def set_source(self, source_id=None):
if source_id is None:<EOL><INDENT>raise NullArgument()<EOL><DEDENT>metadata = Metadata(**settings.METADATA['<STR_LIT>'])<EOL>if metadata.is_read_only():<EOL><INDENT>raise NoAccess()<EOL><DEDENT>if self._is_valid_input(source_id, metadata, array=False):<EOL><INDENT>self._my_map['<STR_LIT>'] = str(source_id)<EOL><DEDENT>else:<EOL><INDENT>raise InvalidArgument()<EOL><DEDENT>
Sets the source.

        :param source_id: the new publisher
        :type source_id: ``osid.id.Id``
        :raise: ``InvalidArgument`` -- ``source_id`` is invalid
        :raise: ``NoAccess`` -- ``Metadata.isReadOnly()`` is ``true``
        :raise: ``NullArgument`` -- ``source_id`` is ``null``

        *compliance: mandatory -- This method must be implemented.*