python-dev Summary for 2005-08-01 through 2005-08-15

Tony Meyer t-meyer at
Tue Aug 30 02:45:30 CEST 2005

[The HTML version of this Summary is available at]


QOTF: Quote of the Fortnight

Some wise words from Donovan Baarda in the PEP 347 discussions:

    It is true that some well designed/developed software becomes reliable
very quickly. However, it still takes heavy use over time to prove that.

Contributing thread:

- `PEP: Migrating the Python CVS to Subversion


Process PEPs

The PEP editors have introduced a new PEP category: "Process", for PEPs that
don't fit into the "Standards Track" and "Informational" categories.  More
detail can be found in `PEP 1`_, which is itself a Process PEP.

.. _PEP 1:

Contributing thread:

- `new PEP type: Process


Tentative Schedule for 2.4.2 and 2.5a1 Releases

Python 2.4.2 is tentatively scheduled for a mid-to-late September release
and a first alpha of Python 2.5 for March 2006 (with a final release around
May/June).  This means that a PEP for the 2.5 release, detailing what will
be included, will likely be created soon; at present there are various
accepted PEPs that have not yet been implemented.

Contributing thread:

- `plans for 2.4.2 and 2.5a1



Moving Python CVS to Subversion

The `PEP 347`_ discussion from last fortnight continued this week, with a
revision of the PEP, and a lot more discussion about possible version
control software (RCS) for the Python repository, and where the repository
should be hosted.  Note that this is not a discussion about bug trackers,
which will remain with Sourceforge (unless a separate PEP is developed for
moving that).

Many revision control systems were extensively discussed, including
`Subversion`_ (SVN), `Perforce`_, `Mercurial`_, and `Monotone`_.  Whichever
system is moved to, it should be able to be hosted somewhere (if
*, then it needs to be easily installable), needs to have
software available to convert a repository from CVS, and ideally would be
open-source; similarity to CVS is also an advantage in that it requires a
smaller learning curve for existing developers.  While Martin isn't willing
to discuss every system there is, he will investigate those that make him
curious, and will add other people's submissions to the PEP, where

The thread included a short discussion about the authentication mechanism
that will use; svn+ssh seems to be a clear winner, and a test
repository will be setup by Martin next fortnight.

The possibility of moving to a distributed revision control system
(particularly `Bazaar-NG`_) was also brought up.  Many people liked the idea
of using a distributed revision control system, but it seems unlikely that
Bazaar-NG is mature enough to be used for the main Python repository at the
current time (a move to it at a later time is possible, but outside the
scope of the PEP).  Distributed RCS are meant to reduce the barrier to
participation (anyone can create their own branches, for example); Bazaar-NG
is also implemented in Python, which is of some benefit.  James Y Knight
pointed out `svk`_, which lets developers create their own branches within

In general, the python-dev crowd is in favour of moving to SVN.  Initial
concern about the demands on the volunteer admins should the repository be
hosted at were addressed by Barry Warsaw, who believes that
the load will be easily managed with the existing volunteers.  Various
alternative hosts were discussed, and if detailed reports about any of them
are created, these can be added to the PEP.

While the fate of all PEPS lie with the BDFL (Guido), it is likely that the
preferences of those that frequently check in changes, the pydotorg admins,
and the release managers (who have all given favourable reports so far),
will have a significant effect on the pronouncement of this PEP.

.. _PEP 347:
.. _svk:
.. _Perforce:
.. _Subversion:
.. _Monotone:
.. _Bazaar-NG:
.. _Mercurial:

Contributing threads:

- `PEP: Migrating the Python CVS to Subversion
- `PEP 347: Migration to Subversion
- `Hosting
- `Fwd: Distributed RCS
- `cvs to bzr?
- `Distributed RCS
- `Fwd: PEP: Migrating the Python CVS to Subversion
- `On distributed vs centralised SCM for Python


PEP 348: Exception Hierarchy in Python 3.0

This fortnight mostly concluded the previous discussion about `PEP 348`_,
which sets out a roadmap for changes to the exception hierarchy in Python
3.0. The proposal was heavily scaled back to retain most of the current
exception hierarchy unchanged.  A new exception, BaseException, will be
introduced above Exception in the current hierarchy, and KeyboardInterrupt
and SystemExit will become siblings of Exception.  The goal here is that::

    except Exception:

will now do the right thing for most cases, that is, it will catch all the
exceptions that you can generally recover from.  The PEP would also move
NotImplementedError out from under RuntimeError, and alter the semantics of
the bare except so that::


is the equivalent of::

    except Exception:

Only BaseException will appear in Python 2.5.  The remaining modifications
will not occur until Python 3.0.

.. _PEP 348:

Contributing threads:

- `Pre-PEP: Exception Reorganization for Python 3.0
- `PEP, take 2: Exception Reorganization for Python 3.0
- `Exception Reorg PEP checked in
- `PEP 348: Exception Reorganization for Python 3.0
- `Major revision of PEP 348 committed
- `Exception Reorg PEP revised yet again
- `PEP 348 and ControlFlow
- `PEP 348 (exception reorg) revised again


Moving towards Unicode

Neil Schemenauer presented `PEP 349`_, which tries to ease the transition to
Python 3.0, in which there will be a bytes() type for byte data and a str()
type for text data.  Currently to convert an object to text, you have one of
three options:

* Call str(). This breaks with a UnicodeEncodeError if the object is of type
unicode (or a subtype) or can only represent itself in unicode and therefore
returns unicode from __str__.
* Call unicode(). This can break external code that is not yet Unicode-safe
and that passed a str object to your code but got a unicode object back.
* Use the "%s" format specifier. This breaks with a UnicodeEncodeError if
the object can only represent itself in unicode and therefore returns
unicode from __str__.

`PEP 349`_ attempts to address this problem by introducing a text() builtin
which returns str or unicode instances unmodified, and returns the result of
calling __str__() on the object otherwise. Guido preferred to instead relax
the restrictions on str() to allow it to return unicode objects. Neil
implemented such a patch, and found that it broke only two test cases. The
discussion stopped shortly after Neil's report however, so it was unclear if
any permanent changes had been agreed upon.

Guido made a few other Python 3.0 suggestions in this thread:

* The bytes() type should be mutable with a corresponding frozenbytes()
immutable type
* Opening a file in binary or text mode would cause it to return bytes() or
str() objects, respectively
* The file type should grow a getpos()/setpos() pair that are identical to
tell()/seek() when a file is open in binary mode, and which work like
tell()/seek() but on characters instead of bytes when a file is opened in
text mode.

However, none of these seemed to be solid commitments.

.. _PEP 349:

Contributing threads:

- `PEP: Generalised String Coercion
- `Generalised String Coercion


PEP 344 and reference cycles

Armin Rigo brought up an issue with `PEP 344`_ which proposes, among other
things, adding a __traceback__ attribute to exceptions to avoid the hassle
of extracting it from sys.exc_info(). Armin pointed out that if exceptions
grow a __traceback__ attribute, every statement::

    except Exception, e:

will create a cycle::


Despite the fact that Python has cyclic garbage collection, there are still
some situations where cycles like this can cause problems. Armin showed an
example of such a case::

    class X:
        def __del__(self):
            except Exception, e:
                e_type, e_value, e_tb = sys.exc_info()

Even in current Python, instances of the X class are uncollectible. When
garbage collection runs and tries to collect an X object, it calls the
__del__() method.  This creates the cycle::


The X object itself is available through this cycle (in
``f_locals['self']``), so the X object's refcount does not drop to 0 when
__del__() returns, so it cannot be collected.  The next time garbage
collection runs, it finds that the X object has not been collected, calls
its __del__() method again and repeats the process.

Tim Peters suggested this problem could be solved by declaring that
__del__() methods are called exactly once. This allows the above X object to
be collected because on the second run of the garbage collection, __del__()
is not called again.  Thus, the refcount of the X object is not incremented,
and so it is collected by garbage collection.  However, guaranteeing that
__del__() is called only once means keeping track somehow of which objects'
__del__() methods were called, which seemed somewhat unattractive.

There was also brief talk about removing __del__ in favor of weakrefs, but
those waters seemed about as murky as the garbage collection ones.

.. _PEP 344:

Contributing thread:

- `__traceback__ and reference cycles


Style for raising exceptions

Guido explained that these days exceptions should always be raised as::

    raise SomeException("some argument")

instead of::

    raise SomeException, "some argument"

The second will go away in Python 3.0, and is only present now for backwards
compatibility.  (It was necessary when strings could be exceptions, in order
to pass both the exception "type" and message.)  PEPs 8_ and 3000_ were
accordingly updated.

.. _8:
.. _3000:

Contributing threads:

- `PEP 8: exception style
- `FW: PEP 8: exception style


Skipping list comprehensions in pdb

When using pdb, the only way to skip to the end of a loop is to set a
breakpoint on the line after the loop.  Ilya Sandler suggested adding an
optimal numeric argument to pdb's "next" comment to indicate how many lines
of code should be skipped.  Martin v. Löwis pointed out that this differs
from gdb's "next <n>" command, which does "next" n times.  Ilya suggested
implementing gdb's "until" command instead, which gained Martin's approval.

It was also pointed out that pdb is one of the less Pythonic modules,
particularly in terms of the ability to subclass/extend, and would be a good
candidate for rewriting, if anyone had the inclination and time.

Contributing threads:

- `pdb: should next command be extended?
- `an alternative suggestion, Re: pdb: should next command be extended?


Sets in Python 2.5

Raymond Hettinger has been checking-in the new implementation for sets in
Python 2.5.  The implementation is based heavily on dictobject.c, the code
for Python dict() objects, and generally deviates only when there is an
obvious gain in doing so.  Raymond posted a proposed C API sets; no comments
were forthcoming and it was implemented for Py2.5 without changes.

Contributing threads:

- `[Python-checkins] python/dist/src/Objects setobject.c, 1.45, 1.46
- `Discussion draft: Proposed Py2.5 C API for set and frozenset objects


Deferred Threads (for next time) ================================

- `SWIG and rlcompleter

Skipped Threads

- `Extension of struct to handle non byte aligned values?
- `Syscall Proxying in Python
- `__autoinit__ (Was: Proposal: reducing self.x=x; self.y=y; self.z=z
boilerplate code)
- `Weekly Python Patch/Bug Summary
- `PEP 342 Implementation
- `String exceptions in Python source
- `[ python-Patches-790710 ] breakpoint command lists in pdb
- `[C++-sig] GCC version compatibility
- `PyTuple_Pack added references undocumented
- `PEP-- Context Managment variant
- `Sourceforge CVS down?
- `PSF grant / contacts
- `Python + Ping
- `Terminology for PEP 343
- `dev listinfo page (was: Re: Python + Ping)
- `set.remove feature/bug
- `Extension to dl module to allow passing strings from native function
- `build problems on macosx (CVS HEAD)
- `request for code review - hashlib - patch #1121611
- `python-dev Summary for 2005-07-16 through 2005-07-31 [draft]
- `string_join overrides TypeError exception thrown in generator
- `implementation of copy standard lib
- `xml.parsers.expat no userdata in callback functions


This is a summary of traffic on the `python-dev mailing list`_ from August
01, 2005 through August 15, 2005. It is intended to inform the wider Python
community of on-going developments on the list on a semi-monthly basis.  An
archive_ of previous summaries is available online.

An `RSS feed`_ of the titles of the summaries is available.
You can also watch comp.lang.python or comp.lang.python.announce for new
summaries (or through their email gateways of python-list or
python-announce, respectively, as found at

Tim Lesher has had to bow out from the summaries for now; many thanks to him
for the contributions he made over the last few months.

This is the 1st summary written by the python-dev summary confederacy of
Steve Bethard and Tony Meyer (Thanks Tim!).

To contact us, please send email:

- Steve Bethard (steven.bethard at
- Tony Meyer (tony.meyer at

Do *not* post to comp.lang.python if you wish to reach us.

The `Python Software Foundation`_ is the non-profit organization that holds
the intellectual property for Python.  It also tries to advance 
the development and use of Python.  If you find the python-dev Summary
helpful please consider making a donation.  You can make a donation at .  Every penny helps so even a small
donation with a credit card, check, or by PayPal helps.  

Commenting on Topics

To comment on anything mentioned here, just post to `comp.lang.python`_ (or
email python-list at which is a gateway to the newsgroup) with a
subject line mentioning what you are discussing.  All python-dev members are
interested in seeing ideas discussed by the community, so don't hesitate to
take a stance on something.  And if all of this really interests you then
get involved and join `python-dev`_!

How to Read the Summaries

The in-development version of the documentation for Python can be found at and should be used when looking up any
documentation for new code; otherwise use the current documentation as found
at .  PEPs (Python Enhancement Proposals) are
located at . To view files in the Python CVS
online, go to .
Reported bugs and suggested patches can be found at the SourceForge_ project

Please note that this summary is written using reStructuredText_. Any
unfamiliar punctuation is probably markup for reST_ (otherwise it is
probably regular expression syntax or a typo =); you can safely ignore it.
We do suggest learning reST, though; it's simple and is accepted for `PEP
markup`_ and can be turned into many different formats like HTML and LaTeX.
Unfortunately, even though reST is standardized, the wonders of programs
that like to reformat text do not allow us to guarantee you will be able to
run the text version of this summary through Docutils_ as-is unless it is
from the `original text file`_.

.. _python-dev:
.. _SourceForge:
.. _python-dev mailing list:
.. _comp.lang.python:
.. _PEP Markup:

.. _Docutils:
.. _reST:
.. _reStructuredText:
.. _PSF:
.. _Python Software Foundation:

.. _last summary:
.. _original text file:
.. _archive:
.. _RSS feed:

More information about the Python-announce-list mailing list