SQLAlchemy ORDER BY FIELD()

This may not be a very satisfying solution, but how about using a case expression instead of order by fields:

sqlalchemy.orm.Query(AlphabetTable) \
    .filter(AlphabetTable.letter.in_("gack")) \
    .order_by(sqlalchemy.sql.expression.case(((AlphabetTable.letter == "g", 1),
                                              (AlphabetTable.letter == "a", 2),
                                              (AlphabetTable.letter == "c", 3),
                                              (AlphabetTable.letter == "k", 4))))

A sqlalchemy func expression can be used to generate the order by field clause:

session.query(AlphabetTable) \
    .filter(AlphabetTable.letter.in_("gack")) \
    .order_by(sqlalchemy.func.field(AlphabetTable.letter, *"gack"))