Using Loguru for Simplified and Powerful Python Logging

Installation

Install Loguru via pip for Python 3: pip3 install loguru .

Basic Usage

Import the pre‑instantiated logger and call its level methods directly:

<code>from loguru import logger

logger.debug('This is debug information')
logger.info('This is info information')
logger.warning('This is warn information')
logger.error('This is error information')</code>

Loguru automatically adds timestamps, level, module name, line number, and colors the output.

Loguru File Configuration

Use logger.add() to add a file sink with optional rotation, retention, encoding, and queueing:

<code>from loguru import logger
logger.add("E:/PythonCode/MOC/log_2021-3-28.log", rotation="500MB", encoding="utf-8", enqueue=True, retention="10 days")
logger.info('This is info information')</code>

String Formatting

Loguru supports f‑string style formatting with keyword arguments:

<code>import platform
from loguru import logger
logger.info('If you are using Python {version}, prefer {feature} of course!', version=platform.python_version(), feature='f-strings')</code>

Common Parameters

sink : file path, file‑like object, coroutine, or logging.Handler.

level : minimum severity to record.

format : template for log messages.

filter : callable, string or dict to decide which records are emitted.

colorize : enable/disable ANSI colors.

serialize : output logs as JSON strings.

backtrace : extend traceback beyond the catch point.

diagnose : show variable values in tracebacks.

enqueue : send logs through a multiprocessing‑safe queue.

catch : automatically catch exceptions in the sink.

Stopping Logging to a File

Remove a sink by its identifier returned from add() :

<code>from loguru import logger
trace = logger.add('2021-3-28.log')
logger.error('This is error information')
logger.remove(trace)
logger.warning('This is warn information')</code>

Output Only to File (No Console)

Remove the default handler and add a file sink:

<code>from loguru import logger
logger.remove(handler_id=None)
trace = logger.add('2021-3-28.log')
logger.error('This is error information')
logger.warning('This is warn information')</code>

Filter Configuration

Define a filter function to keep only ERROR level logs:

<code>from loguru import logger

def error_only(record):
    """Return True only for ERROR level records"""
    return record["level"].name == "ERROR"

logger.add('2021-3-28.log', filter=error_only)
logger.error('This is error information')
logger.warning('This is warn information')</code>

Format Template

Customise the log line format:

<code>from loguru import logger

def format_log():
    trace = logger.add('2021-3-28.log', format="{time:YYYY-MM-DD HH:mm:ss} {level} From {module}.{function} : {message}")
    logger.warning('This is warn information')

if __name__ == '__main__':
    format_log()
</code>

Extra Bind for Structured Logs

Attach additional fields to each record:

<code>from loguru import logger

def format_log():
    trace = logger.add('2021-3-28.log', format="{time:YYYY-MM-DD HH:mm:ss} {extra[ip]}  {extra[username]} {level} From {module}.{function} : {message}")
    extra_logger = logger.bind(ip='192.168.0.1', username='张三')
    extra_logger.info('This is info information')
    extra_logger.bind(username='李四').error('This is error information')
    extra_logger.warning('This is warn information')

if __name__ == '__main__':
    format_log()
</code>

Level Configuration

<code>from loguru import logger
trace = logger.add('2021-3-29.log', level='ERROR')
</code>

Rotation

Rotate logs by size, time of day, or interval:

<code>from loguru import logger
logger.add('2021-3-28.log', rotation='200 MB')
logger.add('2021-3-28.log', rotation='06:00')
logger.add('2021-3-28.log', rotation='2 week')
</code>

Retention

Automatically delete logs older than a given period:

<code>from loguru import logger
logger.add('2021-3-28.log', retention='7 days')
</code>

Compression

Compress closed log files (e.g., zip):

<code>from loguru import logger
logger.add('2021-3-28.log', compression='zip')
</code>

Serialization

Write each log entry as a JSON line for downstream analysis:

<code>from loguru import logger
import platform
rounded_value = round(0.345, 2)
trace = logger.add('2021-3-28.log', serialize=True)
logger.info('If you are using Python {version}, prefer {feature} of course!', version=platform.python_version(), feature='f-strings')
</code>

Traceback (Exception Tracing)

Loguru integrates better_exceptions to provide rich tracebacks. Example using the @logger.catch decorator:

<code>from loguru import logger

trace = logger.add('2021-3-28.log')

@logger.catch
def index_error(custom_list: list):
    for index in range(len(custom_list)):
        index_value = custom_list[index]
        if custom_list[index] < 2:
            custom_list.remove(index_value)
        print(index_value)

if __name__ == '__main__':
    index_error([1,2,3])
</code>

Exception Logging with logger.exception

<code>from loguru import logger
trace = logger.add('2021-3-28.log')

def index_error(custom_list: list):
    for index in range(len(custom_list)):
        try:
            index_value = custom_list[index]
        except IndexError as err:
            logger.exception(err)
            break
        if custom_list[index] < 2:
            custom_list.remove(index_value)

if __name__ == '__main__':
    index_error([1,2,3])
</code>

Class and Static Method Examples

<code>from loguru import logger

trace = logger.add('2021-3-28.log')

class Demo:
    @logger.catch
    def index_error(self, custom_list: list):
        for index in range(len(custom_list)):
            index_value = custom_list[index]
            if custom_list[index] < 2:
                custom_list.remove(index_value)

    @staticmethod
    @logger.catch
    def index_error_static(custom_list: list):
        for index in range(len(custom_list)):
            index_value = custom_list[index]
            if custom_list[index] < 2:
                custom_list.remove(index_value)

if __name__ == '__main__':
    # Demo().index_error([1, 2, 3])
    Demo.index_error_static([1, 2, 3])
</code>

Level Name

Severity Value

Logger Method

TRACE

5

logger.trace()

DEBUG

10

logger.debug()

INFO

20

logger.info()

SUCCESS

25

logger.success()

WARNING

30

logger.warning()

ERROR

40

logger.error()

CRITICAL

50

logger.critical()

For a full list of Loguru configuration keys and their meanings, refer to the documentation table included above.