# itertools  
[官网](https://docs.python.org/3/library/itertools.html)

## batched
```python
# [
#     (0, 1, 2),
#     (3, 4, 5),
#     (6, ),
#     ...
# ]
for batch in itertools.batched(range(7), n=3):  # batch就是一个Tuple
    ...
```

## chain

```python
chain('ABC', 'DEF') --> A B C D E F
chain(range(1, 6), range(4, 0, -1)) --> 1 2 3 4 5 4 3 2 1
```

## [chain.from_iterable](https://docs.python.org/3/library/itertools.html#itertools.chain.from_iterable)

```python
# 和chain差不多， 但是只支持一个参数， 会对此展开后再用chain
def from_iterable(iterables):
    for it in iterables:
        for element in it:
            yield it
```

## count(start, [step])  
从某个数字开始一直循环


```python
from itertools import count
loop = count(10)
next(loop) // 10
next(loop) // 11
next(loop) // 12
...
```

## tee(iterable, n=2)
把一个迭代器变成多个独立的

## `zip_longest
* zip
虽然zip是python的内置函数, 但是估计会经常在这里查看
```
for a, b in zip(iter1, iter2):  # 按照最短的来
    pass
```
* `zip_longest(*iterables, fillvalue=None)
按照最长的来. 其他的会填充fillvalue