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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def UpdateMapping(self, filename, mapping_update):
 """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 """

    if filename not in self._file_mapping:
      raise problems.NonexistentMapping(filename)
    mapping = self._file_mapping[filename]
    mapping.update(mapping_update)