在装饰器里维护一个字典，缓存所有的单例类，只要单例不存在就创建，已经存在直接返回实例对象
def singleton(cls):
instances = {}
def wrapper(*args,**kwargs):
if cls not in instances:
instances[cls] = cls(*args,**kwargs)
return instances[cls]
return wrapper
@singleton
class Foo:
pass
foo1 = Foo()
foo2 = Foo()
print(foo1 is foo2)
实例化对象其实是在__new__方法里，而__init__只做初始化，所以我们重写基类的__new__方法，保证只有一个实例
class Singleton:
def __new__(cls, *args, **kwargs):
if not hasattr(cls,‘_instance‘):
cls._instance = super(Singleton,cls).__new__(cls,*args,**kwargs)
return cls._instance
class Foo(Singleton):
pass
foo1 = Foo()
foo2 = Foo()
print(foo1 is foo2)
元类是用于创建类对象的类，类对象创建实例对象时一定会调用__call__方法，因此在调用__call__时候保证事中值创建一个实例即可，type是python中的类
class Singleton(type):
def __call__(cls, *args, **kwargs):
if not hasattr(cls, ‘_instance‘):
cls._instance = super(Singleton, cls).__call__(*args, **kwargs)
return cls._instance
class Foo(metaclass=Singleton):
pass
foo1 = Foo()
foo2 = Foo()
print(foo1 is foo2)
能想到的应用场景：数据库的连接池，只要一份即可
原文：http://www.cnblogs.com/xinsiwei18/p/5942657.html