python-peps/pep-0348.txt

PEP: 348
Title: Exception Reorganization for Python 3.0
Version: $Revision$
Last-Modified: $Date$
Author: Brett Cannon <brett@python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 28-Jul-2005
Post-History:


Abstract
========

Python, as of version 2.4, has 38 exceptions (including warnings) in
the built-in namespace in a rather shallow hierarchy.  These
classes have come about over the years without a chance to learn from
experience.  This PEP proposes doing a reorganization of the hierarchy
for Python 3.0 when backwards-compatibility is not as much of an
issue.

Along with this reorganization, adding a requirement that all
objects passed to a ``raise`` statement must inherit from a specific
superclass is proposed.  This is to have guarantees about the basic
interface of exceptions and to further enhance the natural hierarchy
of exceptions.

Lastly, bare ``except`` clauses will be removed.  While they had their
usefulness when exceptions could be any object, with the above
proposal
of a required superclass for exceptions bare ``except`` clauses lose
their purpose.


Rationale For Wanting Change
============================

Exceptions are a critical part of Python.  While exceptions are
traditionally used to signal errors in a program, they have also grown
to be used for flow control for things such as iterators.

While their importance is great, there is a lack of structure to them.
This stems from the fact that any object can be raised as an
exception.  Because of this you have no guarantee in terms of what
kind of object will be raised, destroying any possible hierarchy 
raised objects might adhere to.

But exceptions do have a hierarchy, showing the severity of the
exception.  The hierarchy also groups related exceptions together to
simplify catching them in ``except`` clauses.  To allow peopele to
be able to rely on this hierarchy, a common superclass that all
raise objects must inherit from is being proposed.  It also allows
guarantees about the interface to raised objects to be made (see
PEP 344 [#PEP344]_).  A discussion about all of this has occurred
before on python-dev [#Summary2004-08-01]_.

With the requirement of a common superclass for all exceptions, bare
``except`` clauses are impacted.  Currently used as a catch-all for
raised exceptions, its usefulness is terminally weakened.  Now, the
same functionality is possible by catching the required superclass.
Once again, this has been discussed on python-dev [#python-dev3]_.

Finally, slight changes to the exception hierarchy will make it much
more reasonable in terms of structure.  By minor rearranging
exceptions
that should not typically be caught can be allowed to propagate to the
top of the execution stack, terminating the interpreter as intended.


Philosophy of Reorganization
============================

For the reorganization of the hierarchy, there was a general
philosophy followed that developed from discussion of earlier drafts
of this PEP [#python-dev-thread1]_, [#python-dev-thread2]_,
[#python-dev-thread3]_.  First and foremost was to not break anything
that works.  This meant that renaming exceptions was out of the
question unless the name was deemed severely bad.  This
also meant no removal of exceptions unless they were viewed as
truly misplaced. The introduction of new exceptions were only done in
situations where there might be a use for catching a superclass of a
category of exceptions.  Lastly, existing exceptions would have their
inheritance tree changed only if it was felt they were truly
misplaced to begin with.

For all new exceptions, the proper suffix had to be chosen.  For
those that signal an error, "Error" is to be used.  If the exception
is a warning, then "Warning".  "Exception" is to be used when none
of the other suffixes are proper to use and no specific suffix is
a better fit.

After that it came down to choosing which exceptions should and
should not inherit from Exception.  This was for the purpose of
making bare ``except`` clauses more useful.

Lastly, the entire existing hierarchy had to inherit from the new
exception meant to act as the required superclass for all exceptions
to inherit from.


New Hierarchy
=============

.. Note:: Exceptions flagged with "stricter inheritance" will no
   longer inherit from a certain class.  A "broader inheritance" flag
   means a class has been added to the exception's inheritance tree.
   All comparisons are against the Python 2.4 exception hierarchy.

.. parsed-literal::

   +-- BaseException (new; broader inheritance for subclasses)
       +-- Exception
	   +-- GeneratorExit (defined in PEP 342 [#PEP342]_)
	   +-- StandardError
	       +-- ArithmeticError
		   +-- DivideByZeroError
		   +-- FloatingPointError
		   +-- OverflowError
	       +-- AssertionError
	       +-- AttributeError
	       +-- EnvironmentError
		   +-- IOError
		   +-- EOFError
		   +-- OSError
	       +-- ImportError
	       +-- LookupError
		   +-- IndexError
		   +-- KeyError
	       +-- MemoryError
	       +-- NameError
		   +-- UnboundLocalError
	       +-- NotImplementedError (stricter inheritance)
	       +-- SyntaxError
		   +-- IndentationError
		       +-- TabError
	       +-- TypeError
	       +-- RuntimeError
	       +-- UnicodeError
		   +-- UnicodeDecodeError
		   +-- UnicodeEncodeError
		   +-- UnicodeTranslateError
	       +-- ValueError
	       +-- ReferenceError
	   +-- StopIteration
	   +-- SystemError
	   +-- Warning
	       +-- DeprecationWarning
	       +-- FutureWarning
	       +-- PendingDeprecationWarning
	       +-- RuntimeWarning
	       +-- SyntaxWarning
	       +-- UserWarning
	   + -- WindowsError
       +-- TerminatingException (new; stricter inheritance for
				 subclasses)
	   +-- KeyboardInterrupt
	   +-- SystemExit


Differences Compared to Python 2.4
==================================

A more thorough explanation of terms is needed when discussing
inheritance changes.  Inheritance changes result in either broader or
more restrictive inheritance.  "Broader" is when a class has an
inheritance tree like ``cls, A`` and then becomes ``cls, B, A``.
"Stricter" is the reverse.


BaseException
-------------

The superclass that all exceptions must inherit from.  It's name was
chosen to reflect that it is at the base of the exception hierarchy
while being an exception itself.  "Raisable" was considered as a name,
it was passed on because its name did not properly reflect the fact
that it is an exception itself.

Direct inheritance of BaseException is not expected, and will
be discouraged for the general case.  Most user-defined
exceptions should inherit from Exception instead.  This allows
catching Exception to continue to work in the common case of catching
all exceptions that should be caught.  Direct inheritance of
BaseException should only be done in cases where an entirely new
category of exception is desired.

But, for cases where all
exceptions should be caught blindly, ``except BaseException`` will
work.


TerminatingException
--------------------

Superclass for exceptions that are meant to symbolize the termination
of
the interpreter.  It does not inherit from Exception so that the
common
``except Exception`` statement does not catch the exceptions.  This
for ``try`` statements that want to catch all exceptions that are
signals of errors and handle them separately from exceptions that
signify that the interpreter should be terminated::

 try:
     ...
 # Catch all exceptions that are expected to be caught
 except Exception:
     ...
 # Catch exceptions expected to terminate the interpreter
 except TerminatingException:
     ...

Compare this to::

 try:
     ...
 except Exception:
     ...
 except (KeyboardInterrupt, SystemExit):
     ...

While more explicit, it is not necessarily obvious why the two
exceptions are being caught directly.  By providing a common
superclass with a name that explicitly states the exceptions' typical
usage the reasoning behind the catch becomes more apparent.

Comparing it to an even more general ``except`` clause::

 try:
     ...
 except Exception:
     ...
 except BaseException:
     ...

While this will perform the same action and catch all exceptions,
guaranteed, it is once again not obvious why Exception and
BaseException are being caught separately and not just
BaseException based purely on the code.

TerminatingException will also help with transitioning from Python
2.x to 3.0 .  With TerminatingException being a new exception, when
it is used the new inheritance for KeyboardInterrupt and SystemExit
becomes more obvious.

It has been argued that TerminatingException should not be added
because it goes against Flat Is Better Than Nested (FIBTN).
While this new exception will not make the hierarchy as flat is it
could be, the overall depth of the tree is not changed; Exception has
a much deeper inheritance tree below it.

It has also been argued that since KeyboardInterrupt and SystemExit
are not caught together that often in 2.4, there might not be much of
a need for TerminatingException.  But with their new position in the
hierarchy catching them separately from other exceptions should
become more prevalent.

Naming is based on the idea that the interpreter is trying to
terminate when KeyboardInterrupt and SystemExit
are raised.  An earlier proposal suggested
"TerminalException" but
avoidance of any confusion with an actual terminal along with
"terminal" being
more fatalistic than "terminating" led to the current name choice.


NotImplementedError
-------------------

Inherits from Exception instead of from RuntimeError.

Originally inheriting from RuntimeError, NotImplementedError does not
have any direct relation to the exception meant for use in user code
as a quick-and-dirty exception.  Thus it now directly inherits from
Exception.


Required Superclass for ``raise``
=================================

By requiring all objects passed to a ``raise`` statement to inherit
from a specific superclass, all exceptions are guaranteed to have
certain attributes.  If PEP 344 [#PEP344]_ is accepted, the attributes
outlined there will be guaranteed to be on all exceptions raised.
This should help facilitate debugging by making the querying of
information from exceptions much easier.

The proposed hierarchy has BaseException as the required base class.


Implementation
--------------

Enforcement is straightforward.  Modifying ``RAISE_VARARGS`` to do an
inheritance check first before raising an exception should be enough.
For the C API, all functions that set an exception will have the same
inheritance check applied.


Removal of Bare ``except`` Clauses
==================================

Bare ``except`` clauses serve the purpose of catching all exceptions
in Python 2.x .  This is needed thanks to the fact that in 2.x any
exception can be raised as an exception.  But if this PEP is accepted
this will no longer be the case because of a required superclass for
all raised objects.  
This goes against a part of the Zen of Python;
One Way To Do It (OWTDI) [#zen]_.  By having bare ``except`` clauses
keep their semantic meaning, there would be two ways of doing the
same things.  It also goes against Exlpicit Is Better Than Implicit
(EIBTI) by implicitly doing something that can easily be covered by
a more explicit statement.

It has also been proposed that bare ``except`` clauses be changed to
semantically be equivalent to ``except Exception``.  This has been
proposed since this is what most bare ``except`` clauses are meant to
do.  This would make ``except`` clauses have a more reasonable
default behavior.

But this line of reasoning has some issues.  First is that it also
goes against the Zen of Python; both OWTDI and EIBTI by providing
an implicit alternative to ``except Exception``.  Secondly,
backwards-compatibility becomes more difficult.  While removal will
break code, it can mechanically be changed to have the same semantic
meaning as it currently has by finding all occurances of ``except:``
and replacing them with ``except BaseException:``.  But going with
this semantic change makes compatibility more subtle.  Was a bare
``except`` clause meant to catch all exceptions, or actually meant
to catch all reasonable exceptions (i.e., everything but
TerminatingException)?  Every case will need to be carefully
examined, and could easily be incorrectly examined, leading to subtle
bugs.

The shorter typing afforded by bare ``except`` statements also does
not justify its existence.  "Exception", the typical exception that\
will be caught, is only nine characters.  Add in the needed space to
separate "Exception" from "except" you get only 10 more characters to
type.  While this might be a nuisance in quick-and-dirty scripts, the
impact is minimal.


Implementation
--------------

Changing Grammar/Grammar is all that is needed to remove bare
``except`` clauses.


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

Exception Hierarchy Changes
---------------------------

New Exceptions
''''''''''''''

New exceptions can simply be added to the built-in namespace.  Any
pre-existing objects with the same name will mask the new exceptions,
preserving backwards-compatibility.


New Inheritance for Old Exceptions
''''''''''''''''''''''''''''''''''

Using multiple inheritance to our advantage, exceptions whose
inheritance is now more resrictive can be made backwards-compatible.
By inheriting from both the new superclasses as well as the original
superclasses, existing ``except`` clauses will continue to work as
before while allowing the new inheritance to be used for new code.

A PendingDeprecationWarning will be raised based on whether the
bytecode ``COMPARE_OP(10)`` results in an exception being caught that
would not have under the new hierarchy.  This will require hard-coding
in the implementation of the bytecode.


Required Superclass for ``raise``
---------------------------------

A DeprecationWarning will be raised when an object is passed to
``raise`` that does not have the proper inheritance.


Removal of Bare ``except`` Clauses
----------------------------------

A PendingDeprecationWarning will be raised when a bare ``except``
clause is found in code.  One release before they are removed the
warning will be changed to DeprecationWarning.

Doing ``from __future__ import exceptions`` will cause bare
``except`` clauses to be considered syntax errors.


Roadmap
-------

Python 2.x is the first version that contains changes.  Python 3.0
will be when transition will be complete.  Version 3.0-1 represents
one version before 3.0 is released, 3.0-2 two versions before, etc.

* 2.x
    - Add BaseException, TerminatingException
    - Have KeyboardInterrupt and SystemExit inherit from both
      Exception and TerminatingException
    - Introduce ``from __future__ import exceptions``
    - Provide a script that mechanically changes all bare ``except``
      clauses to catch BaseException in a .py file
    - PendingDeprecationWarning for bare ``except`` clauses
    - PendingDeprecationWarning for all objects raised that do not
      inherit from BaseException
    - PendingDeprecationWarning raised when KeyboardInterrupt or
      SystemExit are caught because of their inheritance of Exception
* 3.0-1
    - Turn all introduced PendingDeprecationWarnings into
      DeprecationWarning
* 3.0
    - Remove DeprecationWarnings
    - Have KeyboardInterrupt and SystemExit only inherit from
      TerminatingException
    - Remove ``exceptions`` __future__ import support


Rejected Ideas
==============

Multiple threads on python-dev discussing this PEP have lead to
various ideas being rejected [#python-dev-thread1]_,
[#python-dev-thread2]_, [#python-dev-thread3]_.


DeprecationWarning Inheriting From PendingDeprecationWarning
------------------------------------------------------------

This was originally proposed because a DeprecationWarning can be
viewed as a PendingDeprecationWarning that is being removed in the
next version.  But since enough people thought the inheritance could
logically work the other way around, the idea was dropped.


AttributeError Inheriting From TypeError or NameError
-----------------------------------------------------

Viewing attributes as part of the interface of a type caused the idea
of inheriting from TypeError.  But that partially defeats the thinking
of duck typing and thus the idea was dropped.

Inheriting from NameError was suggested because objects can be viewed
as having their own namespace where the attributes live and when an
attribute is not found it is a namespace failure.  This was also
dropped as a possibility since not everyone shared this view.


Removal of EnvironmentError
---------------------------

Originally proposed based on the idea that EnvironmentError was an
unneeded distinction, the BDFL overruled this idea [#python-dev4]_.


Introduction of MacError and UnixError
--------------------------------------

Proposed to add symmetry to WindowsError, the BDFL said they won't be
used enough [#python-dev4]_.  The idea of then removing WindowsError
was proposed and accepted as reasonable, thus completely negating the
idea of adding these exceptions.


SystemError Subclassing SystemExit
----------------------------------

Proposed because a SystemError is meant to lead to a system exit, the
idea was removed since CriticalError indicates this better.


ControlFlowException Under Exception
------------------------------------

It has been suggested that ControlFlowException should inherit from
Exception.  This idea has been rejected based on the thinking that
control flow exceptions typically do not all need to be caught by a
single ``except`` clause.

Rename NameError to NamespaceError
----------------------------------

NameError is considered more succinct and leaves open no possible
mistyping of
the capitalization of "Namespace" [#python-dev5]_.


Renaming RuntimeError or Introducing SimpleError
''''''''''''''''''''''''''''''''''''''''''''''''

The thinking was that RuntimeError was in no way an obvious name for
an exception meant to be used when a situation did not call for the
creation of a new exception.  The renaming was rejected on the basis
that the exception is already used throughout the interpreter
[#python-dev6]_.
Rejection of SimpleError was founded on the thought that people
should be free to use whatever exception they choose and not have one
so blatently suggested [#python-dev7]_.

Renaming Existing Exceptions
----------------------------

Various renamings were suggested but non garnered more than a +0 vote
(renaming ReferenceError to WeakReferenceError).  The thinking was
that the existing names were fine and no one had actively complained
about them ever.  To minimize backwards-compatibility issues and
causing existing Python programmers extra pain, the renamings were
removed.

Have EOFError Subclass IOError
------------------------------

The original thought was that since EOFError deals directly with I/O,
it should
subclass IOError.  But since EOFError is used more as a signal that an
event
has occurred (the exhaustion of an I/O port), it should not subclass
such a specific error exception.


Have MemoryError and SystemError Have a Common Superclass
---------------------------------------------------------

Both classes deal with the interpreter, so why not have them have a
common
superclass?  Because one of them means that the interpreter is in a
state that it should not recover from while the other does not.


Common Superclass for PendingDeprecationWarning and DeprecationWarning
----------------------------------------------------------------------

Grouping the deprecation warning exceptions together makes intuitive
sense.
But this sensical idea does not extend well when one considers how
rarely either warning is used, let along at the same time.


Removing WindowsError
---------------------

Originally proposed based on the idea that having such a
platform-specific exception should not be in the built-in namespace.
It turns out, though, enough code exists that uses the exception to
warrant it staying.


Acknowledgements
================

Thanks to Robert Brewer, Josiah Carlson, Nick Coghlan, Timothy
Delaney, Jack Diedrich, Fred L. Drake, Jr., Philip J. Eby, Greg Ewing,
James Y. Knight, MA Lemburg, Guido van Rossum, Stephen J. Turnbull,
Raymond Hettinger, and everyone else I missed for participating in the
discussion.


References
==========

.. [#PEP342] PEP 342 (Coroutines via Enhanced Generators)
   http://www.python.org/peps/pep-0342.html

.. [#PEP344] PEP 344 (Exception Chaining and Embedded Tracebacks)
   http://www.python.org/peps/pep-0344.html

.. [#Summary2004-08-01] python-dev Summary (An exception is an
   exception, unless it doesn't inherit from Exception)
   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

.. [#python-dev3] python-dev email (PEP, take 2: Exception
   Reorganization for Python 3.0)
   http://mail.python.org/pipermail/python-dev/2005-August/055116.html

.. [#exceptions-stdlib] exceptions module
   http://docs.python.org/lib/module-exceptions.html

.. [#python-dev-thread1] python-dev thread (Pre-PEP: Exception
   Reorganization for Python 3.0)
   http://mail.python.org/pipermail/python-dev/2005-July/055020.html,
   http://mail.python.org/pipermail/python-dev/2005-August/055065.html

.. [#python-dev-thread2] python-dev thread (PEP, take 2: Exception
   Reorganization for Python 3.0)
   http://mail.python.org/pipermail/python-dev/2005-August/055103.html

.. [#python-dev-thread3] python-dev thread (Reorg PEP checked in)
    http://mail.python.org/pipermail/python-dev/2005-August/055138.html

.. [#python-dev4] python-dev email (Pre-PEP: Exception Reorganization
   for Python 3.0)
   http://mail.python.org/pipermail/python-dev/2005-July/055019.html

.. [#python-dev5] python-dev email (PEP, take 2: Exception Reorganization for
    Python 3.0)
    http://mail.python.org/pipermail/python-dev/2005-August/055159.html

.. [#python-dev6] python-dev email (Exception Reorg PEP checked in)
    http://mail.python.org/pipermail/python-dev/2005-August/055149.html

.. [#python-dev7] python-dev email (Exception Reorg PEP checked in)
    http://mail.python.org/pipermail/python-dev/2005-August/055175.html

.. [#zen] PEP 20 (The Zen of Python)
   http://www.python.org/peps/pep-0020.html


Copyright
=========

This document has been placed in the public domain.


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