单例模式

# 类方法
class Foo:
    __only = None

    def __init__(self, name):
        self.name = name

    @classmethod
    def get_only(cls, *args, **kwargs):
        if cls.__only is None:
            cls.__only = cls(*args, **kwargs)
        return cls.__only


obj1 = Foo.get_only(‘tom‘)
print(obj1.__dict__)  # 结果为 {‘name‘: ‘tom‘}
obj2 = Foo.get_only(‘jan‘)
print(obj1.__dict__)  # 结果为 {‘name‘: ‘tom‘}
print(obj2.__dict__)  # 结果为 {‘name‘: ‘tom‘}
print(obj1 is obj2)  # 结果为 True

# 类的__new__方法
class Foo:
    __only = None

    def __new__(cls, *args, **kwargs):
        if cls.__only is None:
            cls.__only = object.__new__(cls)
        return cls.__only

    def __init__(self, name):
        self.name = name


obj1 = Foo(‘tom‘)
print(obj1.__dict__)  # 结果为 {‘name‘: ‘tom‘}
obj2 = Foo(‘jan‘)
print(obj1.__dict__)  # 结果为 {‘name‘: ‘jan‘}
print(obj2.__dict__)  # 结果为 {‘name‘: ‘jan‘}
print(obj1 is obj2)  # 结果为 True

# 元类控制类的实例化
class MyMeta(type):
    def __new__(cls, *args, **kwargs):
        return super().__new__(cls, *args, **kwargs)

    def __init__(self, class_name, class_base, class_dic):
        super().__init__(class_name, class_base, class_dic)
        self.__only = None

    def __call__(self, *args, **kwargs):
        if self.__only is None:
            obj = self.__new__(self, *args, **kwargs)
            self.__init__(obj, *args, **kwargs)
            self.__only = obj
        return self.__only


class Foo(metaclass=MyMeta):
    def __init__(self, name):
        self.name = name


obj1 = Foo(‘tom‘)
print(obj1.__dict__)  # 结果为 {‘name‘: ‘tom‘}
obj2 = Foo(‘jan‘)
print(obj1.__dict__)  # 结果为 {‘name‘: ‘tom‘}
print(obj2.__dict__)  # 结果为 {‘name‘: ‘tom‘}
print(obj1 is obj2)  # 结果为 True

# 使用装饰器
def passer(cls):
    def wrapper(*args, **kwargs):
        if cls._only is None:
            _only = cls(*args, **kwargs)
            cls._only = _only
        return cls._only

    return wrapper


@passer
class Foo:
    _only = None

    def __init__(self, name):
        self.name = name


obj1 = Foo(‘tom‘)
print(obj1.__dict__)  # 结果为 {‘name‘: ‘tom‘}
obj2 = Foo(‘jan‘)
print(obj1.__dict__)  # 结果为 {‘name‘: ‘jan‘}
print(obj2.__dict__)  # 结果为 {‘name‘: ‘jan‘}
print(obj1 is obj2)  # 结果为 True

# 通过模块导入
import only_obj

obj1 = only_obj._only
print(obj1.__dict__)  # 结果为 {‘name‘: ‘tom‘}
obj2 = only_obj._only
print(obj1.__dict__)  # 结果为 {‘name‘: ‘tom‘}
print(obj2.__dict__)  # 结果为 {‘name‘: ‘tom‘}
print(obj1 is obj2)  # 结果为 True

# 被导入模块
class Foo:
    def __init__(self, name):
        self.name = name


_only = Foo(‘tom‘)