按列表中元素出现的次数对列表进行排序 [英] Sort a list by the number of occurrences of the elements in the list

问题描述

我想按列表中元素的出现次数对列表进行排序.
当我使用这种形式时:

I want to sort a list by the number of occurrences of the elements in the list.
When I use this form:

A=[2,1,3,4,2,2,3]
A.sort(key=lambda x:A.count(x))  
print(A)

结果不是我想要的:[2, 1, 3, 4, 2, 2, 3].
但是，当我使用sorted进行书写时:

the result is not what I want: [2, 1, 3, 4, 2, 2, 3].
But, when I write like it using sorted:

B=sorted(A,key=lambda x:A.count(x))
print(B)

结果正确:[1, 4, 3, 3, 2, 2, 2].
这种行为的原因是什么?

the result is right: [1, 4, 3, 3, 2, 2, 2].
what's the reason for this behavior?

推荐答案

这是故意设计的.在对列表进行排序时，CPython暂时禁止"访问列表，其行为已记录这里:

This is by design and intentional. CPython temporarily "disallows" access to the list while the list is being sorted in place, the behavior is documented here:

CPython实现细节: 在对列表进行排序时， 尝试更改甚至检查列表的效果是 未定义. Python的C实现使列表显示为空 持续时间，如果可以检测到列表，则引发ValueError 在排序过程中发生了变异.

CPython implementation detail: While a list is being sorted, the effect of attempting to mutate, or even inspect, the list is undefined. The C implementation of Python makes the list appear empty for the duration, and raises ValueError if it can detect that the list has been mutated during a sort.

您可以通过在键功能内打印A进行检查-您会得到一个空列表:

You can inspect that by printing A inside the key function - you'll get an empty list:

In [2]: def key_function(x):
    ...:     print(A, x)
    ...:     return A.count(x)
    ...: 

In [3]: A.sort(key=key_function)  
([], 2)
([], 1)
([], 3)
([], 4)
([], 2)
([], 2)
([], 3)

但是，如果您对sorted()执行此操作:

But, if you do that for sorted():

In [4]: sorted(A, key=key_function)
([2, 1, 3, 4, 2, 2, 3], 2)
([2, 1, 3, 4, 2, 2, 3], 1)
([2, 1, 3, 4, 2, 2, 3], 3)
([2, 1, 3, 4, 2, 2, 3], 4)
([2, 1, 3, 4, 2, 2, 3], 2)
([2, 1, 3, 4, 2, 2, 3], 2)
([2, 1, 3, 4, 2, 2, 3], 3)
Out[4]: [1, 4, 3, 3, 2, 2, 2]

---

它也记录在 sort()实现内部:

---

It is also documented inside the sort() implementation:

/* 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).
 */.

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