Python in a Nutshell, 2nd Edition

Exception Objects

Exceptions are instances of subclasses of the built-in, legacy-style Exception class.^[4] An instance of any subclass of Exception has an attribute args, the tuple of arguments used to create the instance. args holds error-specific information, usable for diagnostic or recovery purposes. In Python 2.5, class Exception is new-style, since it inherits from the new built-in new-style class BaseException; this, in turn, makes all built-in exception classes new-style in Python 2.5. For detailed explanations of how the standard exception hierarchy changes in 2.5, and how things will move further in 3.0, see http://python.org/doc/peps/pep-0352/.

The Hierarchy of Standard Exceptions

All exceptions that Python itself raises are instances of subclasses of Exception. The inheritance structure of exception classes is important, as it determines which except clauses handle which exceptions. In Python 2.5, however, classes KeyboardInterrupt and SystemExit inherit directly from the new class BaseException and are not subclasses of Exception: the new arrangement makes it more likely that a general handler clause coded as except Exception: does what’s intended, since you rarely want to catch KeyboardInterrupt and SystemExit (exception handlers are covered in try/except).

In Python 2.3 and 2.4, the SystemExit, StopIteration, and Warning classes inherit directly from Exception. Instances of SystemExit are normally raised by the exit function in module sys (covered in exit in The sys ...