def get_output_names(hf):
    """
    get_output_names(hf)

    Returns a list of the output variables names in the HDF5 file.

    Args:
      hf: An open HDF5 filehandle or a string containing the HDF5
        filename to use.

    Returns:
      A sorted list of the output variable names in the HDF5 file.
    """
    return sorted(map(str, hf['/output/data'].keys()))