How to reuse bind variables in oracledb-python?

Yes, there is. There are a few options available to you.

1. You can use bind by name instead of bind by position. In your example it might look like this:

BEGIN
    UPDATE Competition SET
        abbreviation = :abbrev,
        descriptions = :descr,
        levels = :levels,
        source = :source
    WHERE competitionId = :compet;
    IF ( sql%notfound ) THEN
        INSERT INTO Competition
        VALUES (:abbrev, :descr, :levels, :source, :compet);
    END IF;
END;

2. Since this is an anonymous PL/SQL block, you can also store the bind values in temporary variables, like this:

DECLARE
    t_Abbrev        varchar2(10);
    t_Description   varchar2(100);
    t_Levels        varchar2(50);
    t_Source        varchar2(100);
    t_CompetitionId number;
BEGIN
    t_Abbrev := :1;
    t_Description := :2;
    t_Levels := :3;
    t_Source := :4;
    t_CompetitionId := :5;
    UPDATE Competition SET
        abbreviation = t_Abbrev,
        descriptions = t_Description,
        levels = t_Levels,
        source = t_Source
    WHERE competitionId = t_CompetitionId;
    IF ( sql%notfound ) THEN
        INSERT INTO Competition
        VALUES (t_Abbrev, t_Description, t_Levels, t_Source, t_CompetitionId);
    END IF;
END;

3. Finally, you can use the MERGE statement instead of an anonymous PL/SQL block:

MERGE INTO Competition C USING (
    SELECT
        :1 as ABBREV,
        :2 as DESCRIPTION,
        :3 as LEVELS,
        :4 as SOURCE,
        :5 as COMPETITION_ID
    FROM DUAL
) S
ON (C.CompetitionId = S.COMPETITION_ID)
WHEN MATCHED THEN
    UPDATE COMPETITION SET
        Abbreviation = s.ABBREV,
        Descriptions = s.DESCRIPTION,
        Levels = s.LEVELS,
        Source = s.SOURCE
WHEN NOT MATCHED THEN
    INSERT INTO Competition VALUES (
        s.ABBREV,
        s.DESCRIPTION,
        s.LEVELS,
        s.SOURCE,
        s.COMPETITIONID
    )