Python Exception Handling: try, except, finally

Why Handle Errors?

Code crashes ruin user experience. Exception handling lets you catch errors and respond gracefully.

Basic try/except

try:
    n = int(input("Enter a number: "))
    print(10 / n)
except ValueError:
    print("That's not a number!")
except ZeroDivisionError:
    print("Cannot divide by zero!")

Catching Any Exception

try:
    risky_operation()
except Exception as e:
    print(f"Something went wrong: {e}")

Warning: Avoid catching all exceptions blindly — you hide bugs.

Multiple Exceptions

try:
    result = do_something()
except (TypeError, ValueError) as e:
    print(f"Bad input: {e}")
except IOError as e:
    print(f"File problem: {e}")

try/except/else/finally

try:
    f = open("data.txt")
except FileNotFoundError:
    print("File not found")
else:
    # runs if no exception
    print("File opened")
    f.close()
finally:
    # ALWAYS runs (cleanup)
    print("Done")

Common Built-In Exceptions

Exception	When It Happens
ValueError	Wrong value type (e.g. int("abc"))
TypeError	Wrong type operation (e.g. "a"+1)
KeyError	Missing dictionary key
IndexError	List index out of range
FileNotFoundError	File doesn't exist
ZeroDivisionError	Dividing by zero
AttributeError	Missing attribute/method

Raising Exceptions

def calculate_age(year):
    if year > 2026:
        raise ValueError("Year cannot be in the future")
    return 2026 - year

try:
    calculate_age(2030)
except ValueError as e:
    print(f"Error: {e}")

Practical Input Validation

def get_positive_int(prompt):
    while True:
        try:
            n = int(input(prompt))
            if n <= 0:
                raise ValueError("Must be positive")
            return n
        except ValueError as e:
            print(f"Invalid: {e}. Try again.")

age = get_positive_int("Age: ")

When NOT to Use try/except

• For control flow (use if instead)
• To hide bugs (handle specific errors only)
• For conditions you can check first (e.g. check key exists before accessing)

HKDSE Context

Exception handling appears occasionally in HKDSE, mainly for input validation. Know the basic try/except/ValueError pattern.