This PR is a new one to work around this #6021 Check issue.

issue: #5806

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.

Description of the changes

The error message displayed when a list object, is assigned to the direction parameter of create_study has been changed as follows.

• previous

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

• new

ValueError: For multi-objective optimization, using `directions` instead of `direction`.