为什么从Python3.0中的sort/sorted中删除了cmp参数? [英] Why is the cmp parameter removed from sort/sorted in Python3.0?

问题描述

来自 python Wiki : In Py3.0, the cmp parameter was removed entirely (as part of a larger effort to simplify and unify the language, eliminating the conflict between rich comparisons and the __cmp__ methods).

我不明白为什么在py3.0中删除了cmp的原因

I do not understand the reasoning why cmp is removed in py3.0

考虑以下示例:

>>> def numeric_compare(x, y):
        return x - y
>>> sorted([5, 2, 4, 1, 3], cmp=numeric_compare)
[1, 2, 3, 4, 5]

，现在考虑使用此版本(推荐并与3.0兼容):

and now consider this version (recommended and compatible with 3.0):

def cmp_to_key(mycmp):
    'Convert a cmp= function into a key= function'
    class K(object):
        def __init__(self, obj, *args):
            self.obj = obj
        def __lt__(self, other):
            return mycmp(self.obj, other.obj) < 0
        def __gt__(self, other):
            return mycmp(self.obj, other.obj) > 0
        def __eq__(self, other):
            return mycmp(self.obj, other.obj) == 0
        def __le__(self, other):
            return mycmp(self.obj, other.obj) <= 0
        def __ge__(self, other):
            return mycmp(self.obj, other.obj) >= 0
        def __ne__(self, other):
            return mycmp(self.obj, other.obj) != 0
    return K

>>> sorted([5, 2, 4, 1, 3], key=cmp_to_key(reverse_numeric))
[5, 4, 3, 2, 1]

后者非常冗长，而前者只需一行就可以达到相同的目的.另一方面，我正在编写要为其编写__cmp__方法的自定义类.根据我在网络上的阅读情况，建议写__lt__,__gt__,__eq__,__le__,__ge__,__ne__ and not __cmp__ 同样，为什么要提出这个建议?我不仅可以定义__cmp__来简化生活吗?

The latter is very verbose and the same purpose is achieved in the former with just one line. On another note, I am writing my custom class for which I want to write the __cmp__ method. from my little reading across web, it is recommended to write __lt__,__gt__,__eq__,__le__,__ge__,__ne__ and not __cmp__ Again, why this recommendation? can I not just define __cmp__ making life simpler?

推荐答案

对于两个对象a和b，__cmp__要求 a < b，a == b，并且a > b为true.但这可能并非如此:请考虑集合，其中都不是真的是很常见的，例如{1, 2, 3}与{4, 5, 6}.

For two objects a and b, __cmp__ requires that one of a < b, a == b, and a > b is true. But that might not be the case: consider sets, where it's very common that none of those are true, e.g. {1, 2, 3} vs {4, 5, 6}.

因此介绍了__lt__和朋友.但这给Python留下了两种单独的排序机制，这有点荒谬，因此在Python 3中删除了较不灵活的排序机制.

So __lt__ and friends were introduced. But that left Python with two separate ordering mechanisms, which is kind of ridiculous, so the less flexible one was removed in Python 3.

您实际上不必实现所有六个比较方法.您可以使用 @total_ordering装饰器，并且仅实现__lt__和__eq__

You don't actually have to implement all six comparison methods. You can use the @total_ordering decorator and only implement __lt__ and __eq__.

edit:还请注意，在排序的情况下，key函数可能比cmp更有效:在您给出的示例中，Python可能必须调用Python比较函数O(n²)次.但是key函数仅需要调用O(n)次，并且如果返回值是内置类型(通常是这样)，则O(n²)成对比较将通过C.

edit: Also note that, in the case of sorting, key functions can be more efficient than cmp: in the example you gave, Python may have to call your Python comparison function O(n²) times. But a key function only needs to be called O(n) times, and if the return value is then a builtin type (as it very often is), the O(n²) pairwise comparisons go through C.

这篇关于为什么从Python3.0中的sort/sorted中删除了cmp参数?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！