类方法重载装饰器@singledispatchmethod

@singledispatch可以重载函数接口的第1个参数，@singledispatchmethod同样是重载第1个参数，确切地说，是重载第1个非cls或self参数。

from functools import singledispatchmethod


class clock():

    def __new__(cls):
        raise NotImplementedError('please use classmethod to instantiation')

    @staticmethod
    def init(self, hour, minute, second):
        self.hour = hour
        self.minute = minute
        self.second = second

    @singledispatchmethod
    @classmethod
    def create(cls, hour, minute, second):
        print('int create')
        if not (0 < hour < 23
                and 0 < minute < 60
                and 0 < second < 60):
            return
        obj = object.__new__(cls)
        clock.init(obj, hour, minute, second)
        return obj

    @create.register(str)
    @classmethod
    def _(cls, hour, minute, second):
        print('str create')
        if not (0 < int(hour) < 23
                and 0 < int(minute) < 60
                and 0 < int(second) < 60):
            return
        obj = object.__new__(cls)
        clock.init(obj, int(hour), int(minute), int(second))
        return obj

    @create.register(tuple)
    @classmethod
    def _(cls, tt):
        print('tuple create')
        if not (0 < tt[0] < 23
                and 0 < tt[1] < 60
                and 0 < tt[2] < 60):
            return
        obj = object.__new__(cls)
        clock.init(obj, tt[0], tt[1], tt[2])
        return obj

    @singledispatchmethod
    def show(self, tag):
        print('str-->'+tag, str(self.hour)+':'
                        +str(self.minute)+':'
                        +str(self.second))

    @show.register(int)
    def _(self, tag):
        print('int-->'+str(tag), str(self.hour)+':'
                        +str(self.minute)+':'
                        +str(self.second))


c1 = clock.create(1,2,3)
c1.show('beijing')

c2 = clock.create('3',4,5)
c2.show(24)

c3 = clock.create((5,6,6))
c3.show('a')

try:
    c4 = clock()
except Exception as e:
    print(repr(e))

这段测试代码，将@classmethod，@staticmethod，以及本文的重点@singledispatchmethod都用上了。而且没有定义__init__，并且在__new__内直接raise，需仔细品味。

这样设计，实际上是不允许通过class名称这种常规的方式来创建对象，强迫代码使用者使用create这个被@singledispatchmetho修饰过的classmethod来创建对象。

$ python3 sdm.py
int create
str-->beijing 1:2:3
str create
int-->24 3:4:5
tuple create
str-->a 5:6:6
NotImplementedError('please use classmethod to instantiation')

有了@singledispatchmethod，再配合上@classmethod，我们就可以重载class的factory method，提升代码的可读性和优雅值！