Python sort()函数有哪些参数? [英] What arguments does Python sort() function have?

问题描述

除了key之外是否还有其他参数，例如:value?

Is there any other argument than key, for example: value?

推荐答案

sort和sorted

的参数

sort和sorted都有三个关键字参数:cmp，key和reverse.

Arguments of sort and sorted

Both sort and sorted have three keyword arguments: cmp, key and reverse.

L.sort(cmp=None, key=None, reverse=False) -- stable sort *IN PLACE*;
cmp(x, y) -> -1, 0, 1

sorted(iterable, cmp=None, key=None, reverse=False) --> new sorted list

首选使用key和reverse，因为它们快得多而不是等价的cmp.

Using key and reverse is preferred, because they work much faster than an equivalent cmp.

key应该是一个接受项目并返回值进行比较和排序的函数. reverse允许反转排序顺序.

key should be a function which takes an item and returns a value to compare and sort by. reverse allows to reverse sort order.

您可以使用operator.itemgetter作为关键参数来按元组中的第二，第三等进行排序.

You can use operator.itemgetter as a key argument to sort by second, third etc. item in a tuple.

>>> from operator import itemgetter

>>> a = range(5)
>>> b = a[::-1]
>>> c = map(lambda x: chr(((x+3)%5)+97), a)
>>> sequence = zip(a,b,c)

# sort by first item in a tuple
>>> sorted(sequence, key = itemgetter(0))
[(0, 4, 'd'), (1, 3, 'e'), (2, 2, 'a'), (3, 1, 'b'), (4, 0, 'c')]

# sort by second item in a tuple
>>> sorted(sequence, key = itemgetter(1))
[(4, 0, 'c'), (3, 1, 'b'), (2, 2, 'a'), (1, 3, 'e'), (0, 4, 'd')]

# sort by third item in a tuple
>>> sorted(sequence, key = itemgetter(2))
[(2, 2, 'a'), (3, 1, 'b'), (4, 0, 'c'), (0, 4, 'd'), (1, 3, 'e')]

说明

序列可以包含任何对象，甚至不具有可比性，但是如果我们可以定义一个函数，该函数可以为每个项目进行比较，则可以将该函数在key参数中传递给sort或sorted

Explanation

Sequences can contain any objects, not even comparable, but if we can define a function which produces something we can compare for each of the items, we can pass this function in key argument to sort or sorted.

itemgetter创建这样的函数，该函数从其操作数中获取给定项.文档中的示例:

itemgetter, in particular, creates such a function that fetches the given item from its operand. An example from its documentation:

在f=itemgetter(2)之后，调用f(r)返回r[2].

After, f=itemgetter(2), the call f(r) returns r[2].

迷你基准，key与cmp

出于好奇，key和cmp的性能相比，越小越好:

Mini-benchmark, key vs cmp

Just out of curiosity, key and cmp performance compared, smaller is better:

>>> from timeit import Timer
>>> Timer(stmt="sorted(xs,key=itemgetter(1))",setup="from operator import itemgetter;xs=range(100);xs=zip(xs,xs);").timeit(300000)
6.7079150676727295
>>> Timer(stmt="sorted(xs,key=lambda x:x[1])",setup="xs=range(100);xs=zip(xs,xs);").timeit(300000)
11.609490871429443
>>> Timer(stmt="sorted(xs,cmp=lambda a,b: cmp(a[1],b[1]))",setup="xs=range(100);xs=zip(xs,xs);").timeit(300000)
22.335839986801147

因此，使用key进行排序似乎至少是使用cmp进行排序的两倍.使用itemgetter代替lambda x: x[1]可使排序更快.

So, sorting with key seems to be at least twice as fast as sorting with cmp. Using itemgetter instead of lambda x: x[1] makes sort even faster.

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