在Python中建立单例 - Creating a singleton in Python

问题：

This question is not for the discussion of whether or not the singleton design pattern is desirable, is an anti-pattern, or for any religious wars, but to discuss how this pattern is best implemented in Python in such a way that is most pythonic. 这个问题不是为了讨论是否须要单例设计模式 ，是不是反模式，仍是针对任何宗教战争，而是要讨论如何以最pythonic的方式在Python中最好地实现此模式。 In this instance I define 'most pythonic' to mean that it follows the 'principle of least astonishment' . 在这种状况下，我定义“最pythonic”来表示它遵循“最少惊讶的原理” 。

I have multiple classes which would become singletons (my use-case is for a logger, but this is not important). 我有多个将成为单例的类（个人用例用于记录器，但这并不重要）。 I do not wish to clutter several classes with added gumph when I can simply inherit or decorate. 当我能够简单地继承或修饰时，我不但愿增长gumph来使几个类杂乱无章。

Best methods: 最佳方法：

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Method 1: A decorator 方法1：装饰器

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

Pros 优势

• Decorators are additive in a way that is often more intuitive than multiple inheritance. 装饰器的添加方式一般比多重继承更直观。

Cons 缺点

• While objects created using MyClass() would be true singleton objects, MyClass itself is aa function, not a class, so you cannot call class methods from it. 使用MyClass（）建立的对象将是真正的单例对象，而MyClass自己是一个函数，而不是类，所以您不能从中调用类方法。 Also for m = MyClass(); n = MyClass(); o = type(n)(); 一样对于m = MyClass(); n = MyClass(); o = type(n)(); m = MyClass(); n = MyClass(); o = type(n)(); then m == n && m != o && n != o 而后m == n && m != o && n != o

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Method 2: A base class 方法2：一个基类

class Singleton(object):
    _instance = None
    def __new__(class_, *args, **kwargs):
        if not isinstance(class_._instance, class_):
            class_._instance = object.__new__(class_, *args, **kwargs)
        return class_._instance

class MyClass(Singleton, BaseClass):
    pass

Pros 优势

• It's a true class 这是一个真正的课堂

Cons 缺点

• Multiple inheritance - eugh! 多重继承-好！ __new__ could be overwritten during inheritance from a second base class? __new__是否能够在从第二个基类继承时被覆盖？ One has to think more than is necessary. 人们必须思考的超出了必要。

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Method 3: A metaclass 方法3： 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]

#Python2
class MyClass(BaseClass):
    __metaclass__ = Singleton

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

Pros 优势

• It's a true class 这是一个真正的课堂
• Auto-magically covers inheritance 自动神奇地涵盖继承
• Uses __metaclass__ for its proper purpose (and made me aware of it) 将__metaclass__用于其适当的目的（并使我意识到这一点）

Cons 缺点

• Are there any? 有吗

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Method 4: decorator returning a class with the same name 方法4：装饰器返回具备相同名称的类

def singleton(class_):
    class class_w(class_):
        _instance = None
        def __new__(class_, *args, **kwargs):
            if class_w._instance is None:
                class_w._instance = super(class_w,
                                    class_).__new__(class_,
                                                    *args,
                                                    **kwargs)
                class_w._instance._sealed = False
            return class_w._instance
        def __init__(self, *args, **kwargs):
            if self._sealed:
                return
            super(class_w, self).__init__(*args, **kwargs)
            self._sealed = True
    class_w.__name__ = class_.__name__
    return class_w

@singleton
class MyClass(BaseClass):
    pass

Pros 优势

• It's a true class 这是一个真正的课堂
• Auto-magically covers inheritance 自动神奇地涵盖继承

Cons 缺点

• Is there not an overhead for creating each new class? 建立每一个新类没有开销吗？ Here we are creating two classes for each class we wish to make a singleton. 在这里，咱们为但愿建立单例的每一个类建立两个类。 While this is fine in my case, I worry that this might not scale. 虽然这对我来讲很好，但我担忧这可能没法扩展。 Of course there is a matter of debate as to whether it aught to be too easy to scale this pattern... 固然，要扩展这种模式是否太容易了还有争议。
• What is the point of the _sealed attribute _sealed属性的意义是什么
• Can't call methods of the same name on base classes using super() because they will recurse. 没法使用super()在基类上调用相同名称的方法，由于它们会递归。 This means you can't customize __new__ and can't subclass a class that needs you to call up to __init__ . 这意味着您不能自定义__new__ ，也不能将须要调用__init__类做为子类。

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Method 5: a module 方法5：一个模块

a module file singleton.py 一个模块文件singleton.py

Pros 优势

• Simple is better than complex 简单胜于复杂

Cons 缺点

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解决方案：

参考一： https://stackoom.com/question/SMlJ/在Python中建立单例
参考二： https://oldbug.net/q/SMlJ/Creating-a-singleton-in-Python