[Python-Dev] python-dev Summary for 2005-08-01 through 2005-08-15 [draft]

Tony Meyer t-meyer at ihug.co.nz
Thu Aug 25 10:51:18 CEST 2005

Here's August Part One.  As usual, if anyone can spare the time to proofread
this, that would be great!  Please send any corrections or suggestions to
Steve (steven.bethard at gmail.com) and/or me, rather than cluttering the
list.  Ta!


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
<http://mail.python.org/pipermail/python-dev/2005- August/055105.html>`__


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 it  itself a Process PEP.

.. _PEP 1: http://www.python.org/peps/pep-0001.html

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`_,  and `Monotone`_.  Whichever system is
moved to, it should be able to be hosted somewhere (if  *.python.org, 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 appropriate.

The thread included a short discussion about the authentication mechanism
that svn.python.org 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 the 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 SVN.

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 svn.python.org 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: http://www.python.org/peps/pep-0347.html
.. _svk: http://svk.elixus.org/
.. _Perforce: http://www.perforce.com/
.. _Subversion: http://subversion.tigris.org/
.. _Monotone: http://venge.net/monotone/
.. _Bazaar-NG: http://www.bazaar-ng.org/

Contributing threads:

- `PEP: Migrating the Python CVS to Subversion
<http://mail.python.org/pipermail/python-dev/2005- August/055064.html>`__
- `PEP 347: Migration to Subversion
<http://mail.python.org/pipermail/python-dev/2005- August/055211.html>`__
- `Hosting svn.python.org
- `Fwd: Distributed RCS
- `cvs to bzr?
- `Distributed RCS
- `Fwd: PEP: Migrating the Python CVS to Subversion
<http://mail.python.org/pipermail/python-dev/2005 -August/055388.html>`__
- `On distributed vs centralised SCM for Python
<http://mail.python.org/pipermail/python-dev/2005- August/055432.html>`__


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: http://www.python.org/peps/pep-0348.html

Contributing threads:

- `Pre-PEP: Exception Reorganization for Python 3.0
<http://mail.python.org/pipermail/python-dev/2005 -August/055063.html>`__
- `PEP, take 2: Exception Reorganization for Python 3.0
<http://mail.python.org/pipermail/python- dev/2005-August/055103.html>`__
- `Exception Reorg PEP checked in
<http://mail.python.org/pipermail/python-dev/2005- August/055138.html>`__
- `PEP 348: Exception Reorganization for Python 3.0
<http://mail.python.org/pipermail/python-dev/2005 -August/055162.html>`__
- `Major revision of PEP 348 committed
<http://mail.python.org/pipermail/python-dev/2005- August/055199.html>`__
- `Exception Reorg PEP revised yet again
<http://mail.python.org/pipermail/python-dev/2005- August/055292.html>`__
- `PEP 348 and ControlFlow
- `PEP 348 (exception reorg) revised again
<http://mail.python.org/pipermail/python-dev/2005- August/055412.html>`__


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 open in
text mode

However, none of these seemed to be solid commitments.

.. _PEP 349: http://www.python.org/peps/pep-0349.html

Contributing threads:

- `PEP: Generalised String Coercion
<http://mail.python.org/pipermail/python-dev/2005- August/055186.html>`__
- `Generalised String Coercion
<http://mail.python.org/pipermail/python-dev/2005- August/055194.html>`__


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

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: http://www.python.org/peps/pep-0344.html

Contributing thread:

- `__traceback__ and reference cycles
<http://mail.python.org/pipermail/python-dev/2005- August/055251.html>`__


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: http://www.python.org/peps/pep-0008.html
.. _3000: http://www.python.org/peps/pep-3000.html

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?
<http://mail.python.org/pipermail/python-dev/2005- August/055218.html>`__
- `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 his new API for discussion,  but
there didn't appear to be any comments.

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?
<http://mail.python.org/pipermail/python- dev/2005-August/055062.html>`__
- `Syscall Proxying in Python
- `__autoinit__ (Was: Proposal: reducing self.x=x; self.y=y; self.z=z
boilerplate code)
- `Weekly Python Patch/Bug Summary
<http://mail.python.org/pipermail/python-dev/2005- August/055110.html>`__
- `PEP 342 Implementation
- `String exceptions in Python source
<http://mail.python.org/pipermail/python-dev/2005- August/055155.html>`__
- `[ python-Patches-790710 ] breakpoint command lists in pdb
<http://mail.python.org/pipermail/python -dev/2005-August/055157.html>`__
- `[C++-sig] GCC version compatibility
<http://mail.python.org/pipermail/python-dev/2005- August/055219.html>`__
- `PyTuple_Pack added references undocumented
<http://mail.python.org/pipermail/python-dev/2005- August/055232.html>`__
- `PEP-- Context Managment variant
<http://mail.python.org/pipermail/python-dev/2005- August/055271.html>`__
- `Sourceforge CVS down?
- `PSF grant / contacts
- `Python + Ping
- `Terminology for PEP 343
- `dev listinfo page (was: Re: Python + Ping)
<http://mail.python.org/pipermail/python-dev/2005- August/055348.html>`__
- `set.remove feature/bug
- `Extension to dl module to allow passing strings from native function
- `build problems on macosx (CVS HEAD)
<http://mail.python.org/pipermail/python-dev/2005- August/055385.html>`__
- `request for code review - hashlib - patch #1121611
<http://mail.python.org/pipermail/python- dev/2005-August/055410.html>`__
- `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
<http://mail.python.org/pipermail/python-dev/2005- August/055450.html>`__
- `xml.parsers.expat no userdata in callback functions
<http://mail.python.org/pipermail/python- dev/2005-August/055362.html>`__

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