Python单例模式

1 使用__new__方法

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

class MyClass(Singleton):
    a = 1                                

2 共享属性

创建实例时把所有实例的__dict__指向同一个字典,这样它们具有相同的属性和方法.

class Borg(object):
    _state = {} 
    def __new__(cls, *args, **kw):
    ob = super(Borg, cls).__new__(cls, *args, **kw)
    ob.__dict__ = cls._state
    return ob

class MyClass2(Borg):
    a = 1

3 装饰器版本

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

@singleton
class MyClass:
 ...