iter() 转迭代器

内置函数 iter()，Python 官方文档描述如下：

help(iter)

Help on built-in function iter in module builtins:

iter(...)
    iter(iterable) -> iterator
    iter(callable, sentinel) -> iterator
    
    Get an iterator from an object.  In the first form, the argument must
    supply its own iterator, or be a sequence.
    In the second form, the callable is called until it returns the sentinel.

将一个可迭代对象（iterable）或可调用对象（callable）转换为一个迭代器。

当参数是可调用对象时，需要提供参数 sentinel，生成的迭代器，每次 迭代时都会不带实参地调用 callable，返回 sentinel 时则触 发 StopIteration。

a = iter('abcd')
a

<str_iterator at 0x1c7eea4f910>

next(a),next(a),next(a),next(a)

('a', 'b', 'c', 'd')

a = iter(int, 1)
for i in range(3):
    print(next(a))

0
0
0

a = iter(int, 0)
next(a)

---------------------------------------------------------------------------

StopIteration                             Traceback (most recent call last)

<ipython-input-21-694e44f6d78c> in <module>
      1 a = iter(int, 0)
----> 2 next(a)


StopIteration: