'Return an iterotor over keys for rows of the table.'

@staticmethod
def _values(data, var, as_string):

    if (var == INSTANCEID):
        return (inst.id for inst in data)
    if (var == INDEX):
        return range(len(data))
    col = data.get_column_view(var)[0]
    if (not as_string):
        return col
    if var.is_primitive():
        return ((var.str_val(val) if (not np.isnan(val)) else np.nan) for val in col)
    else:
        return ((str(val) if val else np.nan) for val in col)
