Human friendly name generator for default study names.

Motivation

This issue was originally mentioned at https://github.com/optuna/optuna/pull/799#discussion_r359110361 by @sile.

The format of the current default study name is no-name- + UUID. It is sometimes difficult for users to distinguish each study. For example, we can only see the prefix of the study name like no-name-0 in the following screenshot.

I think we can use more human-friendly name generators like Docker.

Description

Current name generator code for RDBStorage is here.

In my quick survey, Docker seems to employ moby for the name generation and it manages some dictionaries. moby is ported to python as moby-dict, and we may be able to use it. But I’m not sure if moby-dict is the best solution because it is not actively maintained as far as I see its release history. So, I think we need a further survey and design discussion.

[hvy]

We could also add prefixes instead which might allow some sort of sorting. Instead of random numbers we could then use formatted dates or Unix times. It might also be reasonable not being too generous in the name generator and recommend intensive users to actually specify names explicitly.

[sile]

Instead of random numbers we could then use formatted dates or Unix times.

Agree. I personally prefer human understandable datetime format to unixtime format.

It might also be reasonable not being too generous in the name generator and recommend intensive users to actually specify names explicitly.

This choice also sounds acceptable. I think that we could raise an exception and recommend the user to specify the name manually if the retry number of a name generation exceeded a predefined count.