Python Basics — Day 14 Exception Handling (try / except)

01. What is an Error (Exception)?

• An error occurs when something goes wrong during execution → program stops.

• Examples: dividing by zero, invalid index access, missing file.

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

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02. Basic try / except Structure

try:
    # Code to execute
except:
    # Code to run if an error occurs

Example:

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

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03. Handling Specific Exceptions

You can handle different types of exceptions separately.

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

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04. Handling Multiple Exceptions

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.")

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05. Using else and finally

• else → runs only if no exception occurs

• finally → always runs, whether an exception occurs or not

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

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06. Using Exception Objects

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

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07. 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 Reading

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