def __new__(cls, name, bases, attrs):
     attrs['base_fields'] = get_declared_fields(bases, attrs)
     new_class = super(DeclarativeFieldsMetaclass,
                  cls).__new__(cls, name, bases, attrs)
     if 'media' not in attrs:
         new_class.media = media_property(new_class)
     return new_class

    def __new__(cls, name, bases, attrs):
        formfield_callback = attrs.pop('formfield_callback',
                lambda f: f.formfield())
        try:
            parents = [b for b in bases if issubclass(b, ModelForm)]
        except NameError:
            # We are defining ModelForm itself.
            parents = None
        new_class = super(ModelFormMetaclass, cls).__new__(cls, name, bases,
                attrs)
        if not parents:
            return new_class

        if 'media' not in attrs:
            new_class.media = media_property(new_class)
        declared_fields = get_declared_fields(bases, attrs, False)
        opts = new_class._meta = ModelFormOptions(getattr(new_class, 'Meta', None))
        if opts.model:
            # If a model is defined, extract form fields from it.
            fields = fields_for_model(opts.model, opts.fields,
                                      opts.exclude, formfield_callback)
            # Override default model fields with any custom declared ones
            # (plus, include all the other declared fields).
            fields.update(declared_fields)
        else:
            fields = declared_fields
        new_class.declared_fields = declared_fields
        new_class.base_fields = fields
        return new_class

    def __new__(cls, name, bases, attrs):
        formfield_callback = attrs.pop("formfield_callback", None)
        try:
            parents = [b for b in bases if issubclass(b, ModelForm)]
        except NameError:
            # We are defining ModelForm itself.
            parents = None
        declared_fields = get_declared_fields(bases, attrs, False)
        new_class = super(ModelFormMetaclass, cls).__new__(cls, name, bases, attrs)
        if not parents:
            return new_class

        if "media" not in attrs:
            new_class.media = media_property(new_class)
        opts = new_class._meta = ModelFormOptions(getattr(new_class, "Meta", None))
        if opts.model:
            # If a model is defined, extract form fields from it.
            fields = fields_for_model(opts.model, opts.fields, opts.exclude, opts.widgets, formfield_callback)
            # make sure opts.fields doesn't specify an invalid field
            none_model_fields = [k for k, v in fields.iteritems() if not v]
            missing_fields = set(none_model_fields) - set(declared_fields.keys())
            if missing_fields:
                message = "Unknown field(s) (%s) specified for %s"
                message = message % (", ".join(missing_fields), opts.model.__name__)
                raise FieldError(message)
            # Override default model fields with any custom declared ones
            # (plus, include all the other declared fields).
            fields.update(declared_fields)
        else:
            fields = declared_fields
        new_class.declared_fields = declared_fields
        new_class.base_fields = fields
        return new_class

    def __new__(cls, name, bases, attrs):
        attrs['base_fields'] = get_declared_fields(bases, attrs)

        new_class = type.__new__(cls, name, bases, attrs)
        if 'media' not in attrs:
            new_class.media = media_property(new_class)
        return new_class