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Flask logging example
A warning occurred (42 apples)
An error occurred
import logging
from logging.handlers import RotatingFileHandler
from flask import Flask
app = Flask(__name__)
def foo():
app.logger.warning('A warning occurred (%d apples)', 42)
app.logger.error('An error occurred')'Info')
return "foo"
if __name__ == '__main__':
handler = RotatingFileHandler('foo.log', maxBytes=10000, backupCount=1)

Thanks for the tip

Thanks dude

Lovely my friend! Thanks a lot for putting up this code chunk!

webus commented May 11, 2013


umurgdk commented Aug 18, 2013

Thanks its very useful :) Does flask provides a default logger or we have to provide one for it like RotatingFileHandler?

vkotovv commented Sep 20, 2013

You can add this:
handler.setLevel(logging.ERROR) formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
to include time and other info in the log item.

rkrzr commented Nov 8, 2013

If you want line numbers as well in your logs, you can use this formatter:

    formatter = logging.Formatter(
        "[%(asctime)s] {%(pathname)s:%(lineno)d} %(levelname)s - %(message)s")
    handler = RotatingFileHandler(LOG_FILENAME, maxBytes=10000000, backupCount=5)

If you also want to see the log messages emitted by Werkzeug in your log file, you can add the following:

    log = logging.getLogger('werkzeug')

thanks :)

Thank you

Thanks !

Prawyn commented Jun 13, 2014


icron commented Dec 27, 2014


ElMijo commented Jan 7, 2015

Muchas gracias..

KCzar commented Feb 20, 2015

I had to setLevel on app.logger as well to make it log from"...") and app.logger.debug("..."). If I only set the level of the handler, it would only log warn, error, and critical messages.

Here's what worked for me:

handler = TimedRotatingFileHandler('logs/foo.log', when='midnight', interval=1)

Thx~ It help me

Where will be the .log files be saved??

louking commented Mar 27, 2015

Thanks @KCzar -- I was having a devil of a time figuring out why my info messages were not being logged, setting level on app.logger fixed that problem.

It's very useful, thank you!

Thanks, useful code . It will be better if uniform field separator is used, this allows for log processing by automation tools . Eg
formatter = logging.Formatter( "%(asctime)s | %(pathname)s:%(lineno)d | %(funcName)s | %(levelname)s | %(message)s ")

the reason for set log level in app.logger in production is that app.logger set level of app.logger to logging.NOTSET(i.e. 0), and if app.debug is True then all log level above DEBUG will be logged to stderr(StreamHandler). However, if app.debuug is set to False then all the log level above logging.root.level(i.e. logging.WARNING). Check out the code about Flask file you will find what the problem is. Here is the link

Schlik commented May 22, 2015


wujiwh commented Jul 15, 2015

cool !!! very useful, Thx~

swdream commented Jul 21, 2015

When I follow above example:
handler = RotatingFileHandler(maxBytes=10000, backupCount=1)

I got below error:

Traceback (most recent call last):
File "", line 33, in
handler = RotatingFileHandler(maxBytes=10000, backupCount=1)
TypeError: init() takes at least 2 arguments (3 given)

Can you help me to explain about this? thank you very much.

From the example in the beginning try this -

handler = RotatingFileHandler('foo.log', maxBytes=10000, backupCount=1)

great! thanks for this nice little snippet that works

vi3k6i5 commented Oct 15, 2015

How do i pass it to the next file ??

say i have another file

and it has a method

def bar_method():
    print "in bar"

bar_method is called by foo.
I want to use same logging style here. how to do that?

ringxw commented Nov 4, 2015

Thank you to both @ibeex and @rkrzr !

Marabou commented Nov 6, 2015

Great!! :)

rlam3 commented Nov 10, 2015

You have to set the handler to a level lower than the logger record or else it will not emit the message


weaming commented Nov 27, 2015


zws2lll commented Jan 28, 2016


info log cannot be record, we should set


Tallone commented May 24, 2016

i finally could not use print ~~

tOlorun commented Jul 8, 2016

hello everyone

Please is it possible to put this in a function where you can pass the log file to when called

like so

def logga(log_file): handler = TimedRotatingFileHandler('logs/%s.log' % log_file, when='midnight', interval=1) handler.setLevel(logging.DEBUG) formatter = logging.Formatter( "[%(asctime)s] {%(pathname)s:%(lineno)d} | %(funcName)s | %(levelname)s - %(message)s") handler.setFormatter(formatter) loga = logging.getLogger('werkzeug') loga.setLevel(logging.DEBUG) loga.addHandler(handler) app.logger.addHandler(handler)

The above does not currently work . ..
thanks in advance

Great and to the point, just what I needed. Thank you!

This gist seems to print 4 lines for every call to log, e.g.

INFO in app [/usr/src/app/]:
successful query

How do you get it to print on just one line?

sanfx commented Sep 5, 2017

Hi All,

I am using logging in various python apps, your post logging for flask speciically was helpful, however is there a web based log viewer, that I can just login and view logs if any generated by my app ?

oakbani commented Nov 30, 2017

Thanks a lot


narakus commented Jan 19, 2018


Very useful, thank you! Some other logging handlers are here:

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