def _from_deprecated_string(cls, serialized):
        """
        Return an instance of `cls` parsed from its deprecated `serialized` form.

        This will be called only if :meth:`OpaqueKey.from_string` is unable to
        parse a key out of `serialized`, and only if `set_deprecated_fallback` has
        been called to register a fallback class.

        Args:
            cls: The :class:`OpaqueKey` subclass.
            serialized (unicode): A serialized :class:`OpaqueKey`, with namespace already removed.

        Raises:
            InvalidKeyError: Should be raised if `serialized` is not a valid serialized key
                understood by `cls`.
        """
        if serialized.count('/') != 2:
            raise InvalidKeyError(cls, serialized)

        return cls(*serialized.split('/'), deprecated=True)
Return an instance of `cls` parsed from its deprecated `serialized` form.

        This will be called only if :meth:`OpaqueKey.from_string` is unable to
        parse a key out of `serialized`, and only if `set_deprecated_fallback` has
        been called to register a fallback class.

        Args:
            cls: The :class:`OpaqueKey` subclass.
            serialized (unicode): A serialized :class:`OpaqueKey`, with namespace already removed.

        Raises:
            InvalidKeyError: Should be raised if `serialized` is not a valid serialized key
                understood by `cls`.