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 "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[1] 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[2] :

    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[3] and endorsed ([4],[5]) 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 [6], 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[7].

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[8]
using the except fixer[9].

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 ([10], [11]) 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[12] and 53349 [13]. Support
for the new except syntax in 2.6 was implemented in revision 55446[14].

References

Copyright

This document has been placed in the public domain.



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[1] http://docs.python.org/library/sys.html

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

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

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

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

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

[7] http://svn.python.org/view?rev=53342&view=rev

[8] https://hg.python.org/sandbox/guido/file/2.7/Lib/lib2to3/

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

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

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

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

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

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