Exceptions and Handling

An exception is an abnormal situation that occurs while a program is running. It does not always mean the program must crash. With a planned handler, the program can explain the problem, record it, or continue with later logic.

Common exceptions

An exception type identifies the category of failure:

ValueErrorAn unsuitable value, such as converting "3.6" to int.

ZeroDivisionErrorDivision by zero.

IndexErrorA list index is outside its range.

KeyErrorA dictionary key is missing.

FileNotFoundErrorA requested file is missing.

AttributeErrorAn attribute does not exist.

Exceptions should not replace ordinary conditions. Use if for simple rules; exceptions fit failures discovered only while an operation runs.

try, except, else, and finally

try contains operations that may fail. Once an exception occurs, remaining statements in try are skipped. Python finds a matching except and transfers control to its handler. else runs only without an exception; finally performs cleanup on either path.

01try:02    result = a / b03except (ZeroDivisionError, TypeError) as error:04    message = type(error).__name__05else:06    message = result07finally:08    cleaned = True

try:
    dividend = int(input("Enter dividend: "))
    divisor = int(input("Enter divisor: "))
    quotient = dividend / divisor
except ValueError:
    print("Only integers are supported.")
except ZeroDivisionError:
    print("Divisor cannot be 0.")
else:
    print("Result:", quotient)
finally:
    print("This calculation is finished.")

Catch specific exceptions

Do not begin with an overly broad except Exception. Catching everything may swallow genuine bugs. Catch the failures expected from the current operation so the reason and response remain clear.

try:
    score = int(input("Enter score: "))
except ValueError:
    print("Score must be an integer.")

Raise an exception explicitly

raise lets a function report that input violates its requirements. The function detects the problem; its caller decides whether to explain, retry, or record it.

def normalize_score(score):
    if score < 0 or score > 100:
        raise ValueError("Score must be between 0 and 100.")
    return score / 100

try:
    ratio = normalize_score(120)
except ValueError as error:
    print("Invalid input:", error)