Python在排序时访问列表

我可以在list.sort()中 排序 列表时访问它

b = ['b', 'e', 'f', 'd', 'c', 'g', 'a']
f = 'check this'

def m(i):
    print i, b, f
    return None

b.sort(key=m)
print b

这返回

b [] check this
e [] check this
f [] check this
d [] check this
c [] check this
g [] check this
a [] check this

请注意,列表b的各个项目被发送到函数m.但是在列表b为空的情况下,可以看到变量f与列表b的范围相同.为什么函数m打印b为[]？

(CPython,可能是其他实现的不同行为)脚本的奇怪输出变得明显：

/* The list is temporarily made empty, so that mutations performed
* by comparison functions can't affect the slice of memory we're
* sorting (allowing mutations during sorting is a core-dump
* factory, since ob_item may change).
*/
saved_ob_size = Py_SIZE(self);
saved_ob_item = self->ob_item;
saved_allocated = self->allocated;
Py_SIZE(self) = 0;

评论说这一切：当你开始排序,列表被清空.那么在外部观察者的眼中,它是“空的”.

我非常喜欢“core-dump工厂”一词.

比较：

b = ['b','e','f','d','c','g','a']
f = 'check this'


def m(i):
    print i, b, f
    return None

b = sorted(b, key= m)
print b