PEP: 3110 Title: Catching Exceptions in Python 3000 Version: $Revision$ Last-Modified: $Date$ Author: Collin Winter 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 , : should be interpreted as :: except , : or :: except , : Python 2 opts for the latter semantic, at the cost of requiring the former to be parenthesized, like so :: except (, ): 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. 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. 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. 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 ========== .. [#pep352] http://www.python.org/dev/peps/pep-0352/ .. [#zen] http://www.python.org/dev/peps/pep-0020/ .. [#sys-module] http://docs.python.org/library/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://docs.python.org/reference/compound_stmts.html#try .. [#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 .. [#r53342] http://svn.python.org/view/python/branches/p3yk/?view=rev&rev=53342 .. [#r53349] http://svn.python.org/view/python/branches/p3yk/?view=rev&rev=53349 .. [#r55446] http://svn.python.org/view/python/trunk/?view=rev&rev=55446 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 coding: utf-8 End: