# 使用itertools

itertools中的函数大多是返回各种迭代器对象，其中很多函数的作用我们平时要写很多代码才能达到，而在运行效率上反而更低，毕竟人家是系统库。

## itertools.accumulate

``````>>> import itertools
>>> x = itertools.accumulate(range(10))
>>> print(list(x))
[0, 1, 3, 6, 10, 15, 21, 28, 36, 45]``````

## itertools.chain

``````>>> x = itertools.chain(range(3), range(4), [3,2,1])
>>> print(list(x))
[0, 1, 2, 0, 1, 2, 3, 3, 2, 1]``````

## itertools.combinations

``````>>> x = itertools.combinations(range(4), 3)
>>> print(list(x))
[(0, 1, 2), (0, 1, 3), (0, 2, 3), (1, 2, 3)]``````

## itertools.combinations_with_replacement

``````>>> x = itertools.combinations_with_replacement('ABC', 2)
>>> print(list(x))
[('A', 'A'), ('A', 'B'), ('A', 'C'), ('B', 'B'), ('B', 'C'), ('C', 'C')]``````

## itertools.compress

``````>>> x = itertools.compress(range(5), (True, False, True, True, False))
>>> print(list(x))
[0, 2, 3]``````

## itertools.count

``````>>> x = itertools.count(start=20, step=-1)
>>> print(list(itertools.islice(x, 0, 10, 1)))
[20, 19, 18, 17, 16, 15, 14, 13, 12, 11]``````

## itertools.cycle

``````>>> x = itertools.cycle('ABC')
>>> print(list(itertools.islice(x, 0, 10, 1)))
['A', 'B', 'C', 'A', 'B', 'C', 'A', 'B', 'C', 'A']``````

## itertools.dropwhile

``````>>> x = itertools.dropwhile(lambda e: e < 5, range(10))
>>> print(list(x))
[5, 6, 7, 8, 9]``````

## itertools.filterfalse

``````>>> x = itertools.filterfalse(lambda e: e < 5, (1, 5, 3, 6, 9, 4))
>>> print(list(x))
[5, 6, 9]``````

## itertools.groupby

``````>>> x = itertools.groupby(range(10), lambda x: x < 5 or x > 8)
>>> for condition, numbers in x:
...     print(condition, list(numbers))
True [0, 1, 2, 3, 4]
False [5, 6, 7, 8]
True [9] ``````

## itertools.islice

``````>>> x = itertools.islice(range(10), 0, 9, 2)
>>> print(list(x))
[0, 2, 4, 6, 8]  ``````

## itertools.permutations

``````>>> x = itertools.permutations(range(4), 3)
>>> print(list(x))
[(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)] ``````

## itertools.product

``````>>> x = itertools.product('ABC', range(3))
>>>
>>> print(list(x))
[('A', 0), ('A', 1), ('A', 2), ('B', 0), ('B', 1), ('B', 2), ('C', 0), ('C', 1), ('C', 2)]``````

## itertools.repeat

``````>>> x = itertools.repeat(0, 5)
>>> print(list(x))
[0, 0, 0, 0, 0]   ``````

## itertools.starmap

``````>>> x = itertools.starmap(str.islower, 'aBCDefGhI')
>>> print(list(x))
[True, False, False, False, True, True, False, True, False]``````

## itertools.takewhile

``````>>> x = itertools.takewhile(lambda e: e < 5, range(10))
>>> print(list(x))
[0, 1, 2, 3, 4]``````

## itertools.tee

``````>>> x = itertools.tee(range(10), 2)
>>> for letters in x:
...     print(list(letters))
...
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]   ``````

## itertools.zip_longest

``````>>> 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)]     ``````

1 条评论

tee 是复制迭代器