code: def UpdateMapping(self, filename, mapping_update):
      
      if filename not in self._file_mapping:
        raise problems.NonexistentMapping(filename)
      mapping = self._file_mapping[filename]
      mapping.update(mapping_update)
docs: Updates an entry in the list of known filenames.
         An entry is identified by its filename.
  
      Args:
          filename: The filename whose mapping is to be updated
          mapping_update: A dictionary containing the fields to update and their
              new values.
      Raises:
          InexistentMapping if the filename does not exist in the mapping