[Python-Dev] DRAFT: python-dev Summary for 2005年09月16日 to 2005年09月30日
Tony Meyer
tony.meyer at gmail.com
Thu Nov 17 01:36:32 CET 2005
It's been some time (all that concurrency discussion didn't help ;)
but here's the second half of September. Many apologies for the
delay; hopefully you agree with Guido's 'better late than never', and
I promise to try harder in the future. Note that the delay is all my
bad, and epithets should be directed at me and not Steve. As usual,
please read over if you have a chance, and direct comments/
corrections to tony.meyer at gmail.com or steven.bethard at gmail.com.
(One particular question is whether the concurrency summary is too
long).
=============
Announcements
=============
-----------------------------
QOTF: Quotes of the fortnight
-----------------------------
We have two quotes this week, one each from the two biggest threads
of this fortnight: concurrency and conditional expressions. The
first quote, from Donovan Barda, puts Python's approach to threading
into perspective:
The reality is threads were invented as a low overhead way of
easily implementing concurrent applications... ON A SINGLE PROCESSOR.
Taking into account threading's limitations and objectives, Python's
GIL is the best way to support threads. When hardware (seriously)
moves to multiple processors, other concurrency models will start to
shine.
Our second QOTF, by yours truly (hey, who could refuse a nomination
from Guido?), is a not-so-subtle reminder to leave syntax decisions
to Guido:
Please no more syntax proposals! ... We need to leave the syntax
to Guido. We've already proved that ... we can't as a community
agree on a syntax. That's what we have a BDFL for. =)
Contributing threads:
- `GIL, Python 3, and MP vs. UP <http://mail.python.org/pipermail/
python-dev/2005-September/056609.html>`__
- `Adding a conditional expression in Py3.0 <http://mail.python.org/
pipermail/python-dev/2005-September/056617.html>`__
[SJB]
-------------------
Compressed MSI file
-------------------
Martin v. Lˆwis discovered that a little more than a `MiB`_ in the
Python installer by using LZX:21 instead of the standard MSZIP when
compressing the CAB file. After confirmation from several testers
that the new format worked, the change (for Python 2.4.2 and beyond)
was made.
.. _MiB: http://en.wikipedia.org/wiki/Mibibyte
Contributing thread:
- `Compressing MSI files: 2.4.2 candidate? <http://mail.python.org/
pipermail/python-dev/2005-September/056694.html>`__
[TAM]
=========
Summaries
=========
-----------------------
Conditional expressions
-----------------------
Raymond Hettinger proposed that the ``and`` and ``or`` operators be
modified in Python 3.0 to produce only booleans instead of producing
objects, motivating this proposal in part by the common (mis-)use of
``<cond> and <true-expr> or <false-expr>`` to emulate a conditional
expression. In response, Guido suggested that that the conditional
expression discussion of `PEP 308`_ be reopened. This time around,
people seemed almost unanimously in support of adding a conditional
expression, though as before they disagreed on syntax. Fortunately,
this time Guido cut the discussion short and pronounced a new syntax:
``<true-expr> if <cond> else <false-expr>``. Although it has not
been implemented yet, the plan is for it to appear in Python 2.5.
.. _PEP 308: http://www.python.org/peps/pep-0308.html
Contributing threads:
- `"and" and "or" operators in Py3.0 <http://mail.python.org/
pipermail/python-dev/2005-September/056510.html>`__
- `Adding a conditional expression in Py3.0 <http://mail.python.org/
pipermail/python-dev/2005-September/056546.html>`__
- `Conditional Expression Resolution <http://mail.python.org/
pipermail/python-dev/2005-September/056846.html>`__
[SJB]
---------------------
Concurrency in Python
---------------------
Once again, the subject of removing the global interpreter lock (GIL)
came up. Sokolov Yura suggested that the GIL be replaced with a
system where there are thread-local GILs that cooperate to share
writing; Martin v. Lˆwis suggested that he try to implement his
ideas, and predicted that he would find that doing so would be a lot
of work, would require changes to all extension modules (likely to
introduce new bugs, particularly race conditions), and possibly
decrease performance. This kicked off several long threads about
multi-processor coding.
A long time ago (circa Python 1.5), Greg Ward experimented with free
threading, which did yield around a 1.6 times speedup on a dual-
processor machine. To avoid the overhead of multi-processor locking
on a uniprocessor machine, a separate binary could be distributed.
Some of the code apparently did make it into Python 1.5, but the
issue died off because no-one provided working code, or a strategy
for what to do with existing extension modules.
Guido pointed out that it is not clear at this time how multiple
processors will be used as they become the norm. With the treaded
programming model (e.g. in Java) there are problems with concurrent
modification errors (without locking) or deadlocks and livelocks
(with locking). Guido's hunch (and mine, FWIW) is that instead of
writing massively parallel applications, we will continue to write
single-threaded applications that are tied together at the process
level rather than at the thread level. He also pointed out that it's
likely that most problems get little benefit out of multiple processors.
Guido threw down the gauntlet: rather than the endless discussion
about this topic, someone should come up with a GIL-free Python (not
necessarily CPython) and demonstrate its worth. Phillip J. Eby
reminded everyone that Jython, IronPython, and PyPy exist, and that
someone could, for example, create a multiprocessor-friendly backend
for PyPy.
Guido also pointed out that fast threading benefits from fast context
switches, which benefits from small register sets, and that the
current trend in chips is towards larger register sets. In addition,
multiple processors with shared memory don't scale all that well
(multiple processors with explicit interprocess communication (IPC)
channels scale much better). These all favour multi-processing over
multi-threading. Donovan Baarda went so far as to say (a QOTF, as
above), that Python's GIL is the best way to support threads, which
are for single-processor use, and that when multiple-processor
platforms have matured more other concurrency models will likewise
mature. OTOH, Bob Ippolito pointed out that (in many operating
systems) there isn't a lot of difference between threads and
processes, and that threads can typically still use IPC. Bob argued
that the biggest argument for threading is that lots of existing C/C+
+ code uses threads.
Simon Percivall argued that the problem is that Python offers ("out
of the box") some support for multi-threaded programming, but little
for multi-process programming beyond the basics (e.g. data sharing,
communication, control over running processes, dealing out tasks to
be handled). Simon suggested that the best way to stop people
complaining about the GIL is to provide solid, standardized support
for multi-process programming. The idea of a "multiprocess" module
gained a reasonable amount of support.
Phillip J. Eby outlined an idea he is considering PEPifying, in which
one could switch all context variables (such as the Decimal context
and the sys.* variables) simulaneously and instantaneously when
changing execution contexts (like switching between coroutines). He
has a prototype implementation of the basic idea, which is less than
200 lines of Python and very fast. However, he pointed out that it's
not completely PEP-ready at this point, and he needs to continue
considering various parts of the concept.
Bruce Eckel joined the thread, and suggested that low-level threads
people are only now catching up to objects, but as far as concurrency
goes their brains still think in terms of threads, so they naturally
apply thread concepts to objects. He believes that pthread-style
thinking is two steps backwards: you effectively throw open the
innards of the object that you just spent time decoupling from the
rest of your system, and the coupling is not unpredictable.
Bruce and Guido had discussed offlist "active objects": defining a
class as "active" would install a worker thread and concurrent queue
in each object of that class, automatically turn method calls into
tasks and enqueue them, and prevent any other interaction other than
enqueued messages. Guido felt that if multiple active objects could
co-exist in the same process, but be prevented (by the language
implementation) from sharing data except via channels, and dynamic
reallocation of active objects across multiple CPUs were possible,
then this might be a solution. He pointed out that an implementation
would really be needed to prove this.
Phillip and Martin pointed out that preventing any other interacton
other than enqueued messages is the difficult part; each active
object would, for example, have to have its own sys.modules. Phillip
felt that such a solution (which Bruce posed as "a" solution, not
"the" solution) wouldn't help with GIL removal, but would help with
effective use of multiprocessor machines on platforms where fork() is
available, if the API works across processes as well as threads.
Bruce then restarted the discussion, putting forth eight criteria
that he felt would be necessary for the "pythonic" solution to
concurrency. Items on the list were discussed further, with some
disagreement about what was possible. The concurrency discussion
continues next month...
Contributing threads:
- `Variant of removing GIL. <http://mail.python.org/pipermail/python-
dev/2005-September/056423.html>`__
- `GIL, Python 3, and MP vs. UP (was Re: Variant of removing GIL.)
<http://mail.python.org/pipermail/python-dev/2005-September/
056458.html>`__
- `GIL, Python 3, and MP vs. UP <http://mail.python.org/pipermail/
python-dev/2005-September/056498.html>`__
- `Active Objects in Python <http://mail.python.org/pipermail/python-
dev/2005-September/056752.html>`__
- `Pythonic concurrency <http://mail.python.org/pipermail/python-dev/
2005-September/056801.html>`__
- `Pythonic concurrency - cooperative MT <http://mail.python.org/
pipermail/python-dev/2005-September/056860.html>`__
[TAM]
-----------------------------------
Removing nested function parameters
-----------------------------------
Brett Cannon proposed removing support for nested function parameters
so that instead of being able to write::
def f((x, y)):
print x, y
you'd have to write something like::
def f(arg):
x, y = arg
print x, y
Brett (with help from Guido) motivated this removal (for Python 3.0)
by a few factors:
(1) The feature has low visibility: "For every user who is fond of
them there are probably ten who have never even heard of it." - Guido
(2) The feature can be difficult to read for some people.
(3) The feature doesn't add any power to the language; the above
functions emit essentially the same byte-code.
(4) The feature makes function parameter introspection difficult
because tuple unpacking information is not stored in the function
object.
In general, people were undecided on this proposal. While a number
of people said they used the feature and would miss it, many of them
also said that their code wouldn't suffer that much if the feature
was removed. No decision had been made at the time of the summary.
Contributing thread:
- `removing nested tuple function parameters <http://mail.python.org/
pipermail/python-dev/2005-September/056459.html>`__
[SJB]
-----------------------------------------
Evaluating iterators in a boolean context
-----------------------------------------
In Python 2.4 some builtin iterators gained __len__ methods when the
number of remaining items could be made available. This broke some
of Guido's code that tested iterators for their boolean value (to
distinguish them from None). Raymond Hettinger (who supplied the
original patch) argued that `testing for None`_ using boolean tests
was in general a bad idea, and that knowing the length of an
iterator, when possible, had a number of use cases and allowed for
some performance gains. However, Guido felt strongly that iterators
should not supply __len__ methods, as this would lead to some people
writing code expecting this method, which would then break when it
received an iterator which could not determine its own length. The
feature will be rolled back in Python 2.5, and Raymond will likely
move the __len__ methods to private methods in order to maintain the
performance gains.
.. _testing for None: http://www.python.org/peps/
pep-0290.html#testing-for-none
Contributing threads:
- `bool(iter([])) changed between 2.3 and 2.4 <http://mail.python.org/
pipermail/python-dev/2005-September/056576.html>`__
- `bool(container) [was bool(iter([])) changed between 2.3 and 2.4]
<http://mail.python.org/pipermail/python-dev/2005-September/
056879.html>`__
[SJB]
--------------------------------------------------
Properties that only call the getter function once
--------------------------------------------------
Jim Fulton proposed adding a new builtin for a property-like
descriptor that would only call the getter method once, so that
something like::
class Spam(object):
@readproperty
def eggs(self):
... expensive computation of eggs
self.eggs = result
return result
would only do the eggs computation once. Currently, you can't do
this with a property() because the ``self.eggs = result`` statement
tries to call the property's ``fset`` method instead of replacing the
property with the result of the eggs() call. A few other people
commented that they'd needed similar functionality at times, and
Guido seemed moderately interested in the idea, but there was no
final resolution.
Contributing thread:
- `RFC: readproperty <http://mail.python.org/pipermail/python-dev/
2005-September/056769.html>`__
[SJB]
--------
Codetags
--------
Micah Elliott submitted his `Codetags PEP 350`_ (after revisions
following the comp.lang.python discussion) to python-dev for
comment. A common feeling was that this (particularly synonyms) was
over-engineering; Guido pointed out that he only uses XXX, and this
is certainly the most common (although not only) example in the
Python source itself. Some suggestions were made, many of which
Micah integrated into the PEP.
The suggestion was made that an implementation should precede
approval of the PEP. Micah indicated that he would continue
development on the tools, and that he encourages anyone interested in
using a standard set of codetages to give these a try.
.. _Codetags PEP 350: http://python.org/peps/pep-0350.html
- `PEP 350: Codetags <http://mail.python.org/pipermail/python-dev/
2005-September/056744.html>`__
[TAM]
----------------------------
Improving set implementation
----------------------------
Raymond Hettinger suggested a "small, but interesting, C project" to
determine whether the setobject.c implementation would be improved by
recoding the set_lookkey() function to optimize key insertion order
using Brent's variation of Algorithm D (c.f. Knuth vol. III, section
6.4, p525). It has the potential to boost performance for
uniquification applications with duplicate keys being identified more
quickly, and possibly also more frequent retirement of dummy entires
during insertion operations.
Andrew Durdin pointed out that Brent's variation depends on the next
probe position for a key being derivation from the key and it current
position, which is incompatible with the current perturbation system;
Raymond replaced perturbation with a secondary hash with linear
probing. Antoine Pitrou did some `experimenting with this`_,
resulting in a -5% to 2% speedup with various benchmarks.
Raymond has also been experimenting with a simpler approach: whenever
there are more than three probes, always swap the new key into the
first position and then unconditionally re-insert the swapped-out
key. He reported that, most of the time, this gives an improvement,
and it doesn't require changing the perturbation logic. This simpler
approach is cheap to implement, but the benefits are also smaller,
with it improving only the worse collisions.
.. _experimenting with this: http://pitrou.net/python/sets
- `C coding experiment <http://mail.python.org/pipermail/python-dev/
2005-September/055965.html>`__
[TAM]
--------------
Relative paths
--------------
Nathan Bullock suggested a ''relpath(path_a, path_b)'' addition to
os.path that returns a relative path from path_a to path_b. Trent
Mick pointed out that there are a `couple of`_ `recipes for this`_,
as well as `Jason Orendorff's Path module`_. Several people
supported this idea, and hopefully either Nathan or one of the recipe
authors will submit a patch with this functionality.
.. _couple of: http://aspn.activestate.com/ASPN/Cookbook/Python/
Recipe/302594
.. _recipes for this: http://aspn.activestate.com/ASPN/Cookbook/
Python/Recipe/208993
.. _Jason Orendorff's Path module: http://www.jorendorff.com/articles/
python/path/
Contributing threads:
- `os.path.diff(path1, path2) <http://mail.python.org/pipermail/
python-dev/2005-September/056391.html>`__
- `os.path.diff(path1, path2) (and a first post) <http://
mail.python.org/pipermail/python-dev/2005-September/056703.html>`__
[TAM]
----------------------------------
Adding a vendor-packages directory
----------------------------------
Rich Burridge followed up a `comp.lang.python thread`_ about a
"vendor-packages" directory for Python by submitting a `patch`_ and
asking for comments about the proposal on python-dev. General
consensus was that the proposal needed a better rationale, explaining
why this improved on simply adding a .pth file to the site-packages
directory.
Rich explained that the rationale is that Python files supplied by
the vendor (Sun, Apple, RedHat, Microsoft) with their operating
system software should go in a separate base directory to
differentiate them from Python files installed specifically at the
site. However, Bob Ippolito pointed out that, as of OS X 10.4
("Tiger") Apple already does this via a .pth file ("Extras.pth"),
which points to ''/System/Library/Frameworks/Python.framework/
Versions/2.3/Extras/lib/python'' and includes wxPython by default.
Bob also pointed out that such a "vendor-packages.pth" should look
like ''import site; site.addsitedir('/usr/lib/python2.4/vendor-
packages')'' so that packages like Numeric, PIL, and PyObjC, which
take advantage of .pth files themselves, work when installed to the
vendor-packages location.
Phillip J. Eby pointed out that it would be good to have a document
for "Python Distributors" that explained these kind of things, and
suggested that perhaps a volunteer or two could be found within the
distutils-SIG to do this.
.. _comp.lang.python thread: http://mail.python.org/pipermail/python-
list/2005-September/300029.html
.. _patch: http://sourceforge.net/tracker/index.php?
func=detail&aid=1298835&group_id=5470&atid=305470
Contributing thread:
- `vendor-packages directory <http://mail.python.org/pipermail/python-
dev/2005-September/056682.html>`__
[TAM]
=======================
Version numbers on OS X
=======================
Guido asked if platform.system_alias() could be improved on OS X by
mapping uname()'s ''Darwin x.y'' to ''OS X 10.(x-4).y''. Bob
Ippolito and others pointed out that this was not a good idea,
because uname() only reports on the kernel version number and not the
Cocoa API, which is really what OS X 10.x.y refers to. He pointed
out that the correct way to do it using a public API is to used
gestalt, which is what platform.mac_ver() does.
On further inspection, it was discovered that parsing the /System/
Library/CoreServices/SystemVersion.plist property list is also a
supported API, and would not rely on access to the Carbon API set.
Bob and Wilfredo S·nchez Vega provided sample code that would parse
this plist; Marc-Andre Lemburg suggested that a patch be written for
system_alias() that would use this method (if possible) for Mac OS.
Contributing thread:
- `Mapping Darwin 8.2.0 to Mac OS X 10.4.2 in platform.py <http://
mail.python.org/pipermail/python-dev/2005-September/056651.html>`__
[TAM]
================
Deferred Threads
================
- `Python 2.5a1, ast-branch and PEP 342 and 343 <http://
mail.python.org/pipermail/python-dev/2005-September/056449.html>`__
===============
Skipped Threads
===============
- `Visibility scope for "for/while/if" statements <http://
mail.python.org/pipermail/python-dev/2005-September/056669.html>`__
- `inplace operators and __setitem__ <http://mail.python.org/
pipermail/python-dev/2005-September/056766.html>`__
- `Repository for python developers <http://mail.python.org/pipermail/
python-dev/2005-September/056717.html>`__
- `For/while/if statements/comprehension/generator expressions
unification <http://mail.python.org/pipermail/python-dev/2005-
September/056508.html>`__
- `list splicing <http://mail.python.org/pipermail/python-dev/2005-
September/056472.html>`__
- `Compatibility between Python 2.3.x and Python 2.4.x <http://
mail.python.org/pipermail/python-dev/2005-September/056437.html>`__
- `python optimization <http://mail.python.org/pipermail/python-dev/
2005-September/056441.html>`__
- `test__locale on Mac OS X <http://mail.python.org/pipermail/python-
dev/2005-September/056463.html>`__
- `possible memory leak on windows (valgrind report) <http://
mail.python.org/pipermail/python-dev/2005-September/056478.html>`__
- `Mixins. <http://mail.python.org/pipermail/python-dev/2005-
September/056481.html>`__
- `2.4.2c1 fails test_unicode on HP-UX ia64 <http://mail.python.org/
pipermail/python-dev/2005-September/056551.html>`__
- `2.4.2c1: test_macfs failing on Tiger (Mac OS X 10.4.2) <http://
mail.python.org/pipermail/python-dev/2005-September/056558.html>`__
- `test_ossaudiodev hangs <http://mail.python.org/pipermail/python-
dev/2005-September/056559.html>`__
- `unintentional and unsafe use of realpath() <http://mail.python.org/
pipermail/python-dev/2005-September/056616.html>`__
- `Alternative name for str.partition() <http://mail.python.org/
pipermail/python-dev/2005-September/056630.html>`__
- `Weekly Python Patch/Bug Summary <http://mail.python.org/pipermail/
python-dev/2005-September/056713.html>`__
- `Possible bug in urllib.urljoin <http://mail.python.org/pipermail/
python-dev/2005-September/056736.html>`__
- `Trasvesal thought on syntax features <http://mail.python.org/
pipermail/python-dev/2005-September/056741.html>`__
- `Fixing pty.spawn() <http://mail.python.org/pipermail/python-dev/
2005-September/056750.html>`__
- `64-bit bytecode compatibility (was Re: [PEAK] ez_setup on 64-bit
linux problem) <http://mail.python.org/pipermail/python-dev/2005-
September/056811.html>`__
- `C API doc fix <http://mail.python.org/pipermail/python-dev/2005-
September/056827.html>`__
- `David Mertz on CA state e-voting panel <http://mail.python.org/
pipermail/python-dev/2005-September/056840.html>`__
- `[PATCH][BUG] Segmentation Fault in xml.dom.minidom.parse <http://
mail.python.org/pipermail/python-dev/2005-September/056844.html>`__
- `linecache problem <http://mail.python.org/pipermail/python-dev/
2005-September/056856.html>`__
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