python singleton 4种单例

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

@singleton
class Test1:
    pass

# 推荐使用这种 简单好用 线程安全
# ===========================================================
import threading

class Singleton:

    def __init__(self, cls):
        self.__cls = cls
        self.__instance = None
        self._lock = threading.Lock()

    def __call__(self, *args, **kwargs):
        self._lock.acquire()
        if self.__instance is None:
            self.__instance = self.__cls(*args, **kwargs)
        self._lock.release()
        return self.__instance

@Singleton
class Test2:
    pass

# 继承时的执行顺序 base new -> son new -> son init. new 相当于构造方法，init相当于实例后再初始化 先执行父类的new 就好比先是人再是学生
# ==============================================================
class SingleBase(object):
    instance = None

    def __new__(cls, *args, **kwargs):

        # if not instance:
        #     instance = cls.__new__(*args, **kwargs):
        # return instance
        if cls.instance is None:
            cls.instance = object.__new__(cls)
        return cls.instance

    def __init__(self):
        pass

class Test3(SingleBase):

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

    def get_name(self):
        return self._name

# meta 这个我还不是很懂
# =====================================================
class MetaSingleton(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super().__call__(*args, **kwargs)
        return cls._instances[cls]

class Test4(metaclass=MetaSingleton):

    def __init__(self):
        pass

if __name__ == '__main__':
    t1 = Test3('jwang')
    t2 = Test3('hello')
    print(id(t1))
    print(id(t2))
    print(t1.get_name())
    print(t2.get_name())

    test3 = Test4()
    test4 = Test4()
    print(id(test3) == id(test4))