0, 1, 2, 0, 1, 2, 3, 3, 2, 1]
3. 求列表或生成器中指定数目的元素不重复的所有组合
    x = itertools.combinations(range(4), 3)
    print(list(x))
    结果：[(0, 1, 2), (0, 1, 3), (0, 2, 3), (1, 2, 3)]
4. 允许重复元素的组合
    x = itertools.combinations_with_replacement( ABC , 2)
    print(list(x))
    结果：[( A ,  A ), ( A ,  B ), ( A ,  C ), ( B ,  B ), ( B ,  C ), ( C ,  C )]
5. 按照真值表筛选元素
    x = itertools.compress(range(5), (True, False, True, True, False))
    print(list(x))
    结果：[0, 2, 3]
6. 一个计数器,可以指定起始位置和步长
    x = itertools.count(start=20, step=-1)  # X的类型为count
    print(list(itertools.islice(x, 0, 10, 1)))
    结果：[20, 19, 18, 17, 16, 15, 14, 13, 12, 11]
7. 循环指定的列表和迭代器
    x = itertools.cycle( ABC )
    print(list(itertools.islice(x, 0, 10, 1)))
    结果：[ A ,  B ,  C ,  A ,  B ,  C ,  A ,  B ,  C ,  A ]
8. 按照真值函数丢弃掉列表和迭代器前面的元素
    x = itertools.dropwhile(lambda e: e < 5, range(10))
    print(list(x))
    结果：[5, 6, 7, 8, 9]
9. 保留对应真值为False的元素
    x = itertools.filterfalse(lambda e: e < 5, (1, 5, 3, 6, 9, 4))
    print(list(x))
    结果：[5, 6, 9]
10. 按照分组函数的值对元素进行分组
     x = itertools.groupby(range(10), lambda x: x < 5 or x &gt; 8)                                                                                                
     for condition, numbers in x:                                                  
        print(condition, list(numbers))                                                                                                        
    True [0, 1, 2, 3, 4]                                                              
    False [5, 6, 7, 8]                                                                
    True [9]
11. 上文使用过的函数，对迭代器进行切片
    x = itertools.islice(range(10), 0, 9, 2)
    print(list(x))
    结果：[0, 2, 4, 6, 8]
12. 产生指定数目的元素的所有排列(顺序有关)
    x = itertools.permutations(range(4), 3)
    print(list(x))
    结果：[(0, 1, 2), (0, 1, 3), (0, 2, 1), (0, 2, 3), (0, 3, 1), (0, 3, 2), (1, 0, 2), (1, 0, 3), (1, 2, 0), (1, 2, 3), (1, 3, 0), (1, 3, 2), (2, 0, 1), (2, 0,3), (2, 1, 0), (2, 1, 3), (2, 3, 0), (2, 3, 1), (3, 0, 1), (3, 0, 2), (3, 1, 0), (3, 1, 2), (3, 2, 0), (3, 2, 1)]

13. 产生多个列表和迭代器的(积)
    x = itertools.product( ABC , range(3))
    print(list(x))
    结果：[( A , 0), ( A , 1), ( A , 2), ( B , 0), ( B , 1), ( B , 2), ( C , 0), ( C , 1), ( C , 2)]

14. 简单的生成一个拥有指定数目元素的迭代器
    x = itertools.repeat(0, 5)
    print(list(x))
    结果：[0, 0, 0, 0, 0]
15. 类似map
    x = itertools.starmap(str.islower,  aBCDefGhI )
    print(list(x))
    结果：[True, False, False, False, True, True, False, True, False]
16. 与dropwhile相反，保留元素直至真值函数值为假。
    x = itertools.takewhile(lambda e: e < 5, range(10))
    print(list(x))
    结果：[0, 1, 2, 3, 4]
17. 生成指定数目的迭代器
    x = itertools.tee(range(10), 2)
    for letters in x:
        print(list(letters))
    
  结果：[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
        [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
18. 类似于zip，以较长的列表和迭代器的长度为准
    x = itertools.zip_longest(range(3), range(5))
    y = zip(range(3), range(5))
    >>> print(list(x))
        [(0, 0), (1, 1), (2, 2), (None, 3), (None, 4)]
    >>> print(list(y))
        [(0, 0), (1, 1), (2, 2)]