Python3 学习笔记------迭代器
python 迭代器
要理解python迭代器(iterator),先要理解两个概念:Iterable(可迭代对象)
、Iterator(迭代器)
先来help()
一下Iterator:
>>> help(Iterator)
Help on class Iterator in module collections.abc:
class Iterator(Iterable)
| Method resolution order:
| Iterator
| Iterable
| builtins.object
|
| Methods defined here:
|
| __iter__(self)
|
| __next__(self)
| Return the next item from the iterator. When exhausted, raise StopIteration
发现,Iterator是继承自Iterable,迭代器必须实现__iter__()
和__next__()
再来看看Iterable:
>>> help(Iterable)
Help on class Iterable in module collections.abc:
class Iterable(builtins.object)
| Methods defined here:
|
| __iter__(self)
|
Iterable只定义了__iter__()
现在来说说__iter__()
、__next__()
的作用
1、__iter__() 返回一个迭代器对象
2、__next__()返回迭代器对象的下一个item,如果没有更多的item则raise一个StopIteration异常
来做一个实验
>>> from collections import Iterator, Iterable
>>> l = list('hello world')
>>> isinstance(l, Iterator)
Traceback (most recent call last):
File "<input>", line 1, in <module>
NameError: name 'isinstace' is not defined
>>> isinstance(l, Iterable)
True
从上面的执行结果可以看出,list 对象 l 是 Iterable 而不是 Iterator
>>> next(l)
Traceback (most recent call last):
File "<input>", line 1, in <module>
TypeError: 'list' object is not an iterator
>>> iter_l = iter(l)
>>> for i in range(len(l)+1):
... next(iter_l)
...
'h'
'e'
'l'
'l'
'o'
' '
'w'
'o'
'r'
'l'
'd'
Traceback (most recent call last):
File "<input>", line 2, in <module>
StopIteration
next() 只接受 iterator 作为参数,不接受 iterable。要想用 next() 对 l 进行迭代,要先执行 iter(l) 将 l 转为 iterator