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summary: Reason is the light and the light of life.
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import re
from collections.abc import Iterator, Iterable

鸭子类型,通过协议实现

1. Iterable

可迭代协议: __iter__()

import re

class Sentence1:
    def __init__(self, text):
        self.text = text
        self.words = re.findall("\w+", self.text)

    def __iter__(self):
        return self
s1 = Sentence1('Hello world!')
print(f"s is Iterable: {isinstance(s1, Iterable)}")
print(f"s is Iterator: {isinstance(s1, Iterator)}")
s is Iterable: True
s is Iterator: False

2. Iterator

迭代器协议: __iter__(), __next__()

import re

class Sentence2:
    def __init__(self, text):
        self.text = text
        self.words = re.findall("\w+", self.text)

    def __iter__(self):
        return self

    def __next__(self):
        pass
s2 = Sentence2('Hello world!')
print(f"s is Iterable: {isinstance(s2, Iterable)}")
print(f"s is Iterator: {isinstance(s2, Iterator)}")
s is Iterable: True
s is Iterator: True
  • 含有 __iter__() 的类对象是可迭代的。

  • 含有 __iter__() 和 __next__() 是迭代器。

  • 迭代器一定是可迭代的对象,可迭代的对象不一定是迭代器。

  • __iter__()方法返回迭代器。如果是迭代器本身,直接返回self。

  • __next__()方法返回下一个可用元素,否则抛出异常StopIteration

# 内置函数iter()和next(),等价于对象直接调用__iter__()和__next__(),应该避免直接调用。

iter(s2)
s2.__iter__()
next(s2) 
s2.__next__()

3. 一个典型的迭代器

class SentenceIterator:
    def __init__(self, words):
        self.words = words
        self.index = 0

    def __iter__(self):
        return self

    def __next__(self):
        try:
            word = self.words[self.index]
        except IndexError:
            raise StopIteration()
        self.index += 1
        return word

class SentenceIterator:
    def __init__(self, words):
        self.words = words
        self.index = 0

    def __iter__(self):
        return self

    def __next__(self):
        try:
            word = self.words[self.index]
        except IndexError:
            raise StopIteration()
        self.index += 1
        return word
s = Sentence("Hello world!")
print(f"s is Iterable: {isinstance(s, Iterable)}")
print(f"s is Iterator: {isinstance(s, Iterator)}")
s is Iterable: True
s is Iterator: False
# for循环会默认自动调用对象s的__iter__()方法获取迭代器。并且会捕获StopIteration异常。

for w in s:
    print(w)

for w in iter(s):
    print(w)
Hello
world
Hello
world
next(s)
---------------------------------------------------------------------------

TypeError                                 Traceback (most recent call last)

<ipython-input-94-61c30b5fe1d5> in <module>
----> 1 next(s)


TypeError: 'Sentence' object is not an iterator
next(s.__iter__())
'Hello'
next(s.__iter__())
'Hello'
next(iter(s))
'Hello'
next(iter(s))
'Hello'
ss = SentenceIterator(["Hello", "world"])
next(ss)
'Hello'
next(ss)
'world'
next(ss)
---------------------------------------------------------------------------

IndexError                                Traceback (most recent call last)

<ipython-input-68-8245fc14b31c> in __next__(self)
     19         try:
---> 20             word = self.words[self.index]
     21         except IndexError:


IndexError: list index out of range


During handling of the above exception, another exception occurred:


StopIteration                             Traceback (most recent call last)

<ipython-input-112-d2551d6ade9e> in <module>
----> 1 next(ss)


<ipython-input-68-8245fc14b31c> in __next__(self)
     20             word = self.words[self.index]
     21         except IndexError:
---> 22             raise StopIteration()
     23         self.index += 1
     24         return word


StopIteration:
# 没有任何输出,因为之前迭代结束,ss迭代器不能恢复,需要重新构建迭代器。

for w in ss:
    print(w)
ss = SentenceIterator(["Hello", "world"])
for w in ss:
    print(w)
Hello
world

区别可迭代的对象迭代器

  • 可迭代的对象存在__iter__()方法,用于实例化一个新的迭代器返回。

  • 迭代器存在__iter__()方法,返回自身这个迭代器,同时存在__next()__方法,返回单个元素。

4. 更Pythonic的实现方式

用生成器函数替代SentenceIterator迭代器

class Sentence:
    def __init__(self, text):
        self.text = text
        self.words = re.findall("\w+", self.text)
        pass

    def __iter__(self):
        for word in self.words:
            yield word
        return

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