Python如何在内部管理“ for”循环？ [英] How does Python manage a 'for' loop internally?

问题描述

我正在尝试学习Python，我开始玩一些代码：

I'm trying to learn Python, and I started to play with some code:

a = [3,4,5,6,7]
for b in a:
    print(a)
    a.pop(0)

输出为：

[3, 4, 5, 6, 7]
[4, 5, 6, 7]
[5, 6, 7]

我知道这不是 的好习惯，当我在它上循环时更改数据结构，但是我想了解Python如何在此管理迭代器

I know that's not a good practice change data structures while I'm looping on it, but I want to understand how Python manage the iterators in this case.

主要问题是：如果我更改 a的状态，它如何知道必须完成循环？

The principal question is: How does it know that it has to finish the loop if I'm changing the state of a?

推荐答案

kjaquier和Felix讨论了迭代器协议，我们可以在您的实践中看到它情况：

kjaquier and Felix have talked about the iterator protocol, and we can see it in action in your case:

>>> L = [1, 2, 3]
>>> iterator = iter(L)
>>> iterator
<list_iterator object at 0x101231f28>
>>> next(iterator)
1
>>> L.pop()
3
>>> L
[1, 2]
>>> next(iterator)
2
>>> next(iterator)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
StopIteration

由此我们可以推断出 list_iterator .__ next __ 的代码行为类似于：

From this we can infer that list_iterator.__next__ has code that behaves something like:

if self.i < len(self.list):
    return self.list[i]
raise StopIteration

它不能天真地得到物品。这将引发一个 IndexError ，它会冒泡到顶部：

It does not naively get the item. That would raise an IndexError which would bubble to the top:

class FakeList(object):
    def __iter__(self):
        return self

    def __next__(self):
        raise IndexError

for i in FakeList():  # Raises `IndexError` immediately with a traceback and all
    print(i)

实际上，查看listiter_next rel = nofollow noreferrer> CPython来源（感谢Brian Rodriguez）：

Indeed, looking at listiter_next in the CPython source (thanks Brian Rodriguez):

if (it->it_index < PyList_GET_SIZE(seq)) {
    item = PyList_GET_ITEM(seq, it->it_index);
    ++it->it_index;
    Py_INCREF(item);
    return item;
}

Py_DECREF(seq);
it->it_seq = NULL;
return NULL;

尽管我不知道如何返回NULL； 最终转换为 StopIteration 。

Although I don't know how return NULL; eventually translates into a StopIteration.

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