code: def get_wd(self, path):
          
          path = self.__format_path(path)
          for iwd in self._wmd.items():
              if iwd[1].path == path:
                  return iwd[0]
docs: Returns the watch descriptor associated to path. This method
          presents a prohibitive cost, always prefer to keep the WD
          returned by add_watch(). If the path is unknown it returns None.
  
          @param path: Path.
          @type path: str
          @return: WD or None.
          @rtype: int or None