Flask logging - Cannot get it to write to a file

Ok, here's the code where I setup everything:

if __name__ == '__main__':
    app.debug = False

    applogger = app.logger

    file_handler = FileHandler("error.log")
    file_handler.setLevel(logging.DEBUG)

    applogger.setLevel(logging.DEBUG)
    applogger.addHandler(file_handler)

    app.run(host='0.0.0.0')

What happens is

1. error.log gets created
2. Nothing is ever written to it
3. Despite not adding a StreamHandler and setting debug to false I still get everything to STDOUT (this might be correct, but still seems weird)

Am I totally off here somewhere or what is happening?

Why not do it like this:

if __name__ == '__main__':
    init_db()  # or whatever you need to do

    import logging
    logging.basicConfig(filename='error.log',level=logging.DEBUG)

    app.run(host="0.0.0.0")

If you now start you application, you'll see that error.log contains:

INFO:werkzeug: * Running on http://0.0.0.0:5000/

For more info, visit http://docs.python.org/2/howto/logging.html

Okay, as you insist that you cannot have two handler with the method I showed you, I'll add an example that makes this quite clear. First, add this logging code to your main:

import logging, logging.config, yaml
logging.config.dictConfig(yaml.load(open('logging.conf')))

Now also add some debug code, so that we see that our setup works:

logfile    = logging.getLogger('file')
logconsole = logging.getLogger('console')
logfile.debug("Debug FILE")
logconsole.debug("Debug CONSOLE")

All what is left is the "logging.conf" program. Let's use that:

version: 1
formatters:
  hiformat:
    format: 'HI %(asctime)s - %(name)s - %(levelname)s - %(message)s'
  simple:
    format: '%(asctime)s - %(name)s - %(levelname)s - %(message)s'
handlers:
  console:
    class: logging.StreamHandler
    level: DEBUG
    formatter: hiformat
    stream: ext://sys.stdout
  file:
    class: logging.FileHandler
    level: DEBUG
    formatter: simple
    filename: errors.log
loggers:
  console:
    level: DEBUG
    handlers: [console]
    propagate: no
  file:
    level: DEBUG
    handlers: [file]
    propagate: no
root:
  level: DEBUG
  handlers: [console,file]

This config is more complicated than needed, but it also shows some features of the logging module.

Now, when we run our application, we see this output (werkzeug- and console-logger):

HI 2013-07-22 16:36:13,475 - console - DEBUG - Debug CONSOLE
HI 2013-07-22 16:36:13,477 - werkzeug - INFO -  * Running on http://0.0.0.0:5000/

Also note that the custom formatter with the "HI" was used.

Now look at the "errors.log" file. It contains:

2013-07-22 16:36:13,475 - file - DEBUG - Debug FILE
2013-07-22 16:36:13,477 - werkzeug - INFO -  * Running on http://0.0.0.0:5000/

Ok, my failure stemmed from two misconceptions:

1) Flask will apparently ignore all your custom logging unless it is running in production mode

2) debug=False is not enough to let it run in production mode. You have to wrap the app in any sort of WSGI server to do so

After i started the app from gevent's WSGI server (and moving logging initialization to a more appropriate place) everything seems to work fine

The output you see in the console of your app is from the underlying Werkzeug logger that can be accessed through logging.getLogger('werkzeug').

Your logging can function in both development and release by also adding handlers to that logger as well as the Flask one.

More information and example code: Write Flask Requests to an Access Log.