020 列表元素是对象，如何排序

1.如果列表元素是对象，对这样的列表排序有哪些方法？

class MyClass:
    def __init__(self):
        self.value = 0

my1 = MyClass()
my1.value = 20

my2 = MyClass()
my2.value = 10

my3 = MyClass()
my3.value = 30

a = [my1, my2, my3]
print(a)

a.sort()

 

TypeError: '<' not supported between instances of 'MyClass' and 'MyClass'
[<__main__.MyClass object at 0x0000020B8C4F9CC8>, <__main__.MyClass object at 0x0000020B8C4FC248>, <__main__.MyClass object at 0x0000020B8C4FD748>]

 

方法一：利用magic函数：__lt__ 或 __gt__

class MyClass:
    def __init__(self):
        self.value = 0

    def __lt__(self, other):
        return self.value < other.value

    def __gt__(self, other):
        return self.value > other.value

my1 = MyClass()
my1.value = 20

my2 = MyClass()
my2.value = 10

my3 = MyClass()
my3.value = 30

a = [my1, my2, my3]
print(a)

a.sort()

print(a[0].value)
print(a[1].value)
print(a[2].value)

[<__main__.MyClass object at 0x0000016E82E16808>, <__main__.MyClass object at 0x0000016E82E16BC8>, <__main__.MyClass object at 0x0000016E82E16B08>]
10
20
30

方法二：利用operator sort的key

class MyClass:
    def __init__(self):
        self.value = 0

my1 = MyClass()
my1.value = 20

my2 = MyClass()
my2.value = 10

my3 = MyClass()
my3.value = 30

a = [my1, my2, my3]
print(a)

import operator
a.sort(key = operator.attrgetter('value'))

print(a[0].value)
print(a[1].value)
print(a[2].value)

 

方法三：利用operator sorted 的key

class MyClass:
    def __init__(self):
        self.value = 0

my1 = MyClass()
my1.value = 20

my2 = MyClass()
my2.value = 10

my3 = MyClass()
my3.value = 30

a = [my1, my2, my3]
print(a)

import operator
b = sorted(a, key=operator.attrgetter('value'))

print(b)
print(b[0].value)
print(b[1].value)
print(b[2].value)

　　

　　

 

2. 如果列表元素是对象，进行倒序排列的方法有哪些？

最简单方法

import operator
a.sort(key = operator.attrgetter('value'), reverse=True)

 

或者更改__lt__或__gt__

    def __lt__(self, other):
        return self.value > other.value

a.sort()

　　

3. 总结

为类添加__lt__或__gt__方法，可以让该类的实例支持sort方法和sorted函数

 

通过改变__lt__或__gt__方法的返回值，或者设置sort方法和sorted函数的reverse参数，可以让列表倒序排列