Module 5 Lesson 10: Logging and Debugging Files

Using print() for debugging is fine when you're a beginner, but professional apps don't print errors to the screen for users to see. Instead, they write them to a hidden Log File. This allows developers to see exactly what happened days or weeks after an error occurred.

Lesson Overview

In this lesson, we will cover:

• The logging Module: Python's built-in record keeper.
• Log Levels: DEBUG, INFO, WARNING, ERROR, CRITICAL.
• Logging to a File: Creating your own app.log.
• Formatting Logs: Adding timestamps and line numbers automatically.

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1. Why Logging?

Imagine your program crashes at 3:00 AM while you're asleep.

• With print: The error is gone when the program closes. You have no idea what happened.
• With logging: You wake up, open app.log, and see: [03:00:15] ERROR: Database connection lost in database.py line 45.

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2. Setting Up a Basic Logger

To start logging to a file, we use logging.basicConfig().

import logging

# Configure logging to save to 'app.log'
logging.basicConfig(
    filename='app.log', 
    level=logging.DEBUG,
    format='%(asctime)s - %(levelname)s - %(message)s'
)

logging.info("Program started.")
logging.warning("User attempted a low-security action.")
logging.error("Failed to connect to the server.")

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3. The 5 Levels of Logging

1. DEBUG: Detailed info for diagnosing problems.
2. INFO: Confirmation that things are working as expected.
3. WARNING: Something unexpected happened, but the app is still working.
4. ERROR: A serious problem; the app couldn't perform a function.
5. CRITICAL: A fatal error; the program may be unable to continue.

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4. Practical Example: Logging Exceptions

You can use the logging module inside your except blocks to record when crashes happen.

try:
    result = 10 / 0
except ZeroDivisionError:
    logging.error("A user tried to divide by zero!")

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Practice Exercise: The System Monitor

1. Create a script that simulates a system monitor.
2. Log an INFO message when the script starts.
3. Create a loop that "checks" a value (e.g., CPU temperature).
4. If the value is above 70, log a WARNING.
5. If it's above 90, log a CRITICAL message and stop the loop.
6. Open your app.log file to see the results.

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Quick Knowledge Check

1. What is the benefit of logging over printing?
2. Name the five levels of logging in order of severity.
3. What is the purpose of %(asctime)s in a log format?
4. By default, which log level is the minimum (i.e., lower levels won't be shown)?

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Key Takeaways

• Logging provides a persistent history of what your program did.
• The logging module is built-in and very powerful.
• Use different levels (INFO, ERROR, etc.) to categorize your messages.
• Logging to a file is essential for professional software development.

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What’s Next?

We’ve finished the lessons for Module 5! It’s time to put your file-handling and error-catching skills together in our Hands-on Projects where we'll build a Secure Data Archive!