题目：

实现一个单例类，至少用3种方法。

#python #设计模式 #单例类 #metaclass

视频教程：

Python入门题037：实现单例类（3种方法）

代码1：

def singleton(cls):
    instances = {}

    def getinstance(*args, **kwargs):
        if cls not in instances:
            instances[cls] = cls(*args, **kwargs)
        return instances[cls]

    return getinstance


@singleton
class MyClass(object):
    def __init__(self, name):
        self.name = name


class MyClass2(object):
    pass


a = MyClass('hello')
b = MyClass('world')

print('a.name', a.name)  # a.name hello
print('b.name', b.name)  # b.name hello
print('b is a', b is a)  # b is a True
print('type(MyClass2)', type(MyClass2))  # type(MyClass2) <class 'type'>
print('type(MyClass)', type(MyClass))  # type(MyClass) <class 'function'>

代码2：

class Singleton(object):
    _instance = None

    def __new__(cls, *args, **kwargs):
        if not isinstance(cls._instance, cls):
            cls._instance = super().__new__(cls)
        return cls._instance


class MyClass(Singleton):
    def __init__(self, name):
        self.name = name


class MyClass2(object):
    pass


a = MyClass('hello')
b = MyClass('world')  # world 会覆盖 hello

print('a.name', a.name)  # 输出 world
print('b.name', b.name)
print('type(MyClass2)', type(MyClass2))
print('type(MyClass)', type(MyClass))

代码3：

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]


class MyClass(metaclass=Singleton):
    def __init__(self, name):
        self.name = name


class MyClass2(object):
    pass


a = MyClass('hello')
b = MyClass('world')

print('a.name', a.name)
print('b.name', b.name)
print('type(MyClass2)', type(MyClass2))
print('type(MyClass)', type(MyClass))