Flask logging not working at all

Question

I'm trying to log messages in Flask both to file and to stdout. I've been reading the official Flask docs and came up with this:

from flask import Flask
import logging
from logging import Formatter, FileHandler

app = Flask(__name__)



@app.route('/')
def hello_world():
    app.logger.debug('second test message...')
    return 'Hello World!'


if __name__ == '__main__':
    #Setup the logger
    file_handler = FileHandler('output.log')
    handler = logging.StreamHandler()
    file_handler.setLevel(logging.DEBUG)
    handler.setLevel(logging.DEBUG)
    file_handler.setFormatter(Formatter(
        '%(asctime)s %(levelname)s: %(message)s '
        '[in %(pathname)s:%(lineno)d]'
     ))
     handler.setFormatter(Formatter(
        '%(asctime)s %(levelname)s: %(message)s '
        '[in %(pathname)s:%(lineno)d]'
     ))
     app.logger.addHandler(handler)
     app.logger.addHandler(file_handler)
     app.logger.error('first test message...')
     app.run()

There are several problems:

1. No output.log file is generated

2. Only the first logging message works:

   app.logger.error('testing...')

And only in stdout...the one in the view "/" does not even print to stdout... am I doing something wrong?

This is the output from starting the app and going to /:

2015-03-08 11:33:27,183 ERROR: first test message... [in /home/mosquito/python_projects/flask_tst/flask_tst.py:31]
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
127.0.0.1 - - [08/Mar/2015 11:33:43] "GET / HTTP/1.1" 200 -

Solution 1

Your (debug) logging messages are getting suppressed by Flask as you're not running in debug mode. If you set the following flag to True, your code will work.

    app.run(debug=True)

The messages will now appear as expected.

BennyE$ python3 stackoverflow.py 
2015-03-08 12:04:04,650 ERROR: firs test message... [in stackoverflow.py:31]
 * Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)
 * Restarting with stat
2015-03-08 12:04:04,807 ERROR: firs test message... [in stackoverflow.py:31]
--------------------------------------------------------------------------------
DEBUG in stackoverflow [stackoverflow.py:11]:
second test message...
--------------------------------------------------------------------------------
2015-03-08 12:04:13,789 DEBUG: second test message... [in stackoverflow.py:11]
192.168.178.23 - - [08/Mar/2015 12:04:13] "GET / HTTP/1.1" 200 -
--------------------------------------------------------------------------------
DEBUG in stackoverflow [stackoverflow.py:11]:
second test message...
--------------------------------------------------------------------------------
2015-03-08 12:04:14,899 DEBUG: second test message... [in stackoverflow.py:11]
192.168.178.23 - - [08/Mar/2015 12:04:14] "GET / HTTP/1.1" 200 -

This is the output in the associated output file:

BennyE$ cat output.log 
2015-03-08 11:58:22,226 ERROR: firs test message... [in stackoverflow.py:31]
2015-03-08 12:04:04,650 ERROR: firs test message... [in stackoverflow.py:31]
2015-03-08 12:04:04,807 ERROR: firs test message... [in stackoverflow.py:31]
2015-03-08 12:04:13,789 DEBUG: second test message... [in stackoverflow.py:11]
2015-03-08 12:04:14,899 DEBUG: second test message... [in stackoverflow.py:11]

Solution 2

I had the same issue and following worked for me:

app.logger.setLevel(logging.INFO)

Solution 3

Thansk BennyE_HH, it works.

But Flask didn't suppressed ERROR level log message even debug mode is disable(default is disable).

I think we should call app.logger.setLevel(logging.DEBUG) to control log level even debug mode is false.

Comments

[Klaus D.]

Does the process have write permissions to the log file?

[AlejandroVK]

Hello Klaus, yes, process has write permissions at project root folder

[aGuegu]

I got the same problem, even if the loglevel is set to info, all I got is error log. And in production mode, flask is run by uwsgi.

[Amin Alaee]

From the Flask Docs: "Flask will suppress any server error with a generic error page unless it is in debug mode. As such to enable just the interactive debugger without the code reloading, you have to invoke run() with debug=True and use_reloader=False. Setting use_debugger to True without being in debug mode won’t catch any exceptions because there won’t be any to catch."

[Doogle]

Thanks for providing input on this. Really help resolve the issue(Though ignorance/blindspot) faster.

[Maximilian Machedon]

New link to the documentation about run (see Armin's comment) : flask.palletsprojects.com/en/1.1.x/api/#flask.Flask.run

[stenci]

This answer is wrong. The correct answer is below, from kashaziz