Question

I am running a web server, tornado, and I am trying to redirect all the log output to a file using the following command. But I don’t see the output in the file.

/usr/bin/python -u index.py 2>&1 >> /tmp/tornado.log

I pass -u option to python interpreter. I still don’t see any output logged to my log file.

However, I see the output on stdout when I do the following

/usr/bin/python index.py

Answer 1

Tornado uses the built-in logging module. You can easily attach a file handler to the root logger and set its level to NOTSET so it records everything, or some other level if you want to filter.

Reference docs: logging, logging.handlers

Example that works with Tornado’s logging:

import logging
# the root logger is created upon the first import of the logging module

# create a file handler to add to the root logger
filehandler = logging.FileHandler(
    filename = 'test.log', 
    mode = 'a', 
    encoding = None, 
    delay = False
)

# set the file handler's level to your desired logging level, e.g. INFO
filehandler.setLevel(logging.INFO)

# create a formatter for the file handler
formatter = logging.Formatter('%(asctime)s.%(msecs)d  [%(name)s](%(process)d): %(levelname)s: %(message)s')

# add filters if you want your handler to only handle events from specific loggers 
# e.g. "main.sub.classb" or something like that. I'll leave this commented out.
# filehandler.addFilter(logging.Filter(name='root.child'))

# set the root logger's level to be at most as high as your handler's
if logging.root.level > filehandler.level:
    logging.root.setLevel = filehandler.level

# finally, add the handler to the root. after you do this, the root logger will write
# records to file.
logging.root.addHandler(filehandler)

More often than not, I actually wish to suppress tornado’s loggers (because I have my own, and catch their exceptions anyway, and they just end up polluting my logs,) and this is where adding a filter on your filehandlers can come in very handy.