python3: 迭代器与生成器(1)

1. 手动遍历迭代器

你想遍历一个可迭代对象中的所有元素，但是却不想使用for循环。

>>> items = [1, 2, 3]
>>> # Get the iterator
>>> it = iter(items) # Invokes items.__iter__()
>>> # Run the iterator
>>> next(it) # Invokes it.__next__()
1
>>> next(it)
2
>>> next(it)
3
>>> next(it)
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
StopIteration
>>>

2. 代理迭代

3.使用生成器创建新的迭代模式

一个函数中需要有一个 yield 语句即可将其转换为一个生成器。跟普通函数不同的是，生成器只能用于迭代操作

>>> def countdown(n):
...     print('Starting to count from', n)
...     while n > 0:
...         yield n
...         n -= 1
...     print('Done!')
...

>>> # Create the generator, notice no output appears
>>> c = countdown(3)
>>> c
<generator object countdown at 0x1006a0af0>

>>> # Run to first yield and emit a value
>>> next(c)
Starting to count from 3
3

>>> # Run to the next yield
>>> next(c)
2

>>> # Run to next yield
>>> next(c)
1

>>> # Run to next yield (iteration stops)
>>> next(c)
Done!
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
StopIteration
>>>

4. 实现迭代器协议

5. 反向迭代

6.带有外部状态的生成器函数

7.迭代器切片

函数 itertools.islice() 正好适用于在迭代器和生成器上做切片操作。比如：

>>> def count(n):
...     while True:
...         yield n
...         n += 1
...
>>> c = count(0)
>>> c[10:20]
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
TypeError: 'generator' object is not subscriptable

>>> # Now using islice()
>>> import itertools
>>> for x in itertools.islice(c, 10, 20):
...     print(x)
...
10
11
12
13
14
15
16
17
18
19

8. 跳过可迭代对象的开始部分

场景：你想遍历一个可迭代对象，但是它开始的某些元素你并不感兴趣，想跳过它们。

>>> from itertools import dropwhile
>>> with open('/etc/passwd') as f:
...     for line in dropwhile(lambda line: line.startswith('#'), f):
...         print(line, end='')
...

如果你已经明确知道了要跳过的元素的个数的话，那么可以使用 itertools.islice() 来代替。