Python logging - 1

Python's logging module is a powerful tool for tracking events in applications. It provides flexibility in handling log messages and directing them to different outputs, such as files, consoles, or external monitoring services. Here’s a structured way to explore Python logging:

1.Basics of Python Logging

Python’s built-in logging module allows you to log messages with different severity levels.

For example:

import logging

# Configure basic logging
logging.basicConfig(
    filename="app.log",  # Logs will be saved in 'app.log'
    filemode="w",        # Overwrites the file each run
    format="%(asctime)s - %(levelname)s - %(message)s",
    level=logging.DEBUG  # Set log level
)

# Generate log messages
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")

print("Logging completed. Check 'app.log' for output.")

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2️⃣ Run the Script

Open a terminal or command prompt and navigate to the folder where the file is saved.

Run the script with:

shCopyEditpython log_example.py

or, if using Python 3:

shCopyEditpython3 log_example.py

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3️⃣ Check the Output

Console Output

Since we are only logging to a file (not the console), the terminal output will be:

luaCopyEditLogging completed. Check 'app.log' for output.

File Output (app.log)

Open the app.log file in any text editor, and you’ll see:

pgsqlCopyEdit2025-02-22 12:00:01,123 - DEBUG - This is a debug message
2025-02-22 12:00:01,124 - INFO - This is an info message
2025-02-22 12:00:01,125 - WARNING - This is a warning message
2025-02-22 12:00:01,126 - ERROR - This is an error message
2025-02-22 12:00:01,127 - CRITICAL - This is a critical message

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2. Configuring Log Output

You can specify where logs should be written using basicConfig():

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

Then, when you run the script, the log messages will appear in the terminal:

pgsqlCopyEdit2025-02-22 12:05:01,123 - DEBUG - This is a debug message
2025-02-22 12:05:01,124 - INFO - This is an info message
2025-02-22 12:05:01,125 - WARNING - This is a warning message
2025-02-22 12:05:01,126 - ERROR - This is an error message
2025-02-22 12:05:01,127 - CRITICAL - This is a critical message

---

5️⃣ Logging to Both Console and File

Modify your script to send logs to both console and file:

pythonCopyEditimport logging

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

# Create handlers for file and console
file_handler = logging.FileHandler("app.log")
console_handler = logging.StreamHandler()

# Set log levels
file_handler.setLevel(logging.DEBUG)
console_handler.setLevel(logging.WARNING)  # Only WARNING+ logs appear in the console

# Create formatter
formatter = logging.Formatter('%(asctime)s - %(levelname)s - %(message)s')
file_handler.setFormatter(formatter)
console_handler.setFormatter(formatter)

# Add handlers to logger
logger.addHandler(file_handler)
logger.addHandler(console_handler)

# Log messages
logger.debug("Debug message (only in file)")
logger.info("Info message (only in file)")
logger.warning("Warning message (in both)")
logger.error("Error message (in both)")
logger.critical("Critical message (in both)")

Output

📌 Terminal (console) will only show WARNING, ERROR, and CRITICAL messages:

pgsqlCopyEdit2025-02-22 12:10:01,125 - WARNING - Warning message (in both)
2025-02-22 12:10:01,126 - ERROR - Error message (in both)
2025-02-22 12:10:01,127 - CRITICAL - Critical message (in both)

📌 app.log file will contain all log messages:

pgsqlCopyEdit2025-02-22 12:10:01,123 - DEBUG - Debug message (only in file)
2025-02-22 12:10:01,124 - INFO - Info message (only in file)
2025-02-22 12:10:01,125 - WARNING - Warning message (in both)
2025-02-22 12:10:01,126 - ERROR - Error message (in both)
2025-02-22 12:10:01,127 - CRITICAL - Critical message (in both)