L11 Error Handling

⚠️ L11 – Error Handling

Programs don’t always run perfectly—sometimes things go wrong. For example, dividing by zero or trying to open a file that doesn’t exist can cause errors. If we don’t manage these errors, the whole program can crash.

Python gives us a way to catch these errors safely using a try and except block. This helps the program keep running, even when something unexpected happens.

💡 Analogy:
Think of a seatbelt in a car. It doesn’t stop accidents, but it protects you when one happens. Error handling doesn’t stop errors, but it stops your program from crashing.

🎯 Learning Objectives

By the end of this lesson, you will be able to:

✅ Understand what error handling is and why it’s important
✅ Use try and except blocks to catch and manage errors
✅ Handle different types of errors in Python

🗝️ Key Terms

• Error Handling – Catching and managing errors so the program doesn’t crash

• Exception – An error during program execution (like ZeroDivisionError)

• try block – Code that might cause an error

• except block – Code that runs if an error happens

• finally block – Code that always runs, even if there’s an error
  
  

🧪 Examples

Example 1: Basic Try-Except (General Error)

try:

    user_input = input("Type something: ")

    print("You typed:", user_input)

except:

    print("Something went wrong.")

✅ What happens?

• If the user types something normally → it prints it back.

• If something unexpected goes wrong → the program doesn't crash. It prints a message.
  
  

Example 2: Handling Multiple Specific Errors

try:

    num1 = int(input("Enter first number: "))

    num2 = int(input("Enter second number: "))

    result = num1 / num2

    print("Result:", result)

except ZeroDivisionError:

    print("Error: You can't divide by zero!")

except ValueError:

    print("Error: Please enter only numbers.")

✅ What happens?

• Valid numbers → the division is shown

• Zero as second number → catches ZeroDivisionError

• Text input → catches ValueError
  
  

Example 3: Using finally

try:

    file = open("example.txt", "r")

    content = file.read()

    print(content)

except FileNotFoundError:

    print("Error: The file does not exist.")

finally:

    print("Closing program.")

✅ What happens?

• If the file exists → it prints the content

• If the file is missing → it prints an error message

• The finally block always runs → prints "Closing program."
  
  

📋 Common Python Errors You Can Catch

These are some of the most useful error types to know when using except.