内置函数 iter() 如何将 Python 列表转换为迭代器?

问题描述

我已经阅读了我的材料，其中告诉 Python 迭代器必须同时具有 __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__.

我检查了一个列表，发现它没有 __next__ 方法.当在其上使用 iter() 时，它将成为一个迭代器.这是否意味着 iter() 会在列表中添加一个 __next__ 方法以将其转换为迭代器?

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，这是怎么发生的?

If yes, how does this happen?

推荐答案

没有.iter 返回一个迭代器，它不会将列表转换为迭代器.它根本不修改列表，当然，列表没有得到 __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.它们实现了一个 __iter__ 方法，因此它们是可迭代的:

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__，因此它们不是迭代器:

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

根据定义，迭代器本身是可迭代的，因为它们也实现了 __iter__.考虑:

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>

大多数迭代器应该在你对它们使用iter时简单地返回它们自己:

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
>>>

确实，这是迭代器协议的要求:

"迭代器必须有一个返回的 __iter__() 方法迭代器对象本身，因此每个迭代器也是可迭代的，并且可能是在接受其他迭代的大多数地方使用."

"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，它们是同一个迭代器:

>>> 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

所以一个iterator实现了__iter__和__next__，一个iterable仅仅意味着它实现了__iter__.返回 __iter__ 是一个迭代器，所以它必须实现 __next__.