<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def open(self, data_source, *args, **kwargs):
 """ Open filename to get data for data_source. :param data_source: Data source for which the file contains data. :type data_source: str Positional and keyword arguments can contain either the data to use for the data source or the full path of the file which contains data for the data source. """

        if self.sources[data_source]._meta.data_reader.is_file_reader:
            filename = kwargs.get('filename')
            path = kwargs.get('path', '')
            rel_path = kwargs.get('rel_path', '')
            if len(args) > 0:
                filename = args[0]
            if len(args) > 1:
                path = args[1]
            if len(args) > 2:
                rel_path = args[2]
            args = ()
            kwargs = {'filename': os.path.join(rel_path, path, filename)}
            LOGGER.debug('filename: %s', kwargs['filename'])
        # call constructor of data source with filename argument
        self.objects[data_source] = self.sources[data_source](*args, **kwargs)
        # register data and uncertainty in registry
        data_src_obj = self.objects[data_source]
        meta = [getattr(data_src_obj, m) for m in self.reg.meta_names]
        self.reg.register(data_src_obj.data, *meta)