python-peps/pep-0352.txt

PEP: 352
Title: Required Superclass for Exceptions
Version: $Revision$
Last-Modified: $Date$
Author: Brett Cannon, Guido van Rossum
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 27-Oct-2005
Post-History:


Abstract
========

In Python 2.4 and before, any (classic) class can be raised as an
exception.  The plan for 2.5 was to allow new-style classes, but this
makes the problem worse -- it would mean *any* class (or
instance) can be raised! This is a problem as it prevents any
guarantees from being made about the interface of exceptions.
This PEP proposes introducing a new superclass that all raised objects
must inherit from.  Imposing the restriction will allow a standard
interface for exceptions to exist that can be relied upon.  It also
leads to a known hierarchy for all exceptions to adhere to.

One might counter that requiring a specific base class for a
particular interface is unPythonic.  However, in the specific case of
exceptions there's a good reason (which has generally been agreed to
on python-dev): requiring hierarchy helps code that wants to *catch*
exceptions by making it possible to catch *all* exceptions explicitly
by writing ``except BaseException:`` instead of
``except *:``. [#hierarchy-good]_

Introducing a new superclass for exceptions also gives us the chance
to rearrange the exception hierarchy slightly for the better.  As it
currently stands, all exceptions in the built-in namespace inherit
from Exception.  This is a problem since this includes two exceptions
(KeyboardInterrupt and SystemExit) that often need to be excepted from
the application's exception handling: the default behavior of shutting
the interpreter down without a traceback is usually more desirable than
whatever the application might do (with the possible exception of
applications that emulate Python's interactive command loop with
``>>>`` prompt).  Changing it so that these two exceptions inherit
from the common superclass instead of Exception will make it easy for
people to write ``except`` clauses that are not overreaching and not
catch exceptions that should propagate up.

This PEP is based on previous work done for PEP 348 [#pep348]_.


Requiring a Common Superclass
=============================

This PEP proposes introducing a new exception named BaseException that
is a new-style class and has a single attribute, ``args``.  Below
is the code as the exception will work in Python 3.0 (how it will
work in Python 2.x is covered in the `Transition Plan`_ section)::

  class BaseException(object):

      """Superclass representing the base of the exception hierarchy.

      Provides an 'args' attribute that contains all arguments passed
      to the constructor.  Suggested practice, though, is that only a
      single string argument be passed to the constructor.
      
      """

      def __init__(self, *args):
          self.args = args

      def __str__(self):
          if len(self.args) == 1:
              return str(self.args[0])
          else:
              return str(self.args)

      def __repr__(self):
          return "%s(*%s)" % (self.__class__.__name__, repr(self.args))


No restriction is placed upon what may be passed in for ``args``
for backwards-compatibility reasons.  In practice, though, only
a single string argument should be used.  This keeps the string
representation of the exception to be a useful message about the
exception that is human-readable; this is why the ``__str__`` method
special-cases on length-1 ``args`` value.  Including programmatic
information (e.g., an error code number) should be stored as a
separate attribute in a subclass.

The ``raise`` statement will be changed to require that any object
passed to it must inherit from BaseException.  This will make sure
that all exceptions fall within a single hierarchy that is anchored at
BaseException [#hierarchy-good]_.  This also guarantees a basic
interface that is inherited from BaseException.  The change to
``raise`` will be enforced starting in Python 3.0 (see the `Transition
Plan`_ below).

With BaseException being the root of the exception hierarchy,
Exception will now inherit from it.


Exception Hierarchy Changes
===========================

With the exception hierarchy now even more important since it has a
basic root, a change to the existing hierarchy is called for.  As it
stands now, if one wants to catch all exceptions that signal an error
*and* do not mean the interpreter should be allowed to exit, you must
specify all but two exceptions specifically in an ``except`` clause
or catch the two exceptions separately and then re-raise them and
have all other exceptions fall through to a bare ``except`` clause::

 except (KeyboardInterrupt, SystemExit):
     raise
 except:
     ...

That is needlessly explicit.  This PEP proposes moving
KeyboardInterrupt and SystemExit to inherit directly from
BaseException.

::

  - BaseException
    |- KeyboardInterrupt
    |- SystemExit
    |- Exception
       |- (all other current built-in exceptions)

Doing this makes catching Exception more reasonable.  It would catch
only exceptions that signify errors.  Exceptions that signal that the
interpreter should exit will not be caught and thus be allowed to
propagate up and allow the interpreter to terminate.

KeyboardInterrupt has been moved since users typically expect an
application to exit when they press the interrupt key (usually Ctrl-C).
If people have overly broad ``except`` clauses the expected behaviour
does not occur.

SystemExit has been moved for similar reasons.  Since the exception is
raised when ``sys.exit()`` is called the interpreter should normally
be allowed to terminate.  Unfortunately overly broad ``except``
clauses can prevent the explicitly requested exit from occurring.

To make sure that people catch Exception most of the time, various
parts of the documentation and tutorials will need to be updated to
strongly suggest that Exception be what programmers want to use.  Bare
``except`` clauses or catching BaseException directly should be
discouraged based on the fact that KeyboardInterrupt and SystemExit
almost always should be allowed to propagate up.


Transition Plan
===============

Since semantic changes to Python are being proposed, a transition plan
is needed.  The goal is to end up with the new semantics being used in
Python 3.0 while providing a smooth transition for 2.x code.  All
deprecations mentioned in the plan will lead to the removal of the
semantics starting in the version following the initial deprecation.

Here is BaseException as implemented in the 2.x series::

  class BaseException(object):

      """Superclass representing the base of the exception hierarchy.

      The __getitem__ method is provided for backwards-compatibility
      and will be deprecated at some point.  The 'message' attribute
      is also deprecated.

      """

      def __init__(self, *args):
          self.args = args

      def __str__(self):
          return str(self.args[0]
                     if len(self.args) <= 1
                     else self.args)

      def __repr__(self):
          func_args = repr(self.args) if self.args else "()"
          return self.__class__.__name__ + func_args

      def __getitem__(self, index):
          """Index into arguments passed in during instantiation.

          Provided for backwards-compatibility and will be
          deprecated.

          """
          return self.args[index]

      def _get_message(self):
          """Method for 'message' property."""
          warnings.warn("the 'message' attribute has been deprecated "
                          "since Python 2.6")
          return self.args[0] if len(args) == 1 else ''

      message = property(_get_message,
                          doc="access the 'message' attribute; "
                              "deprecated and provided only for "
                              "backwards-compatibility")


Deprecation of features in Python 2.9 is optional.  This is because it
is not known at this time if Python 2.9 (which is slated to be the
last version in the 2.x series) will actively deprecate features that
will not be in 3.0 .  It is conceivable that no deprecation warnings
will be used in 2.9 since there could be such a difference between 2.9
and 3.0 that it would make 2.9 too "noisy" in terms of warnings.  Thus
the proposed deprecation warnings for Python 2.9 will be revisited
when development of that version begins, to determine if they are still
desired.

* Python 2.5 [done]

  - all standard exceptions become new-style classes [done]

  - introduce BaseException [done]

  - Exception, KeyboardInterrupt, and SystemExit inherit from
    BaseException [done]

  - deprecate raising string exceptions [done]

* Python 2.6 [done]

  - deprecate catching string exceptions [done]

  - deprecate ``message`` attribute (see `Retracted Ideas`_) [done]
  
* Python 2.7

  - deprecate raising exceptions that do not inherit from BaseException

  - remove ``message`` attribute

* Python 2.8

  - deprecate catching exceptions that do not inherit from BaseException

* Python 2.9
  
  - deprecate ``__getitem__`` (optional)

* Python 3.0 [done]

  - drop everything that was deprecated above:

    + string exceptions (both raising and catching) [done]

    + all exceptions must inherit from BaseException [done]

    + drop ``__getitem__``, ``message`` [done]


Retracted Ideas
===============

A previous version of this PEP that was implemented in Python 2.5
included a 'message' attribute on BaseException.  Its purpose was to
begin a transition to BaseException accepting only a single argument.
This was to tighten the interface and to force people to use
attributes in subclasses to carry arbitrary information with an
exception instead of cramming it all into ``args``.

Unfortunately, while implementing the removal of the ``args``
attribute in Python 3.0 at the PyCon 2007 sprint
[#pycon2007-sprint-email]_, it was discovered that the transition was
very painful, especially for C extension modules.  It was decided that
it would be better to deprecate the ``message`` attribute in
Python 2.6 (and remove it in Python 2.7 and Python 3.0) and consider a
more long-term transition strategy in Python 3.0 to remove
multiple-argument support in BaseException in preference of accepting
only a single argument.  Thus the introduction of ``message`` and the
original deprecation of ``args`` has been retracted.


References
==========

.. [#pep348] PEP 348 (Exception Reorganization for Python 3.0)
   http://www.python.org/peps/pep-0348.html

.. [#hierarchy-good] python-dev Summary for 2004-08-01 through 2004-08-15
   http://www.python.org/dev/summary/2004-08-01_2004-08-15.html#an-exception-is-an-exception-unless-it-doesn-t-inherit-from-exception

.. [#SF_1104669] SF patch #1104669 (new-style exceptions)
   http://www.python.org/sf/1104669

.. [#pycon2007-sprint-email]  python-3000 email ("How far to go with cleaning up exceptions")
    http://mail.python.org/pipermail/python-3000/2007-March/005911.html


Copyright
=========

This document has been placed in the public domain.



..
   Local Variables:
   mode: indented-text
   indent-tabs-mode: nil
   sentence-end-double-space: t
   fill-column: 70
   End: