python内置函数：sorted中的参数key |
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x.sort和sorted函数中参数key的使用
介绍
python中，列表自带了排序函数sort >>> l = [1, 3, 2] >>> l.sort() >>> l [1, 2, 3]对于其他字典、元组、集合容器，可以使用内置方法sort来做排序，注意返回的结果是列表结构, 字典容器，默认是key进行排序的。 >>> # 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)] >>> 参数key的使用先看一下sorted函数的文档说明 >>> 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.参数key是函数类型，用来支持自定义的排序方式。我们先看一个使用参数key的场景，比如：有一组员工工资单 Name salary age Tom 4000 20 Jerry 4000 24 Bob 5000 28 希望可以按照工资从多到少排序，如果工资一样，按年龄从小到大排序 >>> salary_list = [ ... ("Tom", 4000, 20), ... ("Jerry", 4000, 24), ... ("Bob", 5000, 28), ... ] >>> # 自定义比较函数，返回值为元组(-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，为什么是这样呢？ 我们来实现一个类CmpObject，在它的一些魔法方法中加入调试信息，来看看sorted函数是什么进行比较的 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}" >>> # 初始化工资单 >>> 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)] >>> # 还是用原来的mycmp比较函数 >>> def mycmp(salary_item: tuple) -> (int, int): ... return (-salary_item[1], salary_item[2]) ... >>> # 执行排序 >>> sorted(salary_list, key=mycmp) # 分析一下比较顺序 # sorted函数会先变量保存用于比较的key called __neg__ Tom 4000 called __neg__ Jerry 4000 called __neg__ Bob 5000 # 新比较 -4000 -4000, 即Jerry和Tom的工资，因为是按工资倒序排（工资多的在前），所以比较的是工资的负数 called __eq__ called get_val Jerry -4000 called get_val Tom -4000 # 发现Jerry和Tom的工资相同，再去比较他们的年龄 called __eq__ called get_val Jerry 24 called get_val Tom 20 # 发现Jerry的年龄(24)大于Tom的年龄(20)，Jerry要排在Tom后面 called __lt__ called get_val Jerry 24 called get_val Tom 20 # 然后比较 Bob和Jerry的工资，Bob的工资比Jerry的工资高，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 # 又比较了 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 # 最后比较Bob和Tom, 发现Bob的工资比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 # 最后结果 Bob > Tom > Jerry [('Bob', 5000, 28), ('Tom', 4000, 20), ('Jerry', 4000, 24)] 结论1.sorted函数的排序策略是对于比较用的key，如果是元组形式，从左到右权重递减，(-工资, 年龄)，如果-工资相同，则再比较年龄。 2.比较是用使用__eq__和__lt__进行比较，如果__eq__为真，再比较__lt__，如果__lt__为真，则要调换顺序；否则，不变。 |
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