How to format the name of the logging level

Question

When I start IPython, I see logs like this:

[I 17:03:59.993 NotebookApp] Using MathJax from CDN:       https://cdn.mathjax.org/mathjax/latest/MathJax.js
[W 17:04:00.292 NotebookApp] Terminals not available (error was No    module named terminado)
[I 17:04:00.293 NotebookApp] Serving notebooks from local directory: /home/oleg
[I 17:04:00.293 NotebookApp] 0 active kernels 
[I 17:04:00.293 NotebookApp] The IPython Notebook is running at: http://localhost:8888/
[I 17:04:00.293 NotebookApp] Use Control-C to stop this server and shut down all kernels (twice to skip confirmation).

Here the levels of messages are slightly formatted, that is, you see I instead of INFO, W instead of WARNING etc. In addition, the brackets are colored. I find this cool and would like to write my logs that way too. However, IPython uses tornado logging system.

I use the colorlog module to color the messages. In order to do the formatting, I subclassed the StreamHandler class, as described here: How to get the level of the logging record in a custom logging.Handler in Python?

class FormatLevelHandler(logging.StreamHandler):
    def emit(self, record):
        record.levelname = record.levelname[0]
        logging.StreamHandler.emit(self, record)

But when I do this, coloring does not work anymore.

Is there a way to have both logging level name formatting and coloring? Here is my full code:

import logging
from colorlog import ColoredFormatter
formatter = ColoredFormatter(
    "%(log_color)s[%(levelname)1s %(asctime)s] %(reset)s %(blue)s%(message)s",
    datefmt=None,
    reset=True,
    log_colors={
        'DEBUG':    'cyan',
        'INFO':     'green',
        'WARNING':  'yellow',
        'ERROR':    'red',
        'CRITICAL': 'red,bg_white',
    },
    secondary_log_colors={},
    style='%'
)
logger = logging.getLogger(__name__)

class FormatLevelHandler(logging.StreamHandler):
    def emit(self, record):
        record.levelname = record.levelname[0]
        logging.StreamHandler.emit(self, record)

ch = FormatLevelHandler()
ch.setFormatter(formatter)
logger.addHandler(ch)
logger.setLevel(logging.DEBUG)
logger.info('Hello')
logger.debug('Hi')

Answer 1

Do not change the level in handler.emit(). Instead, truncate the level in the format string itself using %(levelname)1.1s (not %(levelname)1s as in your example).

Or you can use the Tornado logging system whether you use the rest of Tornado or not: just call tornado.log.enable_pretty_logging() at the start of your program.