Error Handling & Debugging

Catch Bugs, Avoid Crashes, and Write Reliable Code

Even the best-written code can face unexpected situations — wrong input, missing files, or network issues. That’s where error handling comes in. Instead of crashing, your program can handle the error gracefully and keep going.

🚫 1. Types of Errors in Python #

🔹 Syntax Errors #

Happen when Python can’t parse your code.

print("Hello"  # Missing closing parenthesis

🔹 Runtime Errors (Exceptions) #

Happen while the program is running.

x = 10 / 0  # ZeroDivisionError

🛡 2. Using try, except Blocks #

Wrap risky code inside a try block and handle potential errors with except.

try:
    num = int(input("Enter a number: "))
    print(10 / num)
except ZeroDivisionError:
    print("❌ Cannot divide by zero.")
except ValueError:
    print("❌ Please enter a valid number.")

🔄 3. else and finally in Try Blocks #

✅ else: Runs if no exception occurs #

🔒 finally: Always runs (good for cleanup) #

try:
    file = open("data.txt", "r")
    content = file.read()
except FileNotFoundError:
    print("❌ File not found!")
else:
    print("📄 File read successfully.")
finally:
    print("🔒 Closing file (if opened).")
    try:
        file.close()
    except:
        pass

🧠 4. Raising Your Own Exceptions #

Sometimes you need to create your own error messages.

age = int(input("Enter your age: "))
if age < 0:
    raise ValueError("Age cannot be negative!")

🧰 5. Debugging Tools in Python #

🧪 Using print() for Debugging (Old School but Effective) #

x = 5
y = 0
print("Debug: x =", x, " y =", y)
print(x / y)  # You'll catch it before crash

🐞 Using pdb (Python Debugger) #

import pdb

pdb.set_trace()  # Execution will pause here
x = 10
y = 5
z = x + y
print(z)

Use commands like:

• n: next line
• c: continue
• q: quit

⚠️ Common Exceptions #

🧠 Real-World Use Case: Login Authenticator with Error Handling #

Let’s create a CLI app that:

• Takes username and password
• Handles incorrect input
• Locks out after 3 failed attempts

✅ Code Example #

def login_system():
    correct_username = "admin"
    correct_password = "1234"
    attempts = 3

    while attempts > 0:
        try:
            username = input("Username: ")
            password = input("Password: ")

            if not username or not password:
                raise ValueError("Fields cannot be empty!")

            if username == correct_username and password == correct_password:
                print("✅ Login successful!")
                return
            else:
                attempts -= 1
                print(f"❌ Invalid credentials. Attempts left: {attempts}")

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

    print("🚫 Too many failed attempts. Account locked.")

# Run it
login_system()

🖥 Output Example #

Username: admin
Password: wrong
❌ Invalid credentials. Attempts left: 2
Username: 
Password: 1234
⚠️ Fields cannot be empty!
Username: admin
Password: 1234
✅ Login successful!

🔍 Key Takeaways #

🔚 Wrap Up #

You can’t predict every problem your code may face — but you can plan how it responds to them. Error handling makes your apps smarter, safer, and way more user-friendly.

🔜 Up Next: #

Modules & Code Organization
Learn how to break your code into reusable modules, import them smartly, and manage larger projects easily — with a Library Book Management App example.