operator.itemgetter和sort()在Python中如何工作? [英] How does operator.itemgetter and sort() work in Python?

问题描述

我有以下代码:

# initialize
a = []

# create the table (name, age, job)
a.append(["Nick", 30, "Doctor"])
a.append(["John",  8, "Student"])
a.append(["Paul", 22, "Car Dealer"])
a.append(["Mark", 66, "Retired"])    

# sort the table by age
import operator
a.sort(key=operator.itemgetter(1))    

# print the table
print(a)

它将创建一个4x3的表格，然后按年龄对其进行排序.我的问题是key=operator.itemgetter(1)到底是做什么的? operator.itemgetter函数是否返回项目的值?为什么我不能在那里输入key=a[x][1]之类的内容?可以吗用运算符如何打印诸如3x2的形式的某个值，即22?

It creates a 4x3 table and then it sorts it by age. My question is, what exactly key=operator.itemgetter(1) does? Does the operator.itemgetter function return the item's value? Why can't I just type something like key=a[x][1] there? Or can I? How could with operator print a certain value of the form like 3x2 which is 22?

1. Python如何对表进行精确排序?我可以对它进行反向排序吗?

1. How does exactly Python sort the table? Can I reverse-sort it?

如何根据两列进行排序(例如第一个年龄，然后如果年龄是相同的b名称)?

How can I sort it based on two columns like first age, and then if age is the same b name?

如果没有operator怎么办?

推荐答案

看起来您对所有这些东西有些困惑.

Looks like you're a little bit confused about all that stuff.

operator是一个内置模块，提供了一组便捷的运算符. operator.itemgetter(n)用两个词构造一个可调用对象，该对象假定一个可迭代对象(例如list，tuple，set)作为输入，并从中获取第n个元素.

operator is a built-in module providing a set of convenient operators. In two words operator.itemgetter(n) constructs a callable that assumes an iterable object (e.g. list, tuple, set) as input, and fetches the n-th element out of it.

因此，您不能在那里使用key=a[x][1]，因为python不知道x是什么.相反，您可以使用lambda函数(elem只是变量名，没有魔术):

So, you can't use key=a[x][1] there, because python has no idea what x is. Instead, you could use a lambda function (elem is just a variable name, no magic there):

a.sort(key=lambda elem: elem[1])

或者只是一个普通功能:

Or just an ordinary function:

def get_second_elem(iterable):
    return iterable[1]

a.sort(key=get_second_elem)

因此，这里有个重要的注意事项:在python函数中，一等公民，因此您可以将它们作为参数传递给其他函数.

So, here's an important note: in python functions are first-class citizens, so you can pass them to other functions as a parameter.

其他问题:

1. 是的，您可以反向排序，只需添加reverse=True:a.sort(key=..., reverse=True)
2. 要按多个列进行排序，可以将itemgetter与多个索引一起使用:operator.itemgetter(1,2)，或者将lambda与lambda elem: (elem[1], elem[2])一起使用.这样，可迭代地为列表中的每个项目构造可迭代对象，然后按lexicographic(?)顺序将它们相互比较(比较第一个元素，如果相等，则比较第二个元素，等等)
3. 您可以使用a[2,1]在[3,2]处获取值(索引从零开始).使用运算符...可能，但是不像索引那样干净.

1. Yes, you can reverse sort, just add reverse=True: a.sort(key=..., reverse=True)
2. To sort by more than one column you can use itemgetter with multiple indices: operator.itemgetter(1,2), or with lambda: lambda elem: (elem[1], elem[2]). This way, iterables are constructed on the fly for each item in list, which are than compared against each other in lexicographic(?) order (first elements compared, if equal - second elements compared, etc)
3. You can fetch value at [3,2] using a[2,1] (indices are zero-based). Using operator... It's possible, but not as clean as just indexing.

有关详细信息，请参阅文档:

Refer to the documentation for details:

1. operator.itemgetter解释
2. 通过Python中的自定义键排序列表

1. operator.itemgetter explained
2. Sorting list by custom key in Python

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