Flask logging with Foreman

Question

I'm trying to set up a Heroku-ready Flask app, but I can't figure out how to turn on logging.

Without Foreman, I could create a helloworld app as described in the Flask tutorial :

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    app.logger.debug('A value for debugging')
    app.logger.warning('A value for warning')
    return "Hello World!"

if __name__ == "__main__":
    app.run(debug=True)

start it like so:

python hello.py

and have logging in stdout.

When I follow the Heroku tutorial , however, there's no app.run line:

import os
from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello():
    app.logger.debug('A value for debugging')
    app.logger.warning('A value for warning')
    return 'Hello World!'

And so I can't figure out how to run in debug mode and/or get logging output:

foreman start -p 5000

Procfile:

web: gunicorn hello:app

Answer 1

The default logging configuration for Flask apps is different in debug vs. production mode.

In your first example you are in debug mode. In this case Flask defines a logging handlers that logs all messages with level logging.DEBUG or higher to stderr .

The second example is not in debug mode. When debug mode is not enabled Flask creates a logger object but does not add any handlers to it, so nothing is printed.

For Foreman and Heroku you need logs to be sent to stdout or stderr , so all you need to do is add a StreamHandler with the logging level of your choice:

import os
from flask import Flask

app = Flask(__name__)

# log to stderr
import logging
from logging import StreamHandler
file_handler = StreamHandler()
app.logger.setLevel(logging.DEBUG)  # set the desired logging level here
app.logger.addHandler(file_handler)

@app.route('/')
def hello():
    app.logger.debug('A value for debugging')
    app.logger.warning('A value for warning')
    return 'Hello World!'

Alternatively, if you prefer you can do none of this and just enable debug mode for the Foreman/Heroku controlled application, though this would not be something I'd recommend for a production app:

from flask import Flask
app = Flask(__name__)
app.debug = True