Exception

Exceptions are runtime errors that interrupt normal program flow. Python provides try/except to handle them safely. Well-handled exceptions make programs more robust and produce clearer error messages for users.

Basic Exception Handling

for raw_value in ["5", "0", "oops"]:
    try:
        value = int(raw_value)
        print(10 / value)
    except ValueError:
        print("Input must be a number")
    except ZeroDivisionError:
        print("Cannot divide by zero")

Catch specific exceptions whenever possible. Avoid broad except Exception: unless you re-raise or log with clear context.

else and finally

• else runs when no exception occurs.
• finally always runs (cleanup code).

f = None

try:
    f = open("data.txt", "r", encoding="utf-8")
except FileNotFoundError:
    print("File not found")
else:
    print(f.read())
finally:
    if f is not None:
        f.close()

For file handling, with open(...) is usually simpler. Use finally when you must guarantee cleanup for resources that are not managed by with.

Raising Exceptions

Use raise when input/state is invalid:

def set_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")
    return age

Raising early keeps invalid data from flowing deeper into your program.

Custom Exceptions

class InvalidUsernameError(Exception):
    pass


def register(username):
    if len(username) < 3:
        raise InvalidUsernameError("Username must be at least 3 characters")

Custom exceptions make error handling clearer in larger applications, especially when different failure cases need different responses.

Common Built-in Exceptions

Assertions (assert)

Assertions are useful for internal sanity checks while developing:

def divide(a, b):
    assert b != 0, "b must not be zero"
    return a / b

Use assertions to catch programmer mistakes, not to validate user input. Assertions can be disabled with optimization flags (for example python -O).

Notes for Python 3

• IOError and EnvironmentError are aliases of OSError.
• StandardError no longer exists in Python 3.
• For concurrent code, Python also supports ExceptionGroup and except*.

Common Errors

• Using bare except: and accidentally hiding real bugs.
• Catching the wrong exception type (handler never runs).
• Swallowing errors without logging, which makes debugging difficult.