python-peps/peps/pep-3110.rst

PEP: 3110
Title: Catching Exceptions in Python 3000
Version: $Revision$
Last-Modified: $Date$
Author: Collin Winter <collinwinter@google.com>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 16-Jan-2006
Python-Version: 3.0
Post-History:


Abstract
========

This PEP introduces changes intended to help eliminate ambiguities
in Python's grammar, simplify exception classes, simplify garbage
collection for exceptions and reduce the size of the language in
Python 3.0.


Rationale
=========

1. ``except`` clauses in Python 2.x present a syntactic ambiguity
   where the parser cannot differentiate whether ::

       except <expression>, <expression>:

   should be interpreted as ::

       except <type>, <type>:

   or ::

       except <type>, <name>:

   Python 2 opts for the latter semantic, at the cost of requiring the
   former to be parenthesized, like so ::

    except (<type>, <type>):

2. As specified in :pep:`352`, the ability to treat exceptions
   as tuples will be removed, meaning this code will no longer work ::

       except os.error, (errno, errstr):

   Because the automatic unpacking will no longer be possible, it is
   desirable to remove the ability to use tuples as ``except`` targets.

3. As specified in :pep:`344`, exception instances in Python 3
   will possess a ``__traceback__`` attribute. The Open Issues section
   of that PEP includes a paragraph on garbage collection difficulties
   caused by this attribute, namely a "exception -> traceback ->
   stack frame -> exception" reference cycle, whereby all locals are
   kept in scope until the next GC run. This PEP intends to resolve
   this issue by adding a cleanup semantic to ``except`` clauses in
   Python 3 whereby the target name is deleted at the end of the
   ``except`` suite.

4. In the spirit of :pep:`"there should be one -- and preferably only one
   -- obvious way to do it" <20>`, it is desirable to consolidate
   duplicate functionality. To this end, the ``exc_value``,
   ``exc_type`` and ``exc_traceback`` attributes of the ``sys``
   module [#sys-module]_ will be removed in favor of
   ``sys.exc_info()``, which provides the same information. These
   attributes are already listed in :pep:`3100` as targeted
   for removal.


Grammar Changes
===============

In Python 3, the grammar for ``except`` statements will change
from [#grammar]_ ::

    except_clause: 'except' [test [',' test]]

to ::

    except_clause: 'except' [test ['as' NAME]]

The use of ``as`` in place of the comma token means that ::

    except (AttributeError, os.error):

can be clearly understood as a tuple of exception classes. This new
syntax was first proposed by Greg Ewing [#firstproposal]_ and
endorsed ([#firstproposal]_, [#renaming]_) by the BDFL.

Further, the restriction of the token following ``as`` from ``test``
to ``NAME`` means that only valid identifiers can be used as
``except`` targets.

Note that the grammar above always requires parenthesized tuples as
exception classes. That way, the ambiguous ::

    except A, B:

which would mean different things in Python 2.x and 3.x -- leading to
hard-to-catch bugs -- cannot legally occur in 3.x code.


Semantic Changes
================

In order to resolve the garbage collection issue related to :pep:`344`,
``except`` statements in Python 3 will generate additional bytecode to
delete the target, thus eliminating the reference cycle.
The source-to-source translation, as suggested by Phillip J. Eby
[#except-translation]_, is ::

    try:
        try_body
    except E as N:
        except_body
    ...

gets translated to (in Python 2.5 terms) ::

    try:
        try_body
    except E, N:
        try:
            except_body
        finally:
            N = None
            del N
    ...

An implementation has already been checked into the py3k (formerly
"p3yk") branch [#translation-checkin]_.


Compatibility Issues
====================

Nearly all ``except`` clauses will need to be changed. ``except``
clauses with identifier targets will be converted from ::

    except E, N:

to ::

    except E as N:

``except`` clauses with non-tuple, non-identifier targets
(e.g., ``a.b.c[d]``) will need to be converted from ::

    except E, T:

to ::

    except E as t:
        T = t

Both of these cases can be handled by Guido van Rossum's ``2to3``
utility [#2to3]_ using the ``except`` fixer [#exceptfixer]_.

``except`` clauses with tuple targets will need to be converted
manually, on a case-by-case basis. These changes will usually need
to be accompanied by changes to the exception classes themselves.
While these changes generally cannot be automated, the ``2to3``
utility is able to point out cases where the target of an ``except``
clause is a tuple, simplifying conversion.

Situations where it is necessary to keep an exception instance around
past the end of the ``except`` suite can be easily translated like so
::

    try:
        ...
    except E as N:
        ...
    ...

becomes ::

    try:
        ...
    except E as N:
        n = N
        ...
    ...

This way, when ``N`` is deleted at the end of the block, ``n`` will
persist and can be used as normal.

Lastly, all uses of the ``sys`` module's ``exc_type``, ``exc_value``
and ``exc_traceback`` attributes will need to be removed. They can be
replaced with ``sys.exc_info()[0]``, ``sys.exc_info()[1]`` and
``sys.exc_info()[2]`` respectively, a transformation that can be
performed by ``2to3``'s ``sysexcattrs`` fixer.

2.6 - 3.0 Compatibility
-----------------------

In order to facilitate forwards compatibility between Python 2.6 and 3.0,
the ``except ... as ...:`` syntax will be backported to the 2.x series. The
grammar will thus change from::

    except_clause: 'except' [test [',' test]]

to::

    except_clause: 'except' [test [('as' | ',') test]]

The end-of-suite cleanup semantic for ``except`` statements will not be
included in the 2.x series of releases.


Open Issues
===========

Replacing or Dropping "sys.exc_info()"
--------------------------------------

The idea of dropping ``sys.exc_info()`` or replacing it with a
``sys.exception`` attribute or a ``sys.get_exception()`` function
has been raised several times on python-3000 ([#drop-excinfo]_,
[#replace-excinfo]_) and mentioned in :pep:`344`'s "Open Issues" section.

While a ``2to3`` fixer to replace calls to ``sys.exc_info()``
and some attribute accesses would be trivial, it would be far more
difficult for static analysis to find and fix functions that expect
the values from ``sys.exc_info()`` as arguments. Similarly, this does
not address the need to rewrite the documentation for all APIs that
are defined in terms of ``sys.exc_info()``.


Implementation
==============

This PEP was implemented in revisions 53342 [#r53342]_ and 53349
[#r53349]_.  Support for the new ``except`` syntax in 2.6 was
implemented in revision 55446 [#r55446]_.


References
==========

.. [#sys-module]
   http://docs.python.org/library/sys.html

.. [#firstproposal]
   https://mail.python.org/pipermail/python-dev/2006-March/062449.html

.. [#renaming]
   https://mail.python.org/pipermail/python-dev/2006-March/062640.html

.. [#grammar]
   http://docs.python.org/reference/compound_stmts.html#try

.. [#except-translation]
   https://mail.python.org/pipermail/python-3000/2007-January/005395.html

.. [#translation-checkin]
   http://svn.python.org/view?rev=53342&view=rev

.. [#2to3]
   https://hg.python.org/sandbox/guido/file/2.7/Lib/lib2to3/

.. [#exceptfixer]
   https://hg.python.org/sandbox/guido/file/2.7/Lib/lib2to3/fixes/fix_except.py

.. [#drop-excinfo]
   https://mail.python.org/pipermail/python-3000/2007-January/005385.html

.. [#replace-excinfo]
   https://mail.python.org/pipermail/python-3000/2007-January/005604.html

.. [#r53342]
   http://svn.python.org/view?view=revision&revision=53342

.. [#r53349]
   http://svn.python.org/view?view=revision&revision=53349

.. [#r55446]
   http://svn.python.org/view/python/trunk/?view=rev&rev=55446


Copyright
=========

This document has been placed in the public domain.



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