Python logging - 2

1️⃣ Logging in a Function (Basic Use Case)

If you want to log function execution, use:

import logging

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

def divide(a, b):
    logging.info(f"Dividing {a} by {b}")
    try:
        result = a / b
        logging.debug(f"Result: {result}")
        return result
    except ZeroDivisionError:
        logging.error("Attempted to divide by zero!", exc_info=True)
        return None

# Run function
divide(10, 2)
divide(10, 0)

📌 Output

2025-02-22 12:15:01,123 - INFO - Dividing 10 by 2
2025-02-22 12:15:01,124 - DEBUG - Result: 5.0
2025-02-22 12:15:01,125 - INFO - Dividing 10 by 0
2025-02-22 12:15:01,126 - ERROR - Attempted to divide by zero!
Traceback (most recent call last):
  File "log_example.py", line 11, in divide
    result = a / b
ZeroDivisionError: division by zero

---

2️⃣ Logging Exceptions to a File

If you want to log only errors to a file:

import logging

# Configure logging to log only errors
logging.basicConfig(filename="error.log", level=logging.ERROR, format="%(asctime)s - %(levelname)s - %(message)s")

try:
    x = 5 / 0
except ZeroDivisionError as e:
    logging.error("Error occurred", exc_info=True)

📌 error.log File Output

2025-02-22 12:16:01,126 - ERROR - Error occurred
Traceback (most recent call last):
  File "log_example.py", line 8, in <module>
    x = 5 / 0
ZeroDivisionError: division by zero

---

3️⃣ Rotating Log Files (Avoid Large Log Files)

If you want to limit log file size and rotate old logs:

import logging
from logging.handlers import RotatingFileHandler

# Create rotating log handler (max size: 1KB, keeps last 3 logs)
handler = RotatingFileHandler("rotating.log", maxBytes=1024, backupCount=3)
handler.setFormatter(logging.Formatter("%(asctime)s - %(levelname)s - %(message)s"))

# Create logger
logger = logging.getLogger("RotatingLogger")
logger.setLevel(logging.DEBUG)
logger.addHandler(handler)

# Generate logs
for i in range(50):
    logger.info(f"Log entry {i}")

📌 How It Works

• When rotating.log reaches 1KB, it renames it as rotating.log.1, rotating.log.2, etc.

• It keeps only the last 3 logs.

---

4️⃣ Logging to JSON File (Structured Logs)

If you want to log structured JSON logs for debugging:

import logging
import json

class JSONFormatter(logging.Formatter):
    def format(self, record):
        log_entry = {
            "timestamp": self.formatTime(record),
            "level": record.levelname,
            "message": record.getMessage(),
        }
        return json.dumps(log_entry)

# Configure logging
logger = logging.getLogger("json_logger")
logger.setLevel(logging.DEBUG)

# Create file handler with JSON formatting
file_handler = logging.FileHandler("logs.json")
file_handler.setFormatter(JSONFormatter())

logger.addHandler(file_handler)

# Generate logs
logger.info("User logged in")
logger.warning("Disk space low")
logger.error("Database connection failed")

📌 logs.json Output

{"timestamp": "2025-02-22 12:18:01", "level": "INFO", "message": "User logged in"}
{"timestamp": "2025-02-22 12:18:02", "level": "WARNING", "message": "Disk space low"}
{"timestamp": "2025-02-22 12:18:03", "level": "ERROR", "message": "Database connection failed"}

---

5️⃣ Logging in a Flask Application

For Flask apps, you can use logging to track errors and requests:

from flask import Flask
import logging

app = Flask(__name__)

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

@app.route("/")
def home():
    app.logger.info("Home page accessed")
    return "Welcome to the Home Page"

@app.route("/error")
def error():
    try:
        1 / 0  # Intentional error
    except ZeroDivisionError:
        app.logger.error("Division by zero error!", exc_info=True)
    return "An error occurred!"

if __name__ == "__main__":
    app.run(debug=True)

📌 Output in flask_app.log

2025-02-22 12:20:01,123 - INFO - Home page accessed
2025-02-22 12:20:05,456 - ERROR - Division by zero error!
Traceback (most recent call last):
  File "app.py", line 12, in error
    1 / 0
ZeroDivisionError: division by zero

---

6️⃣ Sending Logs via Email

If you want to receive critical logs via email, use:

import logging
import smtplib
from logging.handlers import SMTPHandler

# Configure email logging
mail_handler = SMTPHandler(
    mailhost=("smtp.gmail.com", 587),
    fromaddr="[e-mail]",
    toaddrs=["[e-mail]"],
    subject="Critical Error Alert!",
    credentials=("[e-mail]", "your_password"),
    secure=()
)
mail_handler.setLevel(logging.CRITICAL)

# Create logger
logger = logging.getLogger("EmailLogger")
logger.addHandler(mail_handler)

# Log a critical error
logger.critical("Database server is down!")

📌 ⚠️ Note: You may need to enable less secure apps in your email settings or use an app password.

---

7️⃣ Multi-Module Logging (Logging in Multiple Files)

If you have multiple Python files, you can share the same logger.

📜 main.py

import logging
import module

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

logging.info("Starting application")
module.some_function()

📜 module.py

import logging

def some_function():
    logging.info("Function in module executed")

📌 Output

2025-02-22 12:25:01,123 - INFO - Starting application
2025-02-22 12:25:01,124 - INFO - Function in module executed