def _join_names(names):
    'Join the names of a multi-level index with an underscore.'
    levels = (str(name) for name in names if (name != ''))
    return '_'.join(levels)
Join the names of a multi-level index with an underscore.
def _join_names(names):
    
    levels = (str(name) for name in names if (name != ))
    return '_'.join(levels)