Python 单例

def singleton(cls):
    instances = []# 为什么这里不直接为None,因为内部函数没法访问外部函数的非容器变量
    def getinstance(*args, **kwargs):
        if not instances:
            instances.append(cls(*args, **kwargs))
        return instances[0]
    return getinstance
@singleton
class Foo:
    a = 1
f1 = Foo()
f2 = Foo()
print id(f1), id(f2)

class Singleton(object):
    def __new__(cls, *args, **kwargs):
        if not hasattr(cls, '_instance'):
            cls._instance = object.__new__(cls, *args, **kwargs)
        return cls._instance
class Foo(Singleton):
    pass