How to print multiline logs using python logging module?

Question

I want to create multiline logger using python logging module. When I use the code snippet below to format logger:

import logging
logger = logging.getLogger(file_name)
logger.setLevel(logging.DEBUG)
ch = logging.StreamHandler()
ch.setLevel(logging.DEBUG)
formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')
ch.setFormatter(formatter)
logger.addHandler(ch)

I get the output below for multiline logs when I use logger.info("""line 1\nline 2\n line 3""")

2017-07-20 13:21:14,754 - my_logger.py - INFO - line 1
line 2
line 3

I want my output as below:

2017-07-20 13:21:14,754 - my_logger.py - INFO - line 1
2017-07-20 13:21:14,754 - my_logger.py - INFO - line 2
2017-07-20 13:21:14,754 - my_logger.py - INFO - line 3

Is there a reason you can't have multiple calls to logger.info()? — jkr
– jkr, Commented Jul 20, 2017 at 13:57
@Jakub i want to create a logger for a framework and want to have this kind of formatter. — pankmish
– pankmish, Commented Jul 20, 2017 at 14:00

Qeek · Accepted Answer · 2017-07-20 14:32:05Z

6

You can create your own formatter:

import logging
class MultilineFormatter(logging.Formatter):
    def format(self, record: logging.LogRecord):
        save_msg = record.msg
        output = ""
        for line in save_msg.splitlines():
            record.msg = line
            output += super().format(record) + "\n"
        record.msg = save_msg
        record.message = output
        return output

Than modify your logger initialization as follows:

formatter = MultilineFormatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')

answered Jul 20, 2017 at 14:32

Qeek

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2 Comments

studioj Over a year ago

warning over here! test_logger.info("foo %s \nfoo %s", "bar", "bar") if you do multi line % replacement this wont work. I'll look for an extension to this answer

studioj Over a year ago

added mine below

ssm · Accepted Answer · 2017-07-20 14:27:05Z

2

Short answer, you cannot. What you can do is the following:

logger.info('line 1')
logger.info('line 2')
logger.info('line 3')

Or if you have multiline stuff, you can do:

map(logger.info, 'line 1\nline 2\nline 3'.split('\n'))

These won't show up at the same time, but will be very close ...

answered Jul 20, 2017 at 14:27

ssm

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1 Comment

Kyuuhachi Over a year ago

Do note that the latter only works on py2 due to map being lazy; you can do [logger.info(line) for line in 'line 1\nline 2\nline 3'.splitlines()] on py3.

studioj · Accepted Answer · 2021-08-31 09:28:18Z

adjusting @Qeek 's answer so we can support % replacement in all lines

test_logger.info("foo %s foo %s \nfoo %s", "bar","bar", "bar")

taking into account these characters which can be added to % diouxXeEfFgGcrsa

https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting

import logging
import re

class MultilineFormatter(logging.Formatter):
    def format(self, record: logging.LogRecord):
        save_msg = record.msg
        args = list(record.args)
        output = ""
        for line in save_msg.splitlines():
            record.msg = line
            number_of_string_subs = len(re.findall(r"%[diouxXeEfFgGcrsa]", line))
            record.args = tuple(args[:number_of_string_subs])
            args = args[number_of_string_subs:]
            output += super().format(record) + "\n"
        record.msg = save_msg
        record.message = output
        return output

Than modify your logger initialization as follows:

formatter = MultilineFormatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')

macetw · Accepted Answer · 2020-05-26 15:10:19Z

If I am only using a single log level (warning or info or similar), I would prefer using a helper function to do this:

def multiline_warning(header, body):
  for line in body.splitlines():
    logging.warning(f"{header}: {line}")

And in the context, just call:

multiline_warning("stderr", subprocess_result.stderr)

If using more than just one of (CRITICAL, ERROR, WARNING, INFO or DEBUG), you might abstract the log-level and pass it as a parameter, and use logging.log instead.

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