Flask logging example

foo.log

A warning occurred (42 apples)
An error occurred

server.py

import logging
from logging.handlers import RotatingFileHandler

from flask import Flask

app = Flask(__name__)

@app.route('/')
def foo():
    app.logger.warning('A warning occurred (%d apples)', 42)
    app.logger.error('An error occurred')
    app.logger.info('Info')
    return "foo"

if __name__ == '__main__':
    handler = RotatingFileHandler('foo.log', maxBytes=10000, backupCount=1)
    handler.setLevel(logging.INFO)
    app.logger.addHandler(handler)
    app.run()

Thinks

Very useful, thank you! Some other logging handlers are here: https://docs.python.org/2/library/logging.handlers.html

Very helpful, thank you!

Thank you for this gist. In addition if you only want to log to stdout do the following:

import sys
import logging
import flask

app = flask.Flask(__name__)

@app.before_first_request
def setup_logging():
    if not app.debug:
        # In production mode, add log handler to sys.stdout.
        app.logger.addHandler(logging.StreamHandler(stream=sys.stdout))
        app.logger.setLevel(logging.INFO)

This is necessary, because Flask does not log messages in production mode by default. To get Flask's app.logger to log messages, you'll always have to add a logging handler, even for simply logging to stdout.

see

• https://docs.python.org/3/library/logging.handlers.html#streamhandler
• benoitc/gunicorn#379 (comment)

It is not the case (at least anymore) that you'll always have to explicitly add a logging handler to get Flask's app.logger to log messages. According to http://flask.pocoo.org/docs/1.0/logging/, we can simply make a logging.basicConfig(...) call before creating the Flask app, like

import flask
import logging

logging.basicConfig(filename=LOG_FILENAME, format=LOG_FORMAT, level=LOG_LEVEL)

app = Flask(__name__)

This worked for me in a production environment on all log levels.

thanks much

I was using TimedRotatingFileHandler in my Flask app under Gunicorn with multiple workers. I ended up seeing logs from same time being written into different files (i.e one of the workers seem to be writing to a rotated file). Was trying to look thorough if thats the case with others and I see this Blog post:

https://www.packetmischief.ca/2017/10/25/3-ways-to-fail-at-logging-with-flask/?unapproved=27590&moderation-hash=51a1d5344643c9a0f5d47a253641ac53

I'm not sure if the issue is with me unable to get it working or this happens really

Thanks!It really helps me!

Thanks a lot!

is there a way to log errors (ex: timeout connection DB) inside gunicorn logs? Or how should I log errors in a flask app running in production with Gunicorn?