Enable propagation of the library log outputs.

Please disable the Optuna’s default handler to prevent double logging if the root logger has been configured.


Propagate all log output to the root logger in order to save them to the file.

import optuna
import logging

logger = logging.getLogger()

logger.setLevel(logging.INFO)  # Setup the root logger.
logger.addHandler(logging.FileHandler("foo.log", mode="w"))

optuna.logging.enable_propagation()  # Propagate logs to the root logger.
optuna.logging.disable_default_handler()  # Stop showing logs in sys.stderr.

study = optuna.create_study()

logger.info("Start optimization.")
study.optimize(objective, n_trials=10)

with open("foo.log") as f:
    assert f.readline().startswith("A new study created")
    assert f.readline() == "Start optimization.\n"
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