python-dev Summary for 2005-12-01 through 2005-12-15

Tony Meyer tony.meyer at
Fri Jan 20 05:33:33 CET 2006

[The HTML version of this Summary is available at]


Reminder: plain text documentation fixes are accepted

Want to help out with the Python documentation? Don't know LaTeX? No
problem! Plain text or ReST fixes are also welcome. You won't be able
to produce a diff file like with a normal patch, but comments that
explain how to fix the docs are just as good. A form like "in section
XXX right before the paragraph starting with YYY, the documentation
should say ZZZ" will make it easy for the doc maintainers to apply
your fix.

Contributing thread:

 - `c.l.p post on docs


New-style exceptions patch requesting review

With `PEP 352`_ ready_ for Guido's pronouncement, Michael Hudson has
asked for a few more developers to review his patch_ to make all
exceptions new-style. It should be basically ready, but it would be
nice to have a few eyes for sanity checks, documentation, and
compilations on the various platforms.

.. _PEP 352:
.. _ready:
.. _patch:

Contributing threads:

 - `New-style exceptions patch (was Re: PEP 8 updates/clarifications)
 - `New-style exceptions patch


Debugging lib available from ActiveState

Trent Mick confirmed that ActiveState do distribute a separate
distribution of debug DLLs for each Python release.  These can be
found by filling in the version number in the URL<version>

Contributing thread:

 - `Plea to distribute debugging lib



Updating the Python style guide

Ian Bicking kicked off a lengthy discussion about updating various
aspects of `PEP 8`_ (the Python code style guide), which resulted in a
substantial revision of the PEP.

PEP 8 stated that if a module defines a single exception raised for
all sorts of conditions, it is generally called "error" or "Error". 
Ian noted that other than some outlying cases (e.g. os.error,
socket.error), CapWords are always used.  He also wondered if
exceptions should have names that are relatively unique, or simply
unique within their namespace.  Finally, he requested a position be
taken on acronyms (e.g. HHTPRedirect or HttpRedirect).

Barry Warsaw pointed out that since exceptions are now classes, the
rules for naming classes should really apply; his preference is
CapWordsEndingInError rather than Error or error.  He also suggested
that acronym letters should be capitalised.  There was mostly
consensus on this, although some prefer shorter exception class names.

Guido wondered whether the function/method naming convention
(lower_case, mixedCase, or CapWords) was so controversial that the PEP
should not make a recommendation other than of consistency.  In the
end, however, the status quo (lower_case except for consistency
reasons) won out.  He was definite about module, package, and class
names, however, which should be all-lowercase without underscores
(modules/packages), or CapWords (class names).  (He noted that
standard library modules such as and that
break this rule should be changed).

PEP 8 recommended appending an underscore rather than corrupted
spelling when a name conflicts with a reserved word e.g. class\_
rather than klass).  Ian suggested that a specific recommendation for
the class argument to classmethods be made; out of cls, klass and
class\_ he preferred cls.  He further suggested that, as self is not
used outside of the first argument of instance methods, whatever
spelling of the class argument is used should not be used elsewhere
(e.g. cls for the class argument, and class\_ elsewhere).  This was
generally accepted.

A further suggestion from Barry's `Mailman style guide`_ was also
made, that from-imports should follow non-from imports, and dotted
imports should follow non-dotted imports.  Non-dotted imports should
be grouped by increasing length, while dotted imports should be
grouped roughly alphabetically.  However, this was felt to be too
prescriptive; users should follow their own aesthetic taste.

Various other small modifications and improvements were suggested and
made, such as:

* PEP 8 stated that modules designed for use via "from M import \*"
should prefix their globals, internal functions, and internal classes
with an underscore.  Ian suggested that __all__ should be recommended
over using a leading underscore, which was done.
* PEP 8 was fairly dated in discussing inheritance and
public/non-public methods.  Ian suggested that the language should be
updated, and that property() should be discussed, and this was done.
* PEP 8 recommended that class-based exceptions are used rather than
string-based exceptions.  Ian suggested that language be changed to
recommend more strongly against string-based exceptions, which was
* To make the PEP more concise and succinct, references to Emacs were removed.

There was also a sub-thread that considered when to use normal
instance variables and when to use accessor methods.  Jim Fulton
suggested that attributes should be used if the accessors are trivial,
whereas Skip Montanaro preferred more frequent use of accessor
methods, to avoid incorrect usage of classes.  Phillip J. Eby's
opinion was that normal instance variables should be used, until there
is a need to do something when they change, at which point properties
should be introduced.  PEP 8 now recommends simple attribute access
over accessors.

One distinction is how people interpret "public" and "private". 
Skip's opinion is that just because an attribute doesn't start with an
underscore doesn't mean that it is implicitly declared public, and
that people should familiarise themselves with the callable methods of
an object and only use direct access if the necessary methods aren't
provided.  Jim felt that the API should be documented, and people
should use that API (he considered that prepending an underscore
documents that the attribute is internal; although this does not cover
the "subclassing API").

Ian also brought up the section of PEP 8 that discussed private
attributes and double leading underscores.  PEP 8 was unclear whether
double leading attributes should only be used to avoid name conflicts
when subclassing, or whether indicating that an attribute was meant to
be private was a valid use.  This spun off a very long discussion
about private variables.

Several people spoke up against double-underscore prefixes.  On the
other hand, Raymond Hettinger felt that the PEP should not dictate how
variables should be named, especially for a convention with a long
history and language support.  Jim Fulton went so far as to suggest
that __private be deprecated, which gained some support.  However,
Guido stated that he not only liked __private, he liked the current
implementation.  Specifically, he felt that the typically-shallow
class hierarchies found in Python reduces the likelihood of accidental
reuse of class names (he also advocated that subclasses should not
share names with parent classes), and he liked that the name-mangling
algorithm was well-defined and simple (e.g. so that when in the
debugger it is easy to manually mangle/unmangle names). 
Interestingly, this is the main reason that Jeremy Hylton dislikes
mangled names: because the debugger is unaware of the names and he
can't remember how to type them, and also because it's annoying to
have to change every instance of __var to _var if the intended usage
changes (as compared to C++ private variables, where only the
declaration of visibility must change).  He noted, though, that these
are problems that tools (debuggers, editors, IDEs) could solve.

Others felt that keeping mangling was fine, but that it should be
changed so that collisions were harder (e.g. use a GUID instead of the
class name).  However, this violates one of Guido's reasons for liking
the current implementation (although it's possible that having better
support for the feature in common tools would remove this necessity).

Tim Peters gave a nice explanation of why the name mangling is provided::

  If your utility or mixin base class `A` has a data member named `n`,
  nobody deriving from `A` dare name one of their data members `n` too,
  and it's unreasonable to expect everyone deriving from `A` to learn and
  avoid all the names `A` uses internally.  It's even more unreasonable
  for A's author to have to promise, after A's first release, never to
  change the name of, or introduce any new, attribute (A's author dare
  not, lest the new name conflict with a name someone else's subclass used).
  If A's author names the attribute `__n` instead, all those problems go
  away, provided only that some subclass doesn't also name itself `A`.

Contributing threads:

 - `PEP 8 updates/clarifications
 - `Deprecate __ private (was Re: PEP 8 updates/clarifications)
 - `Import order (was Re: PEP 8 updates/clarifications)
 - `Deprecate __ private (was Re: PEP 8updates/clarifications)
 - `PEP 8 updates/clarifications, function/method style

 .. _PEP 8:
 .. _Mailman style guide:


ElementTree in the Python 2.5 stdlib

Skip Montanaro forwarded to python-dev a bit of a comp.lang.python
thread asking why ElementTree wasn't part of the standard library. He
and others argued that it was "best of breed" with a wide user base,
and that it met the other requirements_ for stdlib inclusion like
having an active maintainer, Fredrik Lundh. After a relatively brief
discussion, Fredrik got the ball rolling, setting things up so that:

* The ``external`` directory at the top of the SVN tree contains
snapshots of his celementtree and elementtree packages
* The ``etree`` subpackage of the ``xml`` module contains the
ElementTree, cElementTree, ElementPath and ElementInclude modules

Mike Brown started a brief follow-up thread concerned that the XML-SIG
hadn't been consulted on this inclusion.  Martin v. Löwis and others
indicated that sidestepping XML-SIG had not been intentional, but also
that carrying the discussion on XML-SIG as well would not likely have
changed the outcome due to the strong community support for

As a side effect of this discussion, magic in ``xml/__init__`` was
removed in favor of introducing an ``xmlcore`` package containing all
the xml modules found in the basic standard library, and an ``xml``
module which imports things from ``xmlcore`` or PyXML if it's

.. _requirements:

Contributing threads:

 - `ElementTree - Why not part of the core? (fwd)
 - `stupid package tricks
 - `ElementTree in stdlib
 - `Sharing expat instances
 - `"xml"; package in standard library


More work on the AST

This fortnight saw implementations for the two main models proposed
for revamping the AST memory management. Martin v. Löwis created a
branch_ that converted all AST objects to PyObjects and used the
normal Python reference counting to manage them. This requires that
AST code:

* initialize all PyObjects to NULL
* Py_XDECREF() each PyObject on exit
* goto an error block for Py_DECREF()ing if there is a failure

Jeremy Hylton worked up a separate version of the AST code using an
arena API.  This requires that AST code:

* call PyArena_AddPyObject() for any allocated PyObject
* call PyArena_AddMallocPointer() for any malloc()ed memory

The arena code was merged into the trunk, though it seemed that work
on the PyObject branch would continue in order to be able to compare
the final outcomes of both strategies.

In a related issue, because the AST code is generated from
Parser/, and because subversion sets the timestamp for each
file to the checkout time, trying to build the current trunk on a
machine without Python installed failed even when the AST C files had
been checked into subversion.  It looked like the best solution would
be to introduce a separate make rule for generating the AST code.

.. _branch:

Contributing threads:

 - `ast-objects branch created
 - `Memory management in the AST parser &amp; compiler
 - `PyAST_FromNode returning PyTypeObject* ?
 - `should I really have to install Python before I can build it ?
 - `should I really have to install Python before Ican build it ?
 - `should I really have to install Python before Icanbuild it ?


Python GC Refresher

Gregoire Weber asked for a little more information on Python's garbage
collector since the links in ``Modules/gcmodule.c`` were broken.
Python's garbage collector is a combination of reference counting and
the periodic invocation of a cyclic garbage collector. Python's
reference counting means that each time an object P is referenced by
another object, P's refcount is incremented, and each time one of the
references to P is removed, P's refcount is decremented.  When P's
refcount reaches zero, the interpreter pauses to reclaim P and all
objects that were reachable only from P. In addition to this reference
counting, Python's cyclic garbage collector means that after a large
number of objects have been allocated (and not deallocated though
reference counting), the interpreter will pause to try to clear out
any unreferenced cycles.

Contributing thread:

 - `Documentation about Python's GC, python-dev list messages
referenced in Modules/gcmodule.c not reachable anymore


Improving the documentation process

Skip Montanaro suggested that, in order to lower the barrier for
submitting documentation patches, it might be worth allowing anonymous
bug reports. Guido was against this, indicating that he thought the
problem was more the sourceforge sign-up hassle than the need to
provide an email address, and suggested that it might be time to
switch to roundup_. Martin v. Löwis was concerned about the transition
process - whether or not roundup could properly import the sourceforge
bug reports, and whether or not the<sf bug id> redirect
would continue to work. The discussion trailed off before any final
decisions were made.

.. _roundup:

Contributing threads:

 - `Tracker anonymity


hasattr() and properties

Thomas Lotze noticed that when applied to a class instance (but not an
object of that class), hasattr calls getattr and decides that the
attribute doesn't exist if the call raises any exception.  For

    >>> class Foo(object):
    ...     def get(self):
    ...         raise Exception
    ...     bar = property(get)
    >>> hasattr(Foo, "bar")
    >>> hasattr(Foo(), "bar")

He asked whether it would make more sense to only report a missing
attribute if an AttributeError is raised.  Greg Ewing agreed that this
would be an improvement, but felt that calling the property access
code as a side effect of hasattr seems like a misfeature.

Thomas also wondered whether it would make sense for properties to
look up the attribute in the same way that getattr would rather than
calling getattr.  Greg wondered if descriptors need a forth slot for
hasattr customisation, removing the need to rely on catching
exceptions, so that the logic would be::

   if there is a descriptor for the attribute:
     if the descriptor's hasattr slot is populated:
       return the result of calling it
       return True
     look in the instance dict for the attribute

Guido indicated that he believes that a property that has a side
effect (other than caching, statistics, logging, and so forth) is a
misfeature anyway, so he doesn't have a problem with hasattr() trying
getattr() and reporting False if that raises an exception.  Discussion
died out before anything was resolved.

Contributing thread:

 - `hasattr and properties


Supporting iterables in xmlrpclib

Skip Montanaro `presented a patch`_ to allow all currently supported
iterables (e.g. sets and arrays) to be marshalled as lists with
xmlrpclib.  The goals are to extend the set of sequences that can be
marshalled transparently, and keep the caller from caring as much
about the limitations of the XML-RPC datatypes.  Guido felt that this
was a bad idea, however; he feels that it's better to be aware of the
limitations in what XML-RPC can handle and try to handle it.

Contributing thread:

 - `Broader iterable support for xmlrpclib

.. _presented a patch:


Checking Jython support code into CPython

Fredrik Lundh asked what the policy with respect to Jython-specific
modules in the standard library was.  Guido felt that as long as it
didn't do any harm (likely causing unit tests to fail) to CPython, it
would be fine.  He noted that traditionally Jython-specific code has
been checked into Jython's own source control, however, and Martin v.
Löwis indicated that this is what he would prefer.

Contributing thread:

 - `Jython and CPython


Getting rid of __builtins__ in Python 3.0

Neal Norwitz asked Guido whether improving the confusing system of
having both __builtins__ and __builtin__ could be begun.  Guido
clarified that having both is clearly a bad idea, that he's not sure
that renaming builtin to __builtin__ was correct (and that perhaps it
should be changed back), that __builtins__ always being a dict would
simplify some code but need special handling of vars() in interactive
mode, and that another alternative might be to merge the __builtin__
and __builtins__ functionality (under the __builtin__ name).

Guido asked that people mull this over, but hasn't had any responses so far.

Contributing thread:

 - `__builtin__ vs __builtins__


Subject lines of python-commit emails

If you subscribe to python-checkins, you get email every time
something is updated.  Most people don't care about every change; the
subject lines were made a bit more useful for classification.

Contributing thread:

 - `Subject lines of commit email

[Jim Jewett]

Additional arguments for logging formatters

People would like to pass additional arguments to the logging
formatters.  Now they will be able to also pass a dict containing
extra values.  There were a few questions on the API; particularly on
what to do if the extra dict redefines some of the already documented

Contributing thread:

 - `Proposed additional keyword argument in logging calls

[Jim Jewett]

Correct location for dynmically linked/shared libraries

Since ElementTree is being added, so is cElementTree (not as good for
subclassing, but faster).  This used a statically compiled expat, but
there is already a statically compiled expat distributed with python.
The two uses have been merged, and there was some discussion about where
\*.so or \*.dll files should go if they represent only part of a package.

Contributing thread:

 - `Location of .so files (Was: Sharing expat instances)

[Jim Jewett]

Deferred Threads

 - `Linked lists

Skipped Threads

 - `Python bug day this Sunday
 - `os.normpath may change the meaning of the path if it contains
symbolic links?
 - `SVN backup?
 - `svn problem - can't get log info for a specific revision
 - `Patch reviews &amp; request for patch review
 - `Dynamic Link Library
 - `[Python-checkins] commit of r41586 - in python/trunk:
Lib/ Misc/NEWS
 - `Short-circuiting iterators
 - `Bug bz2.BZ2File(...).seek(0,2) + patch
 - `imaplib module with IDLE implememted via threads
 - `Exception type on handling closed files
 - `A missing piece of information in weakref documentation
 - `Directory for packages maintained outside the core (was Re:
ElementTree - Why not part of the core?)
 - `Incorporating external packages into Python's std distribution
 - `On moving to new-style classes
 - `Weekly Python Patch/Bug Summary
 - `Website cruft
 - `Add timeout to
 - `patch tracker ping: cross compile and mingw support
 - `Needed tester for patch in module


This is a summary of traffic on the `python-dev mailing list`_ from
December 01, 2005 through December 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
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This is the 9th summary written by the python-dev summary gathering of
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