Python单例模式的4种实现方法

#-*- encoding=utf-8 -*- 
print ‘----------------------方法1--------------------------‘
#方法1,实现__new__方法 
#并在将一个类的实例绑定到类变量_instance上, 
#如果cls._instance为None说明该类还没有实例化过,实例化该类,并返回 
#如果cls._instance不为None,直接返回cls._instance 
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
one = MyClass() 
two = MyClass() 
two.a = 3
print one.a 
#3 
#one和two完全相同,可以用id(), ==, is检测 
print id(one) 
#29097904 
print id(two) 
#29097904 
print one == two 
#True 
print one is two 
#True 
print ‘----------------------方法2--------------------------‘
#方法2,共享属性;所谓单例就是所有引用(实例、对象)拥有相同的状态(属性)和行为(方法) 
#同一个类的所有实例天然拥有相同的行为(方法), 
#只需要保证同一个类的所有实例具有相同的状态(属性)即可 
#所有实例共享属性的最简单最直接的方法就是__dict__属性指向(引用)同一个字典(dict) 
#可参看:http://code.activestate.com/recipes/66531/ 
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
one = MyClass2() 
two = MyClass2() 
#one和two是两个不同的对象,id, ==, is对比结果可看出 
two.a = 3
print one.a 
#3 
print id(one) 
#28873680 
print id(two) 
#28873712 
print one == two 
#False 
print one is two 
#False 
#但是one和two具有相同的（同一个__dict__属性）,见: 
print id(one.__dict__) 
#30104000 
print id(two.__dict__) 
#30104000 
print ‘----------------------方法3--------------------------‘
#方法3:本质上是方法1的升级（或者说高级）版 
#使用__metaclass__（元类）的高级python用法 
class Singleton2(type): 
  def __init__(cls, name, bases, dict): 
    super(Singleton2, cls).__init__(name, bases, dict) 
    cls._instance = None
  def __call__(cls, *args, **kw): 
    if cls._instance is None: 
      cls._instance = super(Singleton2, cls).__call__(*args, **kw) 
    return cls._instance 
class MyClass3(object): 
  __metaclass__ = Singleton2 
one = MyClass3() 
two = MyClass3() 
two.a = 3
print one.a 
#3 
print id(one) 
#31495472 
print id(two) 
#31495472 
print one == two 
#True 
print one is two 
#True 
print ‘----------------------方法4--------------------------‘
#方法4:也是方法1的升级（高级）版本, 
#使用装饰器(decorator), 
#这是一种更pythonic,更elegant的方法, 
#单例类本身根本不知道自己是单例的,因为他本身(自己的代码)并不是单例的 
def singleton(cls, *args, **kw): 
  instances = {} 
  def _singleton(): 
    if cls not in instances: 
      instances[cls] = cls(*args, **kw) 
    return instances[cls] 
  return _singleton 
@singleton
class MyClass4(object): 
  a = 1
  def __init__(self, x=0): 
    self.x = x 
one = MyClass4() 
two = MyClass4() 
two.a = 3
print one.a 
#3 
print id(one) 
#29660784 
print id(two) 
#29660784 
print one == two 
#True 
print one is two 
#True 
one.x = 1
print one.x 
#1 
print two.x