code: def _get_version_mode(self, mode=None):
          
          version_mode = self._version_modes.get(mode)
          if not version_mode:
              version_mode = self._version_modes[mode] = VersionMode(name=mode)
          return version_mode
docs: Return a VersionMode for a mode name.
  
          When the mode is None, we are working with the 'base' mode.