I'm trying to add logging to a web application which uses Flask.

When hosted using the built-in server (i.e. python3 server.py), logging works. When hosted using Gunicorn, the log file is not created.

The simplest code which reproduces the problem is this one:

#!/usr/bin/env python

import logging
from flask import Flask
flaskApp = Flask(__name__)


@flaskApp.route('/')
def index():
    flaskApp.logger.info('Log message')
    print('Direct output')
    return 'Hello World\n'


if __name__ == "__main__":
    logHandler = logging.FileHandler('/var/log/demo/app.log')
    logHandler.setLevel(logging.INFO)
    flaskApp.logger.addHandler(logHandler)
    flaskApp.logger.setLevel(logging.INFO)
    flaskApp.run()

The application is called using:

gunicorn server:flaskApp -b :80 -w 4
    --access-gfile /var/log/demo/access.log
    --error-logfile /var/log/demo/error.log

When doing a request to the home page of the site, the following happens:

1. I receive the expected HTTP 200 "Hello World\n" in response.

2. There is a trace of the request in /var/log/demo/access.log.

3. /var/log/demo/error.log stays the same (there are just the boot events).

4. There is the "Direct output" line in the terminal.

5. There is no '/var/log/demo/app.log'. If I create the file prior to launching the application, the file is not modified.

Note that:

• The directory /var/log/demo can be accessed (read, write, execute) by everyone, so this is not the permissions issue.

• If I add StreamHandler as a second handler, there is still no trace of the "Log message" message neither in the terminal, nor in Gunicorn log files.

• Gunicorn is installed using pip3 install gunicorn, so there shouldn't be any mismatch with Python versions.

What's happening?

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?