Deprecated decorator's message does not appear

Expected behavior

When we run a method or class has deprecated decorator of optuna, we expect a warning message is shown. But the current implementation does not show any warning message.

Environment

• Optuna version: the lastest version & additional commit
• Python version: 3.8.4
• OS: MacOSX 10.15.5

Error messages, stack traces, or logs

# error messages, stack traces, or logs

Steps to reproduce

1. pip install git+https://github.com/nzw0301/optuna.git@deprecated-test
2. Run the following examples: they do not show anything.

Reproducible examples (optional)

By the additional commit, IntLogUniformDistribution and IntLogUniformDistribution.single have deprecated decorators. But no warning messages appear when they are called:

import optuna
from optuna.distributions import IntLogUniformDistribution

dist = IntLogUniformDistribution(1, 10)
dist.single()

Or

import optuna
from optuna.distributions import IntLogUniformDistribution

optuna.logging.set_verbosity(optuna.logging.WARNING)

dist = IntLogUniformDistribution(1, 10)
dist.single()

Additional context (optional)

Other optuna’s codes such that it contains DeprecatedWarning such as optuna.structs.py show the warning message by default when we use them.

For example, if I run the following line,

from optuna.structs import FrozenTrial

then the following warning message is printed:

[W 2020-06-24 01:11:56,553] `structs` is deprecated. Classes have moved to the following modules. `structs.StudyDirection`->`study.StudyDirection`, `structs.StudySummary`->`study.StudySummary`, `structs.FrozenTrial`->`trial.FrozenTrial`, `structs.TrialState`->`trial.TrialState`, `structs.TrialPruned`->`exceptions.TrialPruned`.

---

Thank you @HideakiImamura for investigating this issue.

[HideakiImamura]

Hi all. Thank you for all of your contributions.

After the discussion with core team members, we have decided to use only warnings.warn() for the deprecation messages according to https://docs.python.org/3/howto/logging.html, because users can avoid this situation (reported in Issue #1411) by fixing their own code.

However, thanks to the investigation by @nzw0301, we have noticed the following TODOs.

• We should note how to use warnings.warn() and logging.warn() in https://github.com/optuna/optuna/wiki/Coding-Style-Conventions.
• We should refactor the current optuna code base such as optuna/structs.py to use only warnings.warn().

[harupy]

@HideakiImamura Thanks for the reply. Got it!