如何在python中创建带有参数的单例类

Question

我正在寻找正确的方法来创建一个在第一次创建时接受参数的单例类。 我的研究将我引向了 3 种不同的方式：

元类

class Singleton(type):
    instance = None
    def __call__(cls, *args, **kwargs):
        if cls.instance is None:
            cls.instance = super(Singleton, cls).__call__(*args, **kwargs)
        return cls.instance

class ASingleton(metaclass=Singleton):
    pass

__new__

class Singleton(object):
    instance = None
    def __new__(cls, *args, **kwargs):
        if cls.instance is None:
            cls.instance = super().__new__(cls, *args, **kwargs)
        return cls.instance

装饰器

def Singleton(myClass):
    instances={}
    def getInstance(*args, **kwargs):
        if myClass not in instances:
            instances[myClass] = myClass(*args, **kwargs)
        return instances[myClass]
    return getInstance

@Singleton
class SingletonTest(object):
    pass

所有这些都可以正常工作，但是当涉及到启动时（例如在普通课程中使用 __init__），我无法找到正确的实现方法。 我能想到的唯一解决方案是以这种方式使用元类方法：

class Singleton(type):
    instance = None

    def __call__(cls, *args, **kwargs):
        if cls.instance is None:
            cls.instance = super(Singleton, cls).__call__(*args, **kwargs)
        return cls.instance

class ASingleton(metaclass=Singleton):
    def __init__(self,j):
        self.j=j

我不知道这是否是创建接受参数的单例的正确方法。

Answer 1

我发现实现 Singleton 的最佳方式是使用元类：

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]

# Python 2
class MyClass():
    __metaclass__= Singleton

# Python 3
class MyClass(metaclass=Singleton):
     pass

Answer 2

除了@AndriyIvaneyko 的回答，这里有一个线程安全的元类单例实现：

# Based on tornado.ioloop.IOLoop.instance() approach.
# See https://github.com/facebook/tornado
# Whole idea for this metaclass is taken from: https://stackoverflow.com/a/6798042/2402281
class ThreadSafeSingleton(type):
    _instances = {}
    _singleton_lock = threading.Lock()

    def __call__(cls, *args, **kwargs):
        # double-checked locking pattern (https://en.wikipedia.org/wiki/Double-checked_locking)
        if cls not in cls._instances:
            with cls._singleton_lock:
                if cls not in cls._instances:
                    cls._instances[cls] = super(ThreadSafeSingleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]

class YourImplementation(metaclass=ThreadSafeSingleton):
    def __init__(self, *args, **kwargs):
        pass  # your implementation goes here

希望你觉得它有用！