Flask app.logger messages missing from Gunicorn error logs

Answer

No. A warning emitted via app.logger.warning inside a Flask route will not appear in Gunicorn's error output unless you attach a logging handler to app.logger (or to an ancestor logger that Gunicorn configures).

Confirmed facts

• Flask's app.logger is a standard logging.Logger that, by default, has no handlers attached and its propagate flag is True.
• Gunicorn configures its own loggers (gunicorn.error and gunicorn.access) but does **not** attach any handlers to the root logger.
• Because the root logger has no handler, log records that only propagate to the root are discarded.

Steps to make the warning visible

1. Add a handler to the Flask logger before starting Gunicorn, e.g.:

   import logging
   from flask import Flask
   app = Flask(__name__)
   handler = logging.StreamHandler()  # writes to stderr, which Gunicorn captures
   app.logger.addHandler(handler)

2. Start Gunicorn as usual (e.g., gunicorn --log-level info -b 0.0.0.0:8000 myapp:app).
3. Trigger the route; the warning line will now appear in Gunicorn's error output.

When propagation matters

If you have already configured a handler on the root logger (for example, via Gunicorn's --access-logfile or --error-logfile options combined with a custom logging config), then leaving propagate=True would cause the message to appear via the root logger. In that case you should verify whether a root‑logger handler exists; if it does, you can keep propagation enabled, otherwise add a handler directly to app.logger as shown above.

**Missing diagnostic detail:** Do you have any custom logging configuration that attaches a handler to the root logger when running Gunicorn? Knowing this determines whether you need to add a handler to app.logger or can rely on propagation.