298 lines
8.0 KiB
Plaintext
298 lines
8.0 KiB
Plaintext
PEP: 3110
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Title: Catching Exceptions in Python 3000
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Version: $Revision$
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Last-Modified: $Date$
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Author: Collin Winter <collinwinter@google.com>
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Status: Final
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 16-Jan-2006
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Python-Version: 3.0
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Post-History:
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Abstract
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========
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This PEP introduces changes intended to help eliminate ambiguities
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in Python's grammar, simplify exception classes, simplify garbage
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collection for exceptions and reduce the size of the language in
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Python 3.0.
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Rationale
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=========
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1. ``except`` clauses in Python 2.x present a syntactic ambiguity
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where the parser cannot differentiate whether ::
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except <expression>, <expression>:
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should be interpreted as ::
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except <type>, <type>:
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or ::
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except <type>, <name>:
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Python 2 opts for the latter semantic, at the cost of requiring the
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former to be parenthesized, like so ::
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except (<type>, <type>):
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2. As specified in :pep:`352`, the ability to treat exceptions
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as tuples will be removed, meaning this code will no longer work ::
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except os.error, (errno, errstr):
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Because the automatic unpacking will no longer be possible, it is
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desirable to remove the ability to use tuples as ``except`` targets.
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3. As specified in :pep:`344`, exception instances in Python 3
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will possess a ``__traceback__`` attribute. The Open Issues section
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of that PEP includes a paragraph on garbage collection difficulties
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caused by this attribute, namely a "exception -> traceback ->
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stack frame -> exception" reference cycle, whereby all locals are
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kept in scope until the next GC run. This PEP intends to resolve
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this issue by adding a cleanup semantic to ``except`` clauses in
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Python 3 whereby the target name is deleted at the end of the
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``except`` suite.
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4. In the spirit of :pep:`"there should be one -- and preferably only one
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-- obvious way to do it" <20>`, it is desirable to consolidate
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duplicate functionality. To this end, the ``exc_value``,
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``exc_type`` and ``exc_traceback`` attributes of the ``sys``
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module [#sys-module]_ will be removed in favor of
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``sys.exc_info()``, which provides the same information. These
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attributes are already listed in :pep:`3100` as targeted
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for removal.
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Grammar Changes
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===============
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In Python 3, the grammar for ``except`` statements will change
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from [#grammar]_ ::
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except_clause: 'except' [test [',' test]]
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to ::
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except_clause: 'except' [test ['as' NAME]]
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The use of ``as`` in place of the comma token means that ::
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except (AttributeError, os.error):
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can be clearly understood as a tuple of exception classes. This new
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syntax was first proposed by Greg Ewing [#firstproposal]_ and
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endorsed ([#firstproposal]_, [#renaming]_) by the BDFL.
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Further, the restriction of the token following ``as`` from ``test``
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to ``NAME`` means that only valid identifiers can be used as
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``except`` targets.
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Note that the grammar above always requires parenthesized tuples as
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exception classes. That way, the ambiguous ::
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except A, B:
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which would mean different things in Python 2.x and 3.x -- leading to
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hard-to-catch bugs -- cannot legally occur in 3.x code.
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Semantic Changes
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================
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In order to resolve the garbage collection issue related to :pep:`344`,
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``except`` statements in Python 3 will generate additional bytecode to
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delete the target, thus eliminating the reference cycle.
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The source-to-source translation, as suggested by Phillip J. Eby
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[#except-translation]_, is ::
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try:
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try_body
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except E as N:
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except_body
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...
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gets translated to (in Python 2.5 terms) ::
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try:
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try_body
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except E, N:
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try:
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except_body
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finally:
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N = None
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del N
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...
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An implementation has already been checked into the py3k (formerly
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"p3yk") branch [#translation-checkin]_.
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Compatibility Issues
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====================
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Nearly all ``except`` clauses will need to be changed. ``except``
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clauses with identifier targets will be converted from ::
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except E, N:
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to ::
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except E as N:
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``except`` clauses with non-tuple, non-identifier targets
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(e.g., ``a.b.c[d]``) will need to be converted from ::
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except E, T:
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to ::
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except E as t:
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T = t
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Both of these cases can be handled by Guido van Rossum's ``2to3``
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utility [#2to3]_ using the ``except`` fixer [#exceptfixer]_.
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``except`` clauses with tuple targets will need to be converted
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manually, on a case-by-case basis. These changes will usually need
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to be accompanied by changes to the exception classes themselves.
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While these changes generally cannot be automated, the ``2to3``
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utility is able to point out cases where the target of an ``except``
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clause is a tuple, simplifying conversion.
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Situations where it is necessary to keep an exception instance around
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past the end of the ``except`` suite can be easily translated like so
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::
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try:
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...
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except E as N:
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...
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...
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becomes ::
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try:
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...
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except E as N:
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n = N
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...
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...
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This way, when ``N`` is deleted at the end of the block, ``n`` will
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persist and can be used as normal.
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Lastly, all uses of the ``sys`` module's ``exc_type``, ``exc_value``
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and ``exc_traceback`` attributes will need to be removed. They can be
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replaced with ``sys.exc_info()[0]``, ``sys.exc_info()[1]`` and
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``sys.exc_info()[2]`` respectively, a transformation that can be
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performed by ``2to3``'s ``sysexcattrs`` fixer.
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2.6 - 3.0 Compatibility
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-----------------------
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In order to facilitate forwards compatibility between Python 2.6 and 3.0,
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the ``except ... as ...:`` syntax will be backported to the 2.x series. The
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grammar will thus change from::
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except_clause: 'except' [test [',' test]]
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to::
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except_clause: 'except' [test [('as' | ',') test]]
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The end-of-suite cleanup semantic for ``except`` statements will not be
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included in the 2.x series of releases.
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Open Issues
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===========
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Replacing or Dropping "sys.exc_info()"
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--------------------------------------
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The idea of dropping ``sys.exc_info()`` or replacing it with a
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``sys.exception`` attribute or a ``sys.get_exception()`` function
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has been raised several times on python-3000 ([#drop-excinfo]_,
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[#replace-excinfo]_) and mentioned in :pep:`344`'s "Open Issues" section.
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While a ``2to3`` fixer to replace calls to ``sys.exc_info()``
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and some attribute accesses would be trivial, it would be far more
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difficult for static analysis to find and fix functions that expect
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the values from ``sys.exc_info()`` as arguments. Similarly, this does
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not address the need to rewrite the documentation for all APIs that
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are defined in terms of ``sys.exc_info()``.
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Implementation
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==============
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This PEP was implemented in revisions 53342 [#r53342]_ and 53349
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[#r53349]_. Support for the new ``except`` syntax in 2.6 was
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implemented in revision 55446 [#r55446]_.
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References
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==========
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.. [#sys-module]
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http://docs.python.org/library/sys.html
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.. [#firstproposal]
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https://mail.python.org/pipermail/python-dev/2006-March/062449.html
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.. [#renaming]
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https://mail.python.org/pipermail/python-dev/2006-March/062640.html
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.. [#grammar]
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http://docs.python.org/reference/compound_stmts.html#try
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.. [#except-translation]
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https://mail.python.org/pipermail/python-3000/2007-January/005395.html
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.. [#translation-checkin]
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http://svn.python.org/view?rev=53342&view=rev
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.. [#2to3]
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https://hg.python.org/sandbox/guido/file/2.7/Lib/lib2to3/
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.. [#exceptfixer]
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https://hg.python.org/sandbox/guido/file/2.7/Lib/lib2to3/fixes/fix_except.py
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.. [#drop-excinfo]
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https://mail.python.org/pipermail/python-3000/2007-January/005385.html
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.. [#replace-excinfo]
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https://mail.python.org/pipermail/python-3000/2007-January/005604.html
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.. [#r53342]
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http://svn.python.org/view?view=revision&revision=53342
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.. [#r53349]
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http://svn.python.org/view?view=revision&revision=53349
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.. [#r55446]
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http://svn.python.org/view/python/trunk/?view=rev&rev=55446
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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End:
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