問(wèn)題描述

我已經(jīng)閱讀了我的材料，其中告訴 Python 迭代器必須同時(shí)具有 __iter__ 和 __next__ 方法，但迭代器只需要 __iter__.

I have read my materials, which tell that a Python iterator must have both __iter__ and __next__ methods, but an iterable just needs __iter__.

我檢查了一個(gè)列表，發(fā)現(xiàn)它沒(méi)有 __next__ 方法.當(dāng)在其上使用 iter() 時(shí)，它將成為一個(gè)迭代器.這是否意味著 iter() 會(huì)在列表中添加一個(gè) __next__ 方法以將其轉(zhuǎn)換為迭代器?

I check a list and find it has no __next__ method. When using iter() on it, it will become an iterator. Does this mean that iter() will add a __next__ method to a list to convert it to an iterator?

如果是，這是怎么發(fā)生的?

If yes, how does this happen?

推薦答案

沒(méi)有.iter 返回一個(gè)迭代器，它不會(huì)將列表轉(zhuǎn)換為迭代器.它根本不修改列表，當(dāng)然，列表沒(méi)有得到 __next__ 方法.

No. iter returns an iterator, it does not convert the list into an iterator. It doesn't modify the list at all, and certainly, the list does not get a __next__ method.

>>> x = [1,2]
>>> it = iter(x)
>>> it
<list_iterator object at 0x101c021d0>
>>> x.__next__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'list' object has no attribute '__next__'
>>>

列表是iterables，而不是iterators.它們實(shí)現(xiàn)了一個(gè) __iter__ 方法，因此它們是可迭代的:

Lists are iterables, not iterators. They implement a __iter__ method, thus they are iterable:

>>> x.__iter__
<method-wrapper '__iter__' of list object at 0x101bcf248>

但不是 __next__，因此它們不是迭代器:

But not __next__, thus they are not iterators:

>>> next(x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'list' object is not an iterator

根據(jù)定義，迭代器本身是可迭代的，因?yàn)樗鼈円矊?shí)現(xiàn)了 __iter__.考慮:

Iterators themselves are iterable, by definition, since they implement __iter__ as well. Consider:

>>> x = [1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> it = iter(x)
>>> it
<list_iterator object at 0x101c02358>
>>> it.__iter__
<method-wrapper '__iter__' of list_iterator object at 0x101c02358>

大多數(shù)迭代器應(yīng)該在你對(duì)它們使用iter時(shí)簡(jiǎn)單地返回它們自己:

Most iterators should simply return themselves when you use iter on them:

>>> it2 = iter(it)
>>> it, it2
(<list_iterator object at 0x101c02358>, <list_iterator object at 0x101c02358>)
>>> it is it2
True
>>>

確實(shí)，這是迭代器協(xié)議的要求:

"迭代器必須有一個(gè)返回的 __iter__() 方法迭代器對(duì)象本身，因此每個(gè)迭代器也是可迭代的，并且可能是在接受其他迭代的大多數(shù)地方使用."

"Iterators are required to have an __iter__() method that returns the iterator object itself so every iterator is also iterable and may be used in most places where other iterables are accepted."

再次注意，它們是同一個(gè)迭代器:

>>> next(it)
1
>>> next(it2)
2
>>> next(it)
3
>>> next(it)
4
>>> next(it2)
5
>>> list(it)
[6, 7, 8, 9]
>>> next(it2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

所以一個(gè)iterator實(shí)現(xiàn)了__iter__和__next__，一個(gè)iterable僅僅意味著它實(shí)現(xiàn)了__iter__.返回 __iter__ 是一個(gè)迭代器，所以它必須實(shí)現(xiàn) __next__.