def _checkKnownRunOrder(order):
    """
    Check that the given order is a known test running order.

    Does nothing else, since looking up the appropriate callable to sort the
    tests should be done when it actually will be used, as the default argument
    will not be coerced by this function.

    @param order: one of the known orders in C{_runOrders}
    @return: the order unmodified
    """
    if order not in _runOrders:
        raise usage.UsageError(
            "--order must be one of: %s. See --help-orders for details" %
            (", ".join(repr(order) for order in _runOrders),))
    return order

    Check that the given order is a known test running order.

    Does nothing else, since looking up the appropriate callable to sort the
    tests should be done when it actually will be used, as the default argument
    will not be coerced by this function.

    @param order: one of the known orders in C{_runOrders}
    @return: the order unmodified
    
stackoverflow/venv/lib/python3.6/site-packages/twisted/scripts/trial.py