code: def _get_root_files(self, directory):
          
          if len(self.filepaths) is 0:
              if self.filters:
                  root_files = [(directory, f) for f in os.listdir(directory)
                                if os.path.isfile(os.path.join(directory, f))
                                and self.filters.validate(f)
                                and self.filters.get_level(f) == self.filters.max_level]
              else:
                  root_files = [(directory, f) for f in os.listdir(directory)
                                if os.path.isfile(os.path.join(directory, f))]
              self.add_path(root_files)
docs: Retrieve files within the root directory