|Title:||Catching Exceptions in Python 3000|
|Author:||Collin Winter <collinwinter at google.com>|
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.
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>:
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>):
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.
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.
In the spirit of "there should be one -- and preferably only one -- obvious way to do it"  , it is desirable to consolidate duplicate functionality. To this end, the exc_value , exc_type and exc_traceback attributes of the 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.
In Python 3, the grammar for except statements will change from 
except_clause: 'except' [test [',' test]]
except_clause: 'except' [test ['as' NAME]]
The use of as in place of the comma token means that
except (AttributeError, os.error):
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 clases. 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.
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  , 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  .
Nearly all except clauses will need to be changed. except clauses with identifier targets will be converted from
except E, N:
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:
except E as t: T = t
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: ... ...
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() , sys.exc_info() and sys.exc_info() respectively, a transformation that can be performed by 2to3 's sysexcattrs fixer.
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]]
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.
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 (  ,  ) 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() .
|||( 1 , 2 ) http://mail.python.org/pipermail/python-dev/2006-March/062449.html|
This document has been placed in the public domain.