Introduction to the Python logging Module: Why You Should Use Logging Instead of print() and Basic Settings

During development debugging or confirming operation in a production environment, it is common to use the print() function to check “how far the program has progressed” or “what the variable values are.”

However, in full-scale application development, it is strongly recommended to use the standard library logging module instead of print().

This article explains the basic usage of the logging module, the concept of log levels, and how to set output formats.

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Why logging Instead of print()?

Compared to the print() function, the logging module has the following advantages:

1. Control via Log Levels: You can easily filter logs based on importance (error, warning, info, etc.). (e.g., Do not output debug information in production).
2. Changing Output Destinations: You can flexibly change output destinations (handlers) not only to standard output (console) but also to saving to files or sending emails.
3. Unified Format: Automatically adds metadata such as date and time, filename, and line number to unify the log format.

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Understanding Log Levels

The logging module defines five log levels in order of increasing importance. Logs with a level equal to or higher than the set level are output.

Level	Constant Name	Numeric Value	Usage
DEBUG	logging.DEBUG	10	Detailed information during development. For verifying operation.
INFO	logging.INFO	20	Reporting normal operation. Start/end of processing, etc.
WARNING	logging.WARNING	30	Unexpected but executable problem. Warning.
ERROR	logging.ERROR	40	Function cannot be executed due to a serious problem.
CRITICAL	logging.CRITICAL	50	Fatal error causing the program itself to stop.

By default, only logs of WARNING or higher are output, and INFO and DEBUG are ignored.

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Basic Implementation Example

Now, let’s write code to output logs using the logging module.

Here we introduce the standard pattern: using basicConfig for overall log settings and obtaining a module-specific logger with getLogger(__name__).

import logging

# 1. Basic Log Configuration
# level: Specify the minimum level to output (INFO or higher here)
# format: Specify log output format (Time - Logger Name - Level - Message)
logging.basicConfig(
    level=logging.INFO,
    format="%(asctime)s - %(name)s - %(levelname)s - %(message)s"
)

# 2. Get Logger
# Passing __name__ makes it easier to identify which module the log came from
logger = logging.getLogger(__name__)

def perform_task(task_id):
    # Info Level Log (Will be output)
    logger.info(f"Starting process for Task {task_id}.")
    
    # Debug Level Log (Will NOT be output because setting is INFO)
    logger.debug(f"Detailed debug info for Task {task_id}...")

    if task_id < 0:
        # Error Level Log (Will be output)
        logger.error("Invalid Task ID. Aborting process.")
        return

    # Warning Level Log (Will be output)
    if task_id > 1000:
        logger.warning("Task ID is too large. Processing may take time.")

    logger.info(f"Task {task_id} completed.")

# --- Execution ---
print("--- Execute Process ---")
perform_task(10)
print("\n--- Error Case ---")
perform_task(-1)

Output:

--- Execute Process ---
2025-11-29 10:30:05,123 - __main__ - INFO - Starting process for Task 10.
2025-11-29 10:30:05,123 - __main__ - INFO - Task 10 completed.

--- Error Case ---
2025-11-29 10:30:05,124 - __main__ - INFO - Starting process for Task -1.
2025-11-29 10:30:05,124 - __main__ - ERROR - Invalid Task ID. Aborting process.

Explanation of the Code

• logging.basicConfig(...): Performs initial configuration of the logging system. Since level=logging.INFO was set, the content of logger.debug() was not output.
• format="...": Specifies the log output format.
  • %(asctime)s: Date and Time
  • %(name)s: Logger name (here __name__, so __main__)
  • %(levelname)s: Log level name (INFO, ERROR, etc.)
  • %(message)s: Log message body
• logging.getLogger(__name__): Creates a logger object. It is customary to pass the module name (__name__) as an argument. This clarifies which file the log originated from.

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Outputting Logs to a File (Handler)

If you want to save to a file instead of displaying on the screen, simply add the filename argument to basicConfig.

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

When you run this, nothing will be displayed on the console; instead, an app.log file will be created in the execution directory, and logs will be appended to it.

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Summary

• Use the logging module instead of print() for log output.
• Distinguish between the 5 log levels (DEBUG, INFO, WARNING, ERROR, CRITICAL).
• Use logging.basicConfig() to batch set levels and formats.
• Create a logger instance with logger = logging.getLogger(__name__) and use it.