class BookItem(Item):    name=Field()    price=Field() book=BookItem(name='abc',price=43)

在这里BookItem 继承Item类，并且有2个类属性：name和price。都是类Field的实例。

@six.add_metaclass(ItemMeta)class Item(DictItem):    pass

这里表示Item继承于DictItem。并且使用元类ItemMeta创建Item这个类实例。

class ItemMeta(ABCMeta):    def __new__(mcs, class_name, bases, attrs):        classcell = attrs.pop('__classcell__', None)        new_bases = tuple(base._class for base in bases if hasattr(base, '_class'))        _class = super(ItemMeta, mcs).__new__(mcs, 'x_' + class_name, new_bases, attrs)        fields = getattr(_class, 'fields', {})        new_attrs = {}        for n in dir(_class):            v = getattr(_class, n)            if isinstance(v, Field):                fields[n] = v            elif n in attrs:                new_attrs[n] = attrs[n]        new_attrs['fields'] = fields        new_attrs['_class'] = _class        if classcell is not None:            new_attrs['__classcell__'] = classcell        return super(ItemMeta, mcs).__new__(mcs, class_name, bases, new_attrs)

在这里__new__的参数传入分别是(ItemMeta,BookItem,Item,( ('name',{}),('price',()) ))    PS:可参考 www.cnblogs.com/solakevon/p/8894822.html

因为在使用元类创建类实例时，会把类名，父类名，类属性传入。和使用type创建类实例一样。

new_bases是个空元祖。_class是x_item类。类属性和BookItem一样.

在for循环里面会把在attrs里面Field的实例给传到fields这个字典里面。其他非Field实例的类属性都放到new_attrs里面

最后把fields这个字典也放到new_attrs里面。此时fields里面有name和price。

class DictItem(MutableMapping, BaseItem):    fields = {}    def __init__(self, *args, **kwargs):        self._values = {}        if args or kwargs:  # avoid creating dict for most common case            for k, v in six.iteritems(dict(*args, **kwargs)):                self[k] = v    def __getitem__(self, key):        return self._values[key]    def __setitem__(self, key, value):        if key in self.fields:            self._values[key] = value        else:            raise KeyError("%s does not support field: %s" %                (self.__class__.__name__, key))    def __delitem__(self, key):        del self._values[key]    def __getattr__(self, name):        if name in self.fields:            raise AttributeError("Use item[%r] to get field value" % name)        raise AttributeError(name)    def __setattr__(self, name, value):        if not name.startswith('_'):            raise AttributeError("Use item[%r] = %r to set field value" %                (name, value))        super(DictItem, self).__setattr__(name, value)    def __len__(self):        return len(self._values)    def __iter__(self):        return iter(self._values)    __hash__ = BaseItem.__hash__    def keys(self):        return self._values.keys()    def __repr__(self):        return pformat(dict(self))    def copy(self):        return self.__class__(self)

在DictItem将会实现Item对象的一些功能。