Why is Flask application not creating any logs when hosted by Gunicorn?

This approach works for me: Import the Python logging module and add gunicorn's error handlers to it. Then your logger will log into the gunicorn error log file:

import loggingapp = Flask(__name__)gunicorn_error_logger = logging.getLogger('gunicorn.error')app.logger.handlers.extend(gunicorn_error_logger.handlers)app.logger.setLevel(logging.DEBUG)app.logger.debug('this will show in the log')

My Gunicorn startup script is configured to output log entries to a file like so:

gunicorn main:app \    --workers 4 \    --bind 0.0.0.0:9000 \    --log-file /app/logs/gunicorn.log \    --log-level DEBUG \    --reload

When you use python3 server.py you are running the server3.py script.

When you use gunicorn server:flaskApp ... you are running the gunicorn startup script which then imports the module server and looks for the variable flaskApp in that module.

Since server.py is being imported the __name__ var will contain "server", not "__main__" and therefore you log handler setup code is not being run.

You could simply move the log handler setup code outside of the if __name__ == "__main__": stanza. But ensure that you keep flaskApp.run() in there since you do not want that to be run when gunicorn imports server.

More about what does if __name__ == “__main__”: do?

There are a couple of reasons behind this: Gunicorn has its own loggers, and it’s controlling log level through that mechanism. A fix for this would be to add app.logger.setLevel(logging.DEBUG).
But what’s the problem with this approach? Well, first off, that’s hard-coded into the application itself. Yes, we could refactor that out into an environment variable, but then we have two different log levels: one for the Flask application, but a totally separate one for Gunicorn, which is set through the --log-level parameter (values like “debug”, “info”, “warning”, “error”, and “critical”).

A great solution to solve this problem is the following snippet:

import loggingfrom flask import Flask, jsonifyapp = Flask(__name__)@app.route('/')def default_route():    """Default route"""    app.logger.debug('this is a DEBUG message')    app.logger.info('this is an INFO message')    app.logger.warning('this is a WARNING message')    app.logger.error('this is an ERROR message')    app.logger.critical('this is a CRITICAL message')    return jsonify('hello world')if __name__ == '__main__':    app.run(host=0.0.0.0, port=8000, debug=True)else:    gunicorn_logger = logging.getLogger('gunicorn.error')    app.logger.handlers = gunicorn_logger.handlers    app.logger.setLevel(gunicorn_logger.level)

Refrence: Code and Explanation is taken from here