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:
I receive the expected HTTP 200 "Hello World\n" in response.
There is a trace of the request in /var/log/demo/access.log
.
/var/log/demo/error.log
stays the same (there are just the boot events).
There is the "Direct output" line in the terminal.
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?