🔷 Introduction

No program is perfect. Errors are a natural part of programming, especially when dealing with user input, files, APIs, or external systems.

In this lesson, you will learn how to:

✔ Handle errors gracefully using try/except
✔ Use else and finally blocks
✔ Raise your own exceptions
✔ Debug Python programs
✔ Use logging for professional error tracking

These skills are essential for building robust, reliable, and production-ready applications.

🟩 1. What is an Exception?

An exception is an error that occurs during program execution.

Example:

print(10 / 0)  # ZeroDivisionError

🟩 2. Using try / except

Prevent crashes by handling errors:

try:
    x = int(input("Enter a number: "))
    print(10 / x)
except:
    print("An error occurred!")

🔹 Catch specific exceptions

try:
    x = int(input("Enter a number: "))
    print(10 / x)

except ZeroDivisionError:
    print("Cannot divide by zero!")

except ValueError:
    print("Invalid input!")

✔ Best practice: always catch specific exceptions.

🟩 3. else and finally

🔹 else (runs if no error)

try:
    x = int(input())
except ValueError:
    print("Error")
else:
    print("Valid number:", x)

🔹 finally (always runs)

try:
    f = open("file.txt")
except:
    print("Error opening file")
finally:
    print("This always runs")

🟩 4. Raising Exceptions

You can create your own errors using raise.

age = int(input("Enter age: "))

if age < 0:
    raise ValueError("Age cannot be negative")

🟩 5. Custom Exceptions

You can define your own exception classes.

class CustomError(Exception):
    pass

raise CustomError("Something went wrong")

🟩 6. Debugging Techniques

🔹 Use print debugging

print("Value of x:", x)

🔹 Use assert

x = 10
assert x > 0

🔹 Use Python debugger (pdb)

import pdb
pdb.set_trace()

🟩 7. Logging (Professional Debugging)

Use the logging module instead of print.

import logging

logging.basicConfig(level=logging.INFO)

logging.info("Program started")
logging.warning("This is a warning")
logging.error("An error occurred")

✔ Better than print
✔ Used in production apps

🟧 8. Exercises (Hidden Solutions)

Exercise 1 — Handle division error

try:
    x = int(input("Enter a number: "))
    print(10 / x)
except ZeroDivisionError:
    print("Cannot divide by zero")

Exercise 2 — Catch multiple exceptions

try:
    x = int(input())
    print(10 / x)
except (ValueError, ZeroDivisionError):
    print("Error occurred")

Exercise 3 — Use finally block

try:
    f = open("file.txt")
except:
    print("Error")
finally:
    print("Done")

Exercise 4 — Raise an exception

n = int(input())

if n < 0:
    raise ValueError("Negative not allowed")

Exercise 5 — Logging example

import logging

logging.basicConfig(level=logging.INFO)
logging.info("Hello")

🟦 Conclusion

In this lesson, you learned how to:

✔ Handle errors safely
✔ Prevent program crashes
✔ Raise and define custom exceptions
✔ Debug your code effectively
✔ Use logging like a professional developer

These skills are critical for building reliable applications in:

• Web development
• APIs
• Automation scripts
• Data processing

Mini Project — Debug a Student Grade Manager

Project idea

You are given a small Python program that reads student grades, calculates the average, and displays the result.
However, the script contains several common mistakes:

• invalid input handling
• division by zero risk
• wrong variable types
• bad exception handling
• poor debugging structure

The goal is to find the bugs, fix them, and make the program more robust.

Part 1 — The buggy version

grades = []

n = input("How many grades do you want to enter? ")

for i in range(n):
    grade = input("Enter grade: ")
    grades.append(grade)

total = 0
for g in grades:
    total = total + g

average = total / len(grades)

print("Average grade:", average)

Part 2 — Tasks for the learner

Your mission

Find and fix the problems in the program so that it:

1. correctly reads the number of grades
2. correctly converts grades to numbers
3. handles invalid input safely
4. avoids crashing when no grades are entered
5. prints the average correctly

Part 3 — Hints

• input() always returns a string
• range() needs an integer
• grades must be numeric before addition
• division by zero can happen if the list is empty
• use try/except to prevent crashes

Part 4 — Corrected solution

grades = []

try:
    n = int(input("How many grades do you want to enter? "))

    if n <= 0:
        raise ValueError("The number of grades must be greater than 0.")

    for i in range(n):
        while True:
            try:
                grade = float(input(f"Enter grade #{i+1}: "))
                grades.append(grade)
                break
            except ValueError:
                print("Invalid input. Please enter a numeric grade.")

    total = 0
    for g in grades:
        total += g

    average = total / len(grades)

    print("Average grade:", average)

except ValueError as e:
    print("Error:", e)