sorted()函数与lambda()匿名函数

示例代码如下:

根据列表中每个元素的 b键所对应的值进行降序排列。

ls = [{"a": 1, "b": 2, "c": 4}, {"a": 11, "b": 4, "c": 16}, {"a": 11, "b": 4, "c": 16}]
ls_sorted = sorted(ls, key=lambda d: d["b"], reverse=True)
print(ls_sorted)    # [{'a': 11, 'b': 4, 'c': 16}, {'a': 11, 'b': 4, 'c': 16}, {'a': 1, 'b': 2, 'c': 4}]

一： lambda匿名函数:

lambda表达式（又称匿名函数），用于封装有限的逻辑的函数

lambda的主体是一个表达式，而不是一个代码块。仅仅能在lambda表达式中封装有限的逻辑进去

常用方式：

1：函数式编程

my_list = [3, 5, -1, 0, -2, -6]
# 对其绝对值进行排序
my_list = sorted(my_list, key=lambda x: abs(x))
print(my_list)    # [0, -1, -2, 3, 5, -6]

2：结合filter(), map(), reduce() 一起使用的

print(list(map(lambda x:x*x, range(1, 21))))
print(list(filter(lambda x:x%2 == 0, range(1, 21))))


# python 3.0以上的版本需要导入
from functools import reduce
print(reduce(lambda x, y: x+y, range(1, 101)))
print(reduce(lambda x, y:x+y, range(1, 101), 10000))

 3:闭包

def test(n):
    return lambda x: x+n

test1 = test(5)
test2 = test1(30)
print(test2)    # 35

二 sorted()内置函数：

将可迭代的对象的数据重新排序，生成新的列表

sorted(iters, key=None, reverse=None)

iters: 可迭代对象

key: 指定按key的值进行排序

reverse: boolean值，True: 倒序； Flase: 正序

返回值为一个列表。

sorted(L): 返回一个新的列表，原列表不改变

L.sort():  不返回值，改变原列表

ll = [5, -4, 1, -2, 0, 3]
print(sorted(ll))    # [-4, -2, 0, 1, 3, 5]
# 按绝对值排序
print(sorted(ll, key=abs))    # [0, 1, -2, 3, -4, 5]
# 按绝对值倒叙
print(sorted(ll, key=abs, reverse=True))    # [5, -4, 3, -2, 1, 0]

 sorted原理解析

def my_sorted(iters, key=None, reverse=None):
    if key:
        DICT = dict()
        for it in iters:
            # 用函数处理过后的值做键, 处理相同的键加入列表
            DICT.setdefault(key(it), []).append(it)
        INDEX = list(DICT)
        INDEX.sort(reverse=reverse)    # 排序
        return [xx for x in INDEX for xx in DICT[x]]
    else:
        INDEX = list(iters)
        INDEX.sort(reverse=reverse)
        return INDEX
names = ["Tom", "Spike", "Jerry", "Tyke"]
L = sorted(names, key=len, reverse=True)
print(L)    # ['Spike', 'Jerry', 'Tyke', 'Tom']

L = my_sorted(names, key=len, reverse=True)
print(L)    # ['Spike', 'Jerry', 'Tyke', 'Tom']

lambda函数与sorted()结合使用:

# 对列表里的字典排序
L = [{'a': 1, 'b': 4}, {'a': 1111, 'b': 2}, {'a': 1111, 'b': 3}]
l_sorted = sorted(L, key=lambda d: d["b"], reverse=False)
print(l_sorted)
# 对字典进行key排序
d = {'a':25, 'c':27, 'b':20, 'd':22}
l_s = sorted(d.items(), key=lambda x: x[0])
print(l_s)
# 对字典进行values排序
l_s1 = sorted(d.items(), key=lambda x: x[1])
print(l_s1)

降序的另一种方式: ord()函数转为编码值，再 * -1

Data = [['y', 2], ['x', 3], ['z', 4], ['a',1]]
# 对每个列表的第一个值进行正常排序
print(sorted(Data, key=lambda x: x[0]))
# [['a', 1], ['x', 3], ['y', 2], ['z', 4]]
# 对每个列表的第一个值进行降序排序
print(sorted(Data, key=lambda x: x[0], reverse=True))
# [['z', 4], ['y', 2], ['x', 3], ['a', 1]]
print(sorted(Data, key=lambda x: ord(x[0])*-1))  
# [['z', 4], ['y', 2], ['x', 3], ['a', 1]]

# 对每个列表的第二个值进行正常排序
print(sorted(Data, key=lambda x: x[1]))
# [['a', 1], ['y', 2], ['x', 3], ['z', 4]]
# 对每个列表的第二个值进行降序排序
print(sorted(Data, key=lambda x: x[1], reverse=True))
# [['z', 4], ['x', 3], ['y', 2], ['a', 1]]
print(sorted(Data, key=lambda x: x[1]*-1))
# [['z', 4], ['x', 3], ['y', 2], ['a', 1]]

多重排序：

# - 排序规则
#   - 先按照书籍编号降序排序
#   - 再按照书名正序排序
#   - 再按照年份降序排序

number = {
    "test": "B",
    "helloworld": "A",
    "Little price": "C",
    "sg": "A",
    "rb": "A"
}

contents = """2017,126,test
2017,110,helloworld
2017,152,Little price
2017,98,sg
2017,89,rb
2018,116,test
2018,98,helloworld
2018,176,Little price
2018,79,sg
2018,90,rb
2019,122,test
2019,102,helloworld
2019,187,Little price
2019,102,sg
2019,103,rb"""

Dts = [x.split(",") for x in contents.splitlines()]
result = sorted(Dts, key=lambda x: (ord(number[x[2]]) * -1, x[2], int(x[0]) * -1))
# 打印结果
for x in result:
    print(number[x[2]], *x, sep="	")
"""
C	2019	187	Little price
C	2018	176	Little price
C	2017	152	Little price
B	2019	122	test
B	2018	116	test
B	2017	126	test
A	2019	102	helloworld
A	2018	98	helloworld
A	2017	110	helloworld
A	2019	103	rb
A	2018	90	rb
A	2017	89	rb
A	2019	102	sg
A	2018	79	sg
A	2017	98	sg
"""

 itemgetter与attrgetter模块的使用

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

# 按照索引为2的数据倒序排序
print(sorted(student_tuples, key=itemgetter(2), reverse=True))
# [('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
# 多排排序: 先按照索引为1，再按照索引为2排序
print(sorted(student_tuples, key=itemgetter(1, 2)))
# [('john', 'A', 15), ('dave', 'B', 10), ('jane', 'B', 12)]

from operator import attrgetter

class Student:
    def __init__(self, name, grade, age):
        self.name = name
        self.grade = grade
        self.age = age
    def __repr__(self):
        return repr((self.name, self.grade, self.age))

student_tuples = [
    Student('john', 'A', 15),
    Student('jane', 'B', 12),
    Student('dave', 'B', 10)
]
# 按page排序
print(sorted(student_tuples, key=attrgetter("age")))
#  [('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
# 先按grade，再按age
print(sorted(student_tuples, key=attrgetter("grade", "age")))
#  [('john', 'A', 15), ('dave', 'B', 10), ('jane', 'B', 12)]