def keys(self):
        """
        return a list of the content types this set supports.

        this is not a complete list: serializers can accept more than
        one content type. However, it is a good representation of the
        class of content types supported.
        """
        return_value = []
        for s in self.serializers:
            return_value += s.content_type
        return return_value
return a list of the content types this set supports.

        this is not a complete list: serializers can accept more than
        one content type. However, it is a good representation of the
        class of content types supported.