类工厂函数中的一个问题（流畅的python书中的例子）

这是流畅的python一书中关于类工厂函数的代码：

    def factory(cls_name,field_name):
try:
    field_name=field_name.replace(',',' ').spilt(' ')
except AttributeError:
    pass
field_name=tuple(field_name)

def __init__(self,*args,**kwargs):
    attrs=dict(zip(self.__slots__,args))
    attrs.update(kwargs)
    for name,value in attrs.items():
        setattr(self,name,value)

def __iter__(self):
    for name in self.__slots__:
        yield getattr(self,name)

def __repr__(self):
    values=','.join('{}={!r}'.format(*i) for i in zip(self.__slots__,self))
    return '{}({})'.format(self.__class__.__name__,values)

cls_attrs=dict(__slots__=field_name,
               __init__=__init__,
               __iter__=__iter__,
               __repr__=__repr__)
return type(cls_name,(object,),cls_attrs)
Dog=factory('Dog','name weight owner')
dog=Dog('rex',30,'sunshuofeng')
print(dog)

我在运行时其报错了：

    Traceback (most recent call last):
 File "C:\Users\18022863809\Desktop\python_work\11.py", line 27, in <module>
Dog=factory('Dog','name weight owner')
File "C:\Users\18022863809\Desktop\python_work\11.py", line 26, in factory
return type(cls_name,(object,),cls_attrs)
TypeError: __slots__ must be identifiers

这是为什么？

# encoding: utf-8


def factory(cls_name,field_names):
    try:
        field_names=field_names.replace(',',' ').split()
    except AttributeError:
        pass
    field_names=tuple(field_names)

    def __init__(self,*args,**kwards):
        attrs=dict(zip(self.__slots__,args))
        attrs.update(kwards)
        for key,value in attrs.items():
            setattr(self,key,value)

    def __iter__(self):
        for name in self.__slots__:
            yield getattr(self,name)
    def __repr__(self):
        msg=', '.join('{}={!r}'.format(*i) for i in zip(self.__slots__,self))
        return '{}({})'.format(self.__class__.__name__,msg)

    cls_attrs=dict(__slots__=field_names,
                   __init__=__init__,
                  __iter__=__iter__,
                   __repr__=__repr__)

    return type(cls_name,(object,),cls_attrs)

Dog=factory('Dog','name weight owner')
dog=Dog('rex',30,'sunshuofeng')
print(dog)