Python basics - Day 14

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Day 14 – Exception Handling (try / except)

Project: Build a “Safe Calculator” that handles user input errors gracefully.

01. Learning Goal

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

Understand what exceptions (errors) are in Python
Handle errors using try and except blocks
Use else and finally for structured control flow
Prevent your program from crashing unexpectedly

02. Problem Scenario

When users input wrong data or when files are missing, your program crashes.

Your goal today is to handle exceptions safely and ensure the app continues running smoothly.

03. Step 1 – What is an Exception?

An exception occurs when something goes wrong during execution.

If not handled, it will stop the program.

print(10 / 0)   # ZeroDivisionError
numbers = [1, 2, 3]
print(numbers[5])   # IndexError

Examples of common errors:

Division by zero
Accessing an invalid index
File not found

04. Step 2 – Basic try / except Structure

try:
    # Code that may cause an error
except:
    # Code to run if an error occurs

Example:

try:
    x = int(input("Enter a number: "))
    print("You entered:", x)
except:
    print("That's not a valid number!")

05. Step 3 – Handling Specific Exceptions

You can handle different types of errors separately.

try:
    num = int("abc")   # Raises ValueError
except ValueError:
    print("Cannot convert to integer.")

06. Step 4 – Multiple Exception Types

try:
    a = [1, 2, 3]
    print(a[5])         # IndexError
    print(10 / 0)       # ZeroDivisionError
except IndexError:
    print("List index out of range!")
except ZeroDivisionError:
    print("Cannot divide by zero.")

07. Step 5 – Using else and finally

else: Runs only if no exception occurs.
finally: Always runs, even if an exception occurs.

try:
    n = int(input("Enter a number: "))
except ValueError:
    print("Invalid input.")
else:
    print("Valid number:", n)
finally:
    print("Program finished.")

08. Step 6 – Using the Exception Object

You can capture the error message for debugging.

try:
    x = 10 / 0
except Exception as e:
    print("Error occurred:", e)

09. Step 7 – Practice Examples

Example 1: Prevent Division by Zero

try:
    a = int(input("Enter numerator: "))
    b = int(input("Enter denominator: "))
    print(a / b)
except ZeroDivisionError:
    print("Cannot divide by zero.")

Example 2: File Handling

try:
    with open("not_exist.txt", "r", encoding="utf-8") as f:
        data = f.read()
except FileNotFoundError:
    print("File not found!")

10. Step 8 – Mini Project: Safe Calculator

Create a calculator that safely handles user errors (e.g., wrong input or zero division).

while True:
    try:
        a = float(input("Enter first number: "))
        b = float(input("Enter second number: "))
        op = input("Enter operator (+, -, *, /): ")

        if op == "+":
            print("Result:", a + b)
        elif op == "-":
            print("Result:", a - b)
        elif op == "*":
            print("Result:", a * b)
        elif op == "/":
            print("Result:", a / b)
        else:
            print("Unknown operator!")

    except ValueError:
        print("Please enter a valid number.")
    except ZeroDivisionError:
        print("You can’t divide by zero.")
    except Exception as e:
        print("Unexpected error:", e)

    again = input("Try again? (y/n): ")
    if again.lower() != "y":
        print("Goodbye!")
        break