Postgresql bulk upsert in Python (Django)

bulk_upsert.py

def bulk_upsert(model, fields, values, by):
    """
    Return the tuple of (inserted, updated) ids
    """
    result = (None, None)

    if values:
        stmt = """
        WITH data_set AS (
            INSERT INTO %s (%s)
            SELECT DISTINCT ON (%s) *
            FROM  (
                VALUES %s
            ) v(%s)
            ON CONFLICT (%s)
            DO
            UPDATE SET (%s)=(%s)
            RETURNING xmax, id
        )
        SELECT ARRAY_AGG(id) FILTER (WHERE xmax = 0) AS ins,
               ARRAY_AGG(id) FILTER (WHERE xmax::text::int > 0) AS upd
        FROM data_set;
        """
        table_name = model._meta.db_table
        set_fields = ', '.join([f for f in fields if f != by])
        set_values = ', '.join(['EXCLUDED.{0}'.format(f) for f in fields if f != by])
        fields_str = ', '.join(fields)
        values_placeholders = ('%s, ' * len(values))[:-2]
        formatted_sql = stmt % (
            table_name,
            fields_str,
            by,
            values_placeholders,
            fields_str,
            by,
            set_fields,
            set_values
        )

        with connection.cursor() as cursor:
            cursor.execute(formatted_sql, values)
            result = cursor.fetchall()[0]

    return result