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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 GetGtfsClassByFileName(self, filename):
 """Returns the transitfeed class corresponding to a GTFS file. Args: filename: The filename whose class is to be returned Raises: NonStandardMapping if the specified filename has more than one corresponding class """

    if filename not in self._file_mapping:
      return None
    mapping = self._file_mapping[filename]
    class_list = mapping['classes']
    if len(class_list) > 1:
      raise problems.NonStandardMapping(filename)
    else:
      return self._class_mapping[class_list[0]]