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()