单利模式Python下的几种实现方式

singleton

Singleton in Python

本文所有代码未考虑线程安全

大部分代码来源于stackoverflow-Creating a singleton in Python

1.class decorator

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class Singleton:
    def __init__(self, cls):
        self.cls = cls
        self.instance = None
        self.cls()
    def __call__(self, *args, **kwds):
        if self.instance is None:
            self.instance = self.cls(*args, **kwds)
        return self.instance
@Singleton
class MyClass:
    pass

2.function decorator

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def singleton(cls):
    instances = {}
    def get_instance(*args, **kwargs):
        if cls not in instances:
            instances[cls] = cls(*args, **kwargs)
        return instances[cls]
    return get_instance
@singleton
class MyClass:
    pass

3.metaclass

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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):
    pass

5.base class

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

6.class attribute

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class WebSoupClient:
    def __init__(self, api_url):
        self.api_url = api_url
class API:
    _clients = {}
    def __init__(self, api_url, user_id, password):
        super().__init__()
        self.api_url = api_url
    @property
    def client(self):
        if self.api_url not in self._clients:
            self._clients[self.api_url] = WebSoupClient(self.api_url)
        return self._clients[self.api_url]