python singleton design pattern super()

Posted 2020-10-11

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python singleton design pattern

decorate
baseclass
metaclass
import module
super()

一、A decorator

def singleton(class_):
    instances = {}
    def getinstance(*args, **kwargs):
        if class_ not in instances:
            instances[class_] = class_(*args, **kwargs)
        return instances[class_]
    return getinstance

@singleton
class MyClass(BaseClass):
    pass

当用MyClass() 去创建一个对象时这个对象将会是单例的。MyClass 本身已经是一个函数。不是一个类，所以你不能通过它来调用类的方法。所以对于

m=MyClass() n = MyClass() o=type(n)() m==n and m!=o and n != o 将会是True

二、baseclass

class Singleton(object):
    _instance = None
    def __new__(class_, *args, **kwargs):
        if not isinstance(class_._instance, class_):
            # class_._instance = object.__new__(class_)   这行语句和下一行语句作用一样的
            class_._instance=super(Singleton,class_).__new__(class_)
        return class_._instance
class MyClass(Singleton):
    def __init__(self,name):
        self.name = name
        print(name)

pros

　　是真的类

cons:

在多继承的时候要注意

三、metaclass

class Singleton(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]

#Python2
class MyClass(BaseClass):
    __metaclass__ = Singleton

#Python3
class MyClass(BaseClass, metaclass=Singleton):
    pass