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Add PEP 3110: Catching Exceptions in Python 3000

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Collin Winter 2007-02-05 22:05:46 +00:00
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pep-0000.txt
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@ -108,6 +108,7 @@ Index by Category
S 3107 Function Annotations Winter, Lownds S 3107 Function Annotations Winter, Lownds
I 3108 Standard Library Reorganization Cannon I 3108 Standard Library Reorganization Cannon
I 3109 Raising Exceptions in Python 3000 Winter I 3109 Raising Exceptions in Python 3000 Winter
I 3110 Catching Exceptions in Python 3000 Winter
Finished PEPs (done, implemented in Subversion) Finished PEPs (done, implemented in Subversion)
@ -449,6 +450,7 @@ Numerical Index
S 3107 Function Annotations Winter, Lownds S 3107 Function Annotations Winter, Lownds
I 3108 Standard Library Reorganization Cannon I 3108 Standard Library Reorganization Cannon
I 3109 Raising Exceptions in Python 3000 Winter I 3109 Raising Exceptions in Python 3000 Winter
I 3110 Catching Exceptions in Python 3000 Winter
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PEP: 3110
Title: Catching Exceptions in Python 3000
Version: $Revision$
Last-Modified: $Date$
Author: Collin Winter <collinw@gmail.com>
Status: Draft
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 [#pep352]_, 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 [#pep344]_, 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 "there should be one -- and preferably only one
-- obvious way to do it" [#zen]_, 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 [#pep3100]_ 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.
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 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.
Open Issues
===========
"except" Statements in Python 2.x
-----------------------------------
It has been proposed that the grammar for ``except`` statements be
changed to accommodate both Python 2's ``,`` and Python 3's ``as``
between the statement's type and target portions. The grammar
would thus change from ::
except_clause: 'except' [test [',' test]]
to ::
except_clause: 'except' [test [('as' | ',') test]]
It has not been decided whether the proposed end-of-suite cleanup
semantic for ``except`` statements should be included in the 2.x
series.
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()``.
References
==========
.. [#pep352]
http://www.python.org/dev/peps/pep-0352/
.. [#zen]
http://www.python.org/dev/peps/pep-0020/
.. [#sys-module]
http://docs.python.org/lib/module-sys.html
.. [#pep3100]
http://www.python.org/dev/peps/pep-3100/
.. [#pep344]
http://www.python.org/dev/peps/pep-0344/
.. [#firstproposal]
http://mail.python.org/pipermail/python-dev/2006-March/062449.html
.. [#renaming]
http://mail.python.org/pipermail/python-dev/2006-March/062640.html
.. [#grammar]
http://www.python.org/doc/current/ref/try.html
.. [#except-translation]
http://mail.python.org/pipermail/python-3000/2007-January/005395.html
.. [#translation-checkin]
http://svn.python.org/view?rev=53342&view=rev
.. [#2to3]
http://svn.python.org/view/sandbox/trunk/2to3/
.. [#exceptfixer]
http://svn.python.org/view/sandbox/trunk/2to3/fixes/fix_except.py
.. [#drop-excinfo]
http://mail.python.org/pipermail/python-3000/2007-January/005385.html
.. [#replace-excinfo]
http://mail.python.org/pipermail/python-3000/2007-January/005604.html
Copyright
=========
This document has been placed in the public domain.
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