Python Logging - Format a message without printing it

I came across an odd python problem the other day. I wanted to format a log message without printing it with python’s logging module. But I couldn’t find any examples or others who wanted to do the same thing.

You might ask why I would want to do this. In my case, I was using sys.stdout.write() to display the progress of a download as a percentage. Python’s logging module automatically appends a newline to your message and there didn’t seem to be a way to change this without a subclass. Anytime you want to overwrite something on the previous line with python logging, you will run into trouble.

Luckily after a quick look at the source, there’s a relatively simple solution to the problem:

def format_message(log, level, msg, *args):
    fn, lno, func = log.findCaller()
    record = log.makeRecord(, level, fn, lno, msg, args, False, func=func)
    return log.handlers[0].format(record)

log = logging.getLogger('MyCustomLogger')
message = format_message(log, logging.INFO, '%s is happening', 'Something')

# Now you can do stuff like this with the proper formatting
for i in range(0,101):
    percent = '(%0.0f%%)' % i 
    sys.stdout.write(' %s%s' % (percent, '\b' * (len(percent) + 1)))

Depending on your logger’s formatter, the output might look like:

MyCustomLogger INFO | Something is happening (100%)

Be careful, this solution will only format the message based on the first handler in the handlers list. If there are no handlers or multiple handlers, you’ll probably want some additional logic to take care of this.


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