_cli.py: apply expandvars() to storage argument

[nodakai]

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This repository (optuna-dashboard) and Goptuna share common code.
This pull request may therefore be ported to Goptuna.
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What does this implement/fix? Explain your changes.

so that I can pass a database password as an environment variable, like:

$ export PG_PASS=53cr37  # or k8s secret or whatever
$ optuna-dashboard.py 'postgresql://optuna:$PG_PASS@db/optuna'

Note we must not let Bash interpret the $ notation as that'll expose the secret to other users on the machine through commands like ps, top

Apologies, but I haven’t had the opportunity to test it myself...

[c-bata]

@not522 Could you review this PR?

[nodakai]

Hmm, just realized that the optuna CLI (but not APIs) supports $OPTUNA_STORAGE as an experimental feature, accepting a full SQLAlchemy URI.

I think my patch addresses a similar scenario, but it requires the user to explicitly specify '$OPTUNA_STORAGE' (with single quotes). In contrast, with the optuna CLI, it's possible to omit --storage ... when $OPTUNA_STORAGE is set.

[not522]

Thank you for your feedback! After discussing the issue, we've concluded that introducing $OPTUNA_STORAGE as the default database URI is a good approach. Do you think this change will resolve your problem?

[nabenabe0928]

@nodakai If the solution suggested by @not522 works for you, we would like to close this PR.
Could you confirm this with a comment rather than with an emoji?

[nodakai]

Yes, please go ahead with #717 (comment)