python內置函數：sorted中的參數key

x.sort和sorted函數中參數key的使用

介紹

python中，列表自帶了排序函數sort

>>> l = [1, 3, 2]
>>> l.sort()
>>> l
[1, 2, 3]

對於其他字典、元組、集合容器，可以使用內置方法sort來做排序，注意返回的結果是列表結構, 字典容器，默認是key進行排序的。

>>> # tuple sort
>>> t = (1, 3, 2)
>>> sorted(t)
[1, 2, 3]
>>>
>>> # set sort
>>> s = {1, 3, 2}
>>> sorted(s)
[1, 2, 3]
>>>
>>> # dict sort
>>> d = {1:100, 3:200, 2: 0}
>>> sorted(d)
[1, 2, 3]
>>> sorted(d.values())
[0, 100, 200]
>>> sorted(d.items())
[(1, 100), (2, 0), (3, 200)]
>>>

參數key的使用

先看一下sorted函數的文檔說明

>>> help(sorted)
Help on built-in function sorted in module builtins:

sorted(iterable, /, *, key=None, reverse=False)
    Return a new list containing all items from the iterable in ascending order.

    A custom key function can be supplied to customize the sort order, and the
    reverse flag can be set to request the result in descending order.

參數key是函數類型，用來支持自定義的排序方式。我們先看一個使用參數key的場景，比如：有一組員工工資單

Name	salary	age
Tom	4000	20
Jerry	4000	24
Bob	5000	28

希望可以按照工資從多到少排序，如果工資一樣，按年齡從小到大排序

>>> salary_list = [
... ("Tom", 4000, 20),
... ("Jerry", 4000, 24),
... ("Bob", 5000, 28),
... ]
>>> # 自定義比較函數，返回值為元組(-salary, age)
>>> def mycmp(salary_item: tuple) -> (int, int):
...     return (-salary_item[1], salary_item[2])
...
>>> sorted(salary_list, key=mycmp)
[('Bob', 5000, 28), ('Tom', 4000, 20), ('Jerry', 4000, 24)]

所以，為什么是這樣呢？
我們來實現一個類CmpObject，在它的一些魔法方法中加入調試信息，來看看sorted函數是什么進行比較的

class CmpObject:
    def __init__(self, name, val):
        self.name = name
        self.val = val
    
    def __neg__(self):
        print("called __neg__", self.name, self.val)
        return CmpObject(self.name, -self.val)
   
    def __eq__(self, other):
        print("called __eq__")
        return self.get_val() == other.get_val()
    
    def __lt__(self, other):
        print("called __lt__")
        return self.get_val() < other.get_val()
    
    def __gt__(self, other):
        print("called __gt__")
        return self.get_val() > other.get_val()
    
    def get_val(self):
        print("called get_val", self.name, self.val)
        return self.val
     
    def __repr__(self):
        return f"{self.val}"

>>> # 初始化工資單
>>> salary_list = [
... ("Tom", CmpObject("Tom", 4000), CmpObject("Tom", 20)),
... ("Jerry", CmpObject("Jerry", 4000), CmpObject("Jerry", 24)),
... ("Bob", CmpObject("Bob", 5000), CmpObject("Bob", 28)),
... ]
>>> salary_list
 [('Tom', 4000, 20), ('Jerry', 4000, 24), ('Bob', 5000, 28)]
>>> # 還是用原來的mycmp比較函數
>>> def mycmp(salary_item: tuple) -> (int, int):
...     return (-salary_item[1], salary_item[2])
...
>>> # 執行排序
>>> sorted(salary_list, key=mycmp)
# 分析一下比較順序
# sorted函數會先變量保存用於比較的key
called __neg__ Tom 4000
called __neg__ Jerry 4000
called __neg__ Bob 5000
# 新比較 -4000 -4000, 即Jerry和Tom的工資，因為是按工資倒序排（工資多的在前），所以比較的是工資的負數
called __eq__
called get_val Jerry -4000
called get_val Tom -4000
# 發現Jerry和Tom的工資相同，再去比較他們的年齡
called __eq__
called get_val Jerry 24
called get_val Tom 20
# 發現Jerry的年齡(24)大於Tom的年齡(20)，Jerry要排在Tom后面
called __lt__
called get_val Jerry 24
called get_val Tom 20
# 然后比較 Bob和Jerry的工資，Bob的工資比Jerry的工資高，Jerry要排在Bob后面
called __eq__
called get_val Bob -5000
called get_val Jerry -4000
called __lt__
called get_val Bob -5000
called get_val Jerry -4000
# 又比較了 Bob和Jerry
called __eq__
called get_val Bob -5000
called get_val Jerry -4000
called __lt__
called get_val Bob -5000
called get_val Jerry -4000
# 最后比較Bob和Tom, 發現Bob的工資比Tom多，Tom要排在Bob后面
called __eq__
called get_val Bob -5000
called get_val Tom -4000
called __lt__
called get_val Bob -5000
called get_val Tom -4000

# 最后結果 Bob > Tom > Jerry
[('Bob', 5000, 28), ('Tom', 4000, 20), ('Jerry', 4000, 24)]

結論

1.sorted函數的排序策略是對於比較用的key，如果是元組形式，從左到右權重遞減，(-工資, 年齡)，如果-工資相同，則再比較年齡。
2.比較是用使用__eq__和__lt__進行比較，如果__eq__為真，再比較__lt__，如果__lt__為真，則要調換順序；否則，不變。