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Rahul Gupta
Rahul Gupta

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

#python #programming #learning #100daysofcode

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. 🐍

📦 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

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

✅ 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.")
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🔍 Output examples:

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

🧪 Catching All Exceptions (Not Recommended Often) 

try: risky_code() except Exception as e: print("Something went wrong:", e)
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Useful for logging or debugging, but try to handle specific exceptions when possible.

🧠 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

🧠 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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