why exceptions are raised without logged?

[Dongzhenpu]

I've encountered a problem that my alembic_version table's data was deleted, but there are migrations history in migrations/versions directory. So when I execute flask db migrate there will be an exception which is Target database is not up to date, in api.py.
But by default, alembic will raise exception without logging it, so what I saw was

INFO  [alembic.runtime.migration] Context impl MySQLImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.

without an ERROR msg, after seen the source code, I'm curious why alembic only raise error without logging it?

[zzzeek]

logging of errors using log.error() is intended to be performed when exceptions are caught, not when they are thrown. that's why Python logging includes an exc_info option; exceptions have a lot more detail when they are caught rather than thrown. Therefore if the flask-alembic module is using the Alembic API, catching an exception, and then squashing it without it being logged, that's a bug in that application and you should report it over there.

[Dongzhenpu]

Many thanks! @zzzeek
I've found it's my mistake that I didn't use the latest version of flask_migrate, the problem was the same as here miguelgrinberg/Flask-Migrate#384.
The old version has a bug as this comment.
Thanks again for your explanation, make me I understand why exceptions are thrown without being logged here.