Day 18/100: Exception Handling with try-except in Python

Welcome to Day 18 of the 100 Days of Python series!
Today we tackle a critical skill for writing robust and error-resistant Python programs — Exception Handling using try, except, and more.

We all make mistakes — and so does code. Instead of letting programs crash, Python gives us tools to catch and handle errors gracefully.

Let’s dive in. 🐍

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📦 What You’ll Learn

• What exceptions are and how they happen
• How to use try and except blocks
• Optional tools: else and finally
• How to handle multiple exceptions
• Real-world examples

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⚠️ What Is an Exception?

An exception is a runtime error that interrupts the normal flow of a program. Common examples include:

• Dividing by zero
• Accessing an invalid index
• Converting a string to a number improperly
• File not found

Without Handling:

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

If the user enters 0 or a string, the program crashes. Let's fix that.

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✅ Basic try-except Block

try:
    num = int(input("Enter a number: "))
    result = 10 / num
    print(result)
except ZeroDivisionError:
    print("You can't divide by zero!")
except ValueError:
    print("Please enter a valid number.")

🔍 Output examples:

• Input: 0 → "You can't divide by zero!"
• Input: "abc" → "Please enter a valid number."

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🧪 Catching All Exceptions (Not Recommended Often)

try:
    risky_code()
except Exception as e:
    print("Something went wrong:", e)

Useful for logging or debugging, but try to handle specific exceptions when possible.

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🧠 Using else and finally

• else runs if no exception occurs
• finally runs no matter what (great for cleanup)

try:
    num = int(input("Enter number: "))
    result = 10 / num
except ZeroDivisionError:
    print("Can't divide by zero.")
else:
    print("Division successful:", result)
finally:
    print("This always runs.")

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🔁 Handling Multiple Errors

try:
    numbers = [1, 2, 3]
    print(numbers[5])  # IndexError
except (IndexError, ValueError) as e:
    print("An error occurred:", e)

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🚀 Real-World Example: Safe User Input

def get_age():
    try:
        age = int(input("Enter your age: "))
        print("You are", age, "years old.")
    except ValueError:
        print("Invalid age. Please enter a number.")

get_age()

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🧰 Custom Exceptions (Advanced)

You can define your own exceptions for specific business logic:

class AgeTooLowError(Exception):
    pass

def check_age(age):
    if age < 18:
        raise AgeTooLowError("You must be at least 18.")

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🧼 Best Practices

• ✅ Catch specific exceptions (ValueError, ZeroDivisionError)
• ✅ Use finally for cleanup (e.g., closing files)
• ✅ Avoid bare except: unless absolutely necessary
• ✅ Log or display meaningful error messages
• 🚫 Don’t use exceptions for normal control flow

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🧠 Recap

Today you learned:

• What exceptions are
• How to use try, except, else, and finally
• How to handle multiple and custom exceptions
• Real-world examples to make your programs crash-resistant

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