Python by example

Logging in Python

Logging is a way to track events that happen when your program runs. It’s much better than using print() statements because you can control the level of detail and where the log messages go.

Basic Logging

Python’s logging module provides a simple way to add logging to your programs:

import logging

logging.debug("This is a debug message")
logging.info("This is an info message")
logging.warning("This is a warning message")
logging.error("This is an error message")
logging.critical("This is a critical message")

# Output (only warning and above are shown by default):
# WARNING:root:This is a warning message
# ERROR:root:This is an error message
# CRITICAL:root:This is a critical message

Setting the Log Level

You can control which messages are displayed by setting the log level:

import logging

# Set the minimum level to INFO
logging.basicConfig(level=logging.INFO)

logging.debug("This won't be shown")
logging.info("This will be shown")
logging.warning("This will be shown")

# Output:
# INFO:root:This will be shown
# WARNING:root:This will be shown

Formatting Log Messages

You can customize how log messages appear:

import logging

logging.basicConfig(
    level=logging.INFO,
    format="%(asctime)s - %(levelname)s - %(message)s"
)

logging.info("Application started")
logging.warning("Low memory warning")

# Output:
# 2024-01-15 10:30:45,123 - INFO - Application started
# 2024-01-15 10:30:45,456 - WARNING - Low memory warning

Logging to a File

Instead of printing to the console, you can save logs to a file:

import logging

logging.basicConfig(
    filename="app.log",
    level=logging.INFO,
    format="%(asctime)s - %(levelname)s - %(message)s"
)

logging.info("User logged in")
logging.error("Database connection failed")

# Messages are saved to app.log file instead of console

Using Loggers

For more control, you can create your own logger:

import logging

# Create a logger
logger = logging.getLogger("my_app")
logger.setLevel(logging.DEBUG)

# Create a console handler
console_handler = logging.StreamHandler()
console_handler.setLevel(logging.INFO)

# Create a formatter
formatter = logging.Formatter("%(name)s - %(levelname)s - %(message)s")
console_handler.setFormatter(formatter)

# Add handler to logger
logger.addHandler(console_handler)

logger.debug("This won't show (below INFO level)")
logger.info("Application initialized")
logger.error("Something went wrong")

# Output:
# my_app - INFO - Application initialized
# my_app - ERROR - Something went wrong

Logging Variables

You can include variables in your log messages:

import logging

logging.basicConfig(level=logging.INFO)

username = "alice"
age = 25

logging.info(f"User {username} logged in, age: {age}")
# Or using % formatting (older style)
logging.info("User %s logged in, age: %d", username, age)

# Output:
# INFO:root:User alice logged in, age: 25

Log Levels Explained
Next example: Multi-threading