iter()函数就是生成一个迭代器，迭代器必须用__next__方法才会调用一次，如下
>>> myTuple = (123，‘xyz’，45.67)
>>> i = iter(myTuple)
>>> i.next()
123
>>> i.next()
‘xyz’
>>> i.next()
45.67
>>> i.next()
Traceback (most recent call last):
File “<stdin>”，line 1，in <module>
StopIteration
>>>lst = [1, 2, 3]
>>> for i in iter(lst):
... print(i)
...
1
2
3
