单例模式

• 使用__new__方法

class Singleton(object):
    def __new__(cls, *args, **kwargs):
        if not hasattr(cls, '_instance'):
            bef = super(Singleton, cls)
            cls._instance = bef.__new__(cls, *args, **kwargs)
        return cls._instance

class sClass(Singleton):
    a = 1

• 共享属性

class Singleton(object):
    _state = {}
    def __new__(cls, *args, **kwargs):
        bef = super(Singleton, cls).__new__(cls, *args, **kwargs)
        bef.__dict__ = cls._state       # 创建实例时把所有实例的`__dict__`指向同一个字典
        return bef 

class sClass(Singleton):
    a = 1

• 装饰器版本

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

@Singleton
class sClass:
    ...

• import方法

# pSingleton.py
class Singleton(object):
    def foo(self):
        pass

mSingleton = Singleton()

# Using
from pSingleton import mSingleton

mSingleton.foo()

（最近更新：2019年05月20日）