UnicodeDecodeError in optuna 3.5.0 with db.sqlite3 storage

[HDembinski]

Expected behavior

As newbie on optuna, I wrongly tried to use this call in the CLI, where db.sqlite3 is a file on my drive in the same folder.

optuna --storage db.sqlite3 study-names

This call currently gives a complicated error message about a UnicodeDecodeError that does not indicate the true mistake; I should have used the full URL sqlite:///db.sqlite3. For me it is not obvious that I have to use this URL, and I expected this simpler call to work. I propose two solutions, with preference in that order:

1. For user convenience, make the call above work: optuna detects that db.sqlite3 is the path to a filename and handles this call correctly.
2. Optuna should detect that the storage is given without an URL prefix and complain with an error message that points the user towards this issue.

[nzw0301]

According to the first line of the log message, I suspect the CLI reads the SQLite file as journal storage. Could you try either way

optuna --storage sqlite:///db.sqlite3 study-names

or

optuna --storage db.sqlite3 --storage-class RDBStorage study-names

EDIT: sorry the second example does not work. Could you try only the first one?

[HDembinski]

Thank you for the fix, the first line works. Still, I think this a UX bug. Please provide an error message for this case which points the user in the right direction or make the CLI smarter to handle the call optuna --storage db.sqlite3 study-names correctly. You should not expect that your users are familiar with the details of sqlite.

[nzw0301]

Thank you for your input. Could you update the title and description? Then I will re-open this issue and change the label to a more appropriate one.

[HDembinski]

Thanks, ok, done.