Lesson 24: Logging and Debugging in Python

🎯 Lesson Objective

To understand how to find and fix errors efficiently using debugging tools and how to track program execution using Python’s logging module.

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🧩 1. What Is Debugging?

Debugging is the process of identifying and fixing bugs (errors) in your code.

Common debugging techniques:

• Using print() statements
• Using Python’s built-in pdb (Python Debugger)
• Using IDEs like VS Code or PyCharm with breakpoints
• Using the logging module for professional tracking

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⚙️ 2. Common Types of Errors

Error Type	Description	Example
SyntaxError	Invalid Python syntax	print("Hello"
NameError	Variable not defined	print(x)
TypeError	Invalid type operation	"5" + 2
ValueError	Wrong type of value	int("abc")
ZeroDivisionError	Division by zero	10 / 0

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🧠 3. Simple Debugging Using print()

def divide(a, b):
    print(f"Dividing {a} by {b}")
    return a / b

print(divide(10, 2))
print(divide(10, 0))  # Error: division by zero

Problem: Works for quick checks but not for large or production programs.
✅ Solution: Use the logging module.

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🧾 4. Introduction to Logging

Logging records events that happen during program execution, making it easier to:

• Diagnose issues
• Record user activity
• Track performance and errors

import logging

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🧱 5. Basic Logging Example

import logging

logging.basicConfig(level=logging.INFO)
logging.info("Program started successfully")
logging.warning("Low disk space")
logging.error("File not found!")

Output:

INFO:root:Program started successfully
WARNING:root:Low disk space
ERROR:root:File not found!

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🧰 6. Logging Levels

Level	Description	Usage
DEBUG	Detailed information (for developers)	Code tracing
INFO	Confirmation that things are working	Status updates
WARNING	Something unexpected happened	Potential problems
ERROR	Serious issue, program may continue	Error handling
CRITICAL	Very serious error	Program crash alerts

Example:

logging.debug("This is a debug message")
logging.info("Program is running fine")
logging.warning("Low memory")
logging.error("An error occurred")
logging.critical("System failure!")

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🪶 7. Writing Logs to a File

import logging

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

logging.info("Application started")
logging.warning("Disk usage high")
logging.error("Error while processing data")

Creates a file: app.log
Example log:

2025-10-25 10:15:23,123 - INFO - Application started
2025-10-25 10:15:23,456 - WARNING - Disk usage high
2025-10-25 10:15:23,789 - ERROR - Error while processing data

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🧮 8. Logging in Try–Except Blocks

import logging

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

try:
    result = 10 / 0
except ZeroDivisionError as e:
    logging.error("Division by zero error: %s", e)

Output:

2025-10-25 12:34:56,789 - ERROR - Division by zero error: division by zero

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🐞 9. Debugging with pdb (Python Debugger)

You can use the Python debugger to pause execution and inspect variables step-by-step.

import pdb

def add_numbers(a, b):
    pdb.set_trace()  # Debug breakpoint
    return a + b

print(add_numbers(5, 10))

🔹 Commands inside pdb:

Command	Meaning
n	Next line
c	Continue execution
p variable	Print variable value
q	Quit debugging

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💡 10. Example – Logging in a Real Program

import logging

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

class BankAccount:
    def __init__(self, name, balance):
        self.name = name
        self.balance = balance
        logging.info(f"Account created for {self.name} with balance {self.balance}")

    def deposit(self, amount):
        self.balance += amount
        logging.info(f"{self.name} deposited {amount}. New balance: {self.balance}")

    def withdraw(self, amount):
        if amount > self.balance:
            logging.warning(f"{self.name} tried to withdraw {amount} but insufficient funds.")
        else:
            self.balance -= amount
            logging.info(f"{self.name} withdrew {amount}. Remaining: {self.balance}")

acc = BankAccount("Sameer", 1000)
acc.deposit(500)
acc.withdraw(2000)

Log File (bank.log):

2025-10-25 12:40:05,123 - INFO - Account created for Sameer with balance 1000
2025-10-25 12:40:05,456 - INFO - Sameer deposited 500. New balance: 1500
2025-10-25 12:40:05,789 - WARNING - Sameer tried to withdraw 2000 but insufficient funds.

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