python metaclass的使用

item.py

"""
Scrapy Item

See documentation in docs/topics/item.rst
"""

from pprint import pformat
from UserDict import DictMixin

from scrapy.utils.trackref import object_ref


class BaseItem(object_ref):
    """Base class for all scraped items."""
    pass


class Field(dict):
    """Container of field metadata"""


class ItemMeta(type):

    def __new__(mcs, class_name, bases, attrs):
        fields = {}
        new_attrs = {}
        for n, v in attrs.iteritems():
            if isinstance(v, Field):
                fields[n] = v
            else:
                new_attrs[n] = v

        cls = type.__new__(mcs, class_name, bases, new_attrs)
        cls.fields = cls.fields.copy()
        cls.fields.update(fields)
        return cls


class DictItem(DictMixin, BaseItem):

    fields = {}

    def __init__(self, *args, **kwargs):
        self._values = {}
        if args or kwargs: # avoid creating dict for most common case
            for k, v in dict(*args, **kwargs).iteritems():
                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 keys(self):
        return self._values.keys()

    def __repr__(self):
        return pformat(dict(self))

    def copy(self):
        return self.__class__(self)


class Item(DictItem):

    __metaclass__ = ItemMeta