python-使用键功能对序列进行排序 [英] python - sorting a sequence with key function

问题描述

我不清楚以下内容如何工作:

It is unclear to me how the following works:

In [1]: student_tuples = [('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]
In [2]: sorted(student_tuples, key=lambda student: student[2])
Out [2]: [('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)] # sort by age

但是

In [3]: st = lambda student: student[2]
In [4]: st(student_tuples)
Out [4]: ('dave', 'B', 10)

为什么前一个示例中的[2]引用单个元组的索引，而在lambda函数中，它返回列表中的第二个元组?

Why does the [2] in the former sample refer to the index for the individual tuples, when in a lambda function it returns the 2nd tuple in the list?

推荐答案

由于排序时，将对要排序的列表中的每个元素调用一次key函数.这就是为什么它不是lambda student:而不是lambda student_tuples:的原因(不是参数的命名有任何改变，只是在解释命名选择).

Because when you're sorting, the key function is called once for every element of the list being sorted. That's why it's lambda student: not lambda student_tuples: (not that the naming of parameters changes anything, just explaining the naming choice).

您可以通过打印键函数的参数直接看到此信息:

You can see this directly by printing the argument of the key function:

student_tuples = [('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]

def my_key(student):
    print(student)
    return student[2]

sorted(student_tuples, key=my_key)
# calls to my_key print:
# ('john', 'A', 15)
# ('jane', 'B', 12)
# ('dave', 'B', 10)

my_key(student_tuples)
# prints (not returns):
# [('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]

这篇关于python-使用键功能对序列进行排序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！