python 四种单例模式

 1 使用__new__方法
 Python
 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._instanceclass MyClass(Singleton): a = 1

 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.

 Python
 class Borg(object): _state = {} def __new__(cls, *args, **kw): ob = super(Borg, cls).__new__(cls, *args, **kw) ob.__dict__ = cls._state return obclass MyClass2(Borg): a = 1

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

 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:
   ...
 4 import方法
 作为python的模块是天然的单例模式

 Python
 # mysingleton.pyclass My_Singleton(object): def foo(self): passmy_singleton = My_Singleton()# to usefrom mysingleton import my_singletonmy_singleton.foo()

 # mysingleton.py
 class My_Singleton(object):
     def foo(self):
         pass

 my_singleton = My_Singleton()

 # to use
 from mysingleton import my_singleton

 my_singleton.foo()