Motivation

Users sometimes makes the mistake of passing the list of directions to the direction argument. The error message for this mistake is hard to understand what is the problem.

• Correct

optuna.create_study(directions=["minimize", "minimize"])

• Wrong

Using direction instead of directions

optuna.create_study(direction=["minimize", "minimize"])

ValueError: Please set either 'minimize' or 'maximize' to direction. You can also set the corresponding `StudyDirection` member.

Suggestion

We can make the error message more helpful for the mistake. It can be something like "for multi-objective optimization, using directions instead of direction".

Additional context (optional)

No response