Exception Handling

try, except, finally, raising exceptions, and writing robust code.

What is an Exception?

An exception is a runtime error that interrupts normal program flow. Without handling, it prints a traceback and crashes. Common exceptions include ValueError, ZeroDivisionError, FileNotFoundError, and IndexError.

try / except

try:
    num = int(input("Enter a number: "))
    result = 100 / num
    print(f"Result: {result}")
except ValueError:
    print("That's not a valid integer.")
except ZeroDivisionError:
    print("Cannot divide by zero.")

else and finally

try:
    f = open("data.txt")
except FileNotFoundError:
    print("File not found!")
else:
    print(f.read())    # runs only if no exception
finally:
    print("Done.")    # always runs

Raising Exceptions

def set_age(age):
    if age < 0 or age > 120:
        raise ValueError(f"Invalid age: {age}")
    return age

try:
    set_age(-5)
except ValueError as e:
    print(f"Error: {e}")