def merge_asof(left, right, on=None,
               left_on=None, right_on=None,
               left_index=False, right_index=False,
               by=None, left_by=None, right_by=None,
               suffixes=('_x', '_y'),
               tolerance=None,
               allow_exact_matches=True,
               direction='backward'):
    op = _AsOfMerge(left, right,
                    on=on, left_on=left_on, right_on=right_on,
                    left_index=left_index, right_index=right_index,
                    by=by, left_by=left_by, right_by=right_by,
                    suffixes=suffixes,
                    how='asof', tolerance=tolerance,
                    allow_exact_matches=allow_exact_matches,
                    direction=direction)
    return op.get_result()