Logging Fundamentals

Logging provides a mechanism to track software runtime events. Developers instrument their code with logging calls to capture system activities. Each event consists of a descriptive message that may include variable data and is assigned a severity level.

Log Severity Levels

Level	Usage
DEBUG	Detailed diagnostic information for troubleshooting
INFO	Confirmation of expected operation
WARNING	Unexpected but non-critical conditions
ERROR	Functional impairment in specific features
CRITICAL	System failure preventing continued operation

Log levels follow an increasing severity hierarchy where higher levels represent more critical events with less verbose output.

Basic Logging Implementation

import logging

logging.debug("Detailed trace information")
logging.info("Operation confirmation")
logging.warning("Non-critical issue detected")
logging.error("Functional failure occurred")
logging.critical("System failure condition")

"""
Output:
WARNING:root:Non-critical issue detected
ERROR:root:Functional failure occurred
CRITICAL:root:System failure condition
"""

Log Formatting Attributes

Attribute	Format	Description
timestamp	%(asctime)s	Event occurrence time
level	%(levelname)s	Severity designation
source	%(filename)s	Origin script name
position	%(lineno)d	Code line reference
message	%(message)s	Descriptive content

Structured Logging Implementation

import logging

# Initialize logger
app_logger = logging.getLogger('application.core')
app_logger.setLevel(logging.DEBUG)

# Configure output handlers
console_output = logging.StreamHandler()
file_output = logging.FileHandler('app_activity.log', mode='w')

# Define message format
log_format = logging.Formatter('%(asctime)s|%(levelname)8s|%(filename)s[:%(lineno)d]|%(message)s')

# Apply formatting
console_output.setFormatter(log_format)
file_output.setFormatter(log_format)

# Register handlers
app_logger.addHandler(console_output)
app_logger.addHandler(file_output)

# Generate log events
user = 'admin'; id = 42
app_logger.debug('User %s initiated session, ID %d', user, id)
app_logger.info('Configuration loaded for %s', user)
app_logger.warning('Resource threshold exceeded: %d%%', 85)

Configuration File Implementation

# app_log.conf
[loggers]
keys=root,app

[handlers]
keys=file_handler,console_handler

[formatters]
keys=standard_format

[logger_root]
level=INFO
handlers=console_handler

[logger_app]
level=DEBUG
handlers=file_handler,console_handler
qualname=app
propagate=0

[handler_console_handler]
class=StreamHandler
args=(sys.stdout,)
level=INFO
formatter=standard_format

[handler_file_handler]
class=handlers.TimedRotatingFileHandler
args=('app.log','midnight',1,0)
level=DEBUG
formatter=standard_format

[formatter_standard_format]
format=%(asctime)s|%(levelname)8s|%(filename)s[:%(lineno)d]|%(message)s
datefmt=%Y-%m-%d %H:%M:%S

Configuration Loading

import logging
import logging.config

logging.config.fileConfig('app_log.conf')

system_log = logging.getLogger()
app_log = logging.getLogger('app')

app_log.debug('Debugging component initialization')
app_log.info('Application started')

try:
    result = 10 / 0
except Exception as e:
    app_log.exception('Calculation error: %s', e)