def _opposite_side(pos):
    """Return opposite of 'top', 'bottom', 'left', 'right'."""
    opposite = {
        'top': 'bottom',
        'bottom': 'top',
        'right': 'left',
        'left': 'right',
    }
    if pos not in opposite:
        raise ValueError(
            'Pos needs to be one of [{0}].'.format(
                ', '.join('"{0}"'.format(position) for position in opposite),
            ),
        )

    return opposite[pos]