PyMOTW: atexit

The atexit module provides a simple interface to register
functions to be called when a program closes down normally. The
sys module also provides a hook, sys.exitfunc, but only one
function can be registered there. The atexit registry can be used
by multiple modules and libraries simultaneously.

Read more at atexit

PyMOTW: subprocess

The subprocess module provides a consistent interface to creating and
working with additional processes. It offers a higher-level interface
than some of the other available modules, and is intended to replace
functions such as os.system, os.spawn*, os.popen*, popen2.* and
commands.*. To make it easier to compare subprocess with those other
modules, this week I will re-create earlier examples using the functions
being replaced.

Read more at subprocess

PyMOTW: pickle and cPickle

The pickle module implements an algorithm for turning an arbitrary
Python object into a series of bytes (“serializing” the object). The
byte stream can then be transmitted or stored, and later reconstructed
to create a new object with the same characteristics.

The cPickle module implements the same algorithm, in C instead of
Python. It is many times faster than the Python implementation, but does
not allow the user to subclass from Pickle. If sub-classing is not
important for your use, you probably want to use cPickle.

Read more at pickle

DjangoKit help?

I spent a little time last night trying to assemble an application
using DjangoKit without much success.

I’m running Python 2.5 on a PowerBook with Mac OS 10.4. I downloaded
and installed PyObjC from source so it would compile (I thought) against
the right version of Python, then installed DjangoKit using
python install. Everything seemed to be working, and I was
able to build an application. But when I ran that app, it produced an
error about the version of the SQLite libraries being used (2 instead of
3) and missing libraries.

I gave up on Python 2.5, re-installed PyObjC and DjangoKit for 2.4 and
tried again. Same error.

Just for grins, I copied the app over to my wife’s laptop (she has a
MacBook Pro). The result was, of course, a new error about the platform.
No universal binaries? Really?

I’m sure there are options, or something, that I’m leaving out when I
build the app. This was mostly an experiment, and I was in a hurry, so I
gave up easily and just installed the django code I wanted on an
existing (Linux) web server and let her use that instead of messing with
a desktop application.

Has anyone else had more success building portable Python apps, esp.
with django, on Mac OS X?


Of course I knew better than to post in frustration when I posted this
originally. In my haste, I didn’t post sample code, the error message,
or much of the rest of the information I would have wanted if I was the
DjangoKit author trying to help someone out. Nonetheless, Tom did some
digging anyway and offered suggestions. Others did as well. Thanks! I
finally found time to follow up, and am coming closer to an answer.

Here are the full details

The application is very, very simple. The model just contains 2
classes for creating an index of a pile of Cook’s Illustrated magazine
we have laying around the house. There is no front-end, since the admin
views already provide the functionality she wanted. I thought I would be
cute and bundle it as a desktop app for Ms. PyMOTW, instead of setting
the app up on my web server. I have packaged the sample code and placed
it on my server.

I have included 2 separate files ( and I couldn’t package the source using the DjangoKit
version of setup:

$ python sdist --force-manifest
Loading 'initial_data' fixtures...
No fixtures found.
running sdist
warning: sdist: missing required meta-data: name, url
warning: sdist: manifest template '' does not exist (using default file list)
error: dist/Cook's Illustrated File name too long

I only seem to get the error if my dist directory includes the
application, too. Otherwise I get a minimal package with the name
‘UNKNOWN’. So, the tarball was packaged with a regular distutils That’s not a big problem, since it is easy to use separate

When I ran python py2app, the first time it
reported this error:

*** creating application bundle: Cook's Illustrated Index ***
error: can't copy 'media': doesn't exist or not a regular file

I eventually figured out (guessed) that even though I don’t have any
external media, I need a media directory at the same level in the
directory tree as the setup file. Creating the directory let me create
the app. Running that app gives me this traceback:

Traceback (most recent call last):
  File "/Users/dhellmann/Devel/personal/CooksIndex/trunk/dist/Cook's Illustrated", line 31, in
  File "/Users/dhellmann/Devel/personal/CooksIndex/trunk/dist/Cook's Illustrated", line 28, in _run
    execfile(path, globals(), globals())
  File "/Users/dhellmann/Devel/personal/CooksIndex/trunk/dist/Cook's Illustrated", line 9, in
    from pysqlite2 import dbapi2 as sqlite
ImportError: No module named pysqlite2

That brings the error reporting up to date, without trying any of the
suggestions in the comments, yet. As I mentioned, the code itself works
if I run django outside of the packaged application (from the command
line, etc.). So I’m confident that my own imports are valid, etc.

Based on a hint from Tom (in the comments, he suggests that I install
pysqlite2), I tried editing the file created inside the
application to import from sqlite3 instead of sqlite2. Editing the file
directly didn’t do it. Editing the copy already in my application
changed the error message to:

Traceback (most recent call last):
  File "/Users/dhellmann/Devel/personal/CooksIndex/trunk/dist/Cook's Illustrated", line 31, in
  File "/Users/dhellmann/Devel/personal/CooksIndex/trunk/dist/Cook's Illustrated", line 28, in _run
    execfile(path, globals(), globals())
  File "/Users/dhellmann/Devel/personal/CooksIndex/trunk/dist/Cook's Illustrated", line 10, in
    from sqlite3 import dbapi2 as sqlite
ImportError: No module named sqlite3

Next I tried editing the version of in
After removing the application and rebuilding, I see the same error.

So I finally broke down and installed the pysqlte2 package Tom
pointed out for me. The package seems to imply that it is for Python
2.4, and I’m running 2.5, but I installed it anyway.

The application Packaged with python 2.5 gave me “No module named
pysqlite2” when I ran it. Repackaged using “python2.4 py2app”, I got the app to run but it does not seem
to actually work. The console log shows this:

2007-06-23 15:07:28.403 Cook's Illustrated Index[14739] creating support folder /Users/dhellmann/Library/Application Support/DjangoKit/CooksIndex
2007-06-23 15:07:28.405 Cook's Illustrated Index[14739] installing default database
Starting web server on port 10557
Unhandled exception in thread started by
Traceback (most recent call last):
  File "/Users/dhellmann/Devel/personal/CooksIndex/trunk/dist/Cook's Illustrated", line 81, in startWebServer
    handler = AdminMediaHandler(WSGIHandler(), path)
TypeError: __init__() takes exactly 2 arguments (3 given)

So, I am a lot closer but not quite where I would like to be. I don’t
really care whether I package under 2.4 or 2.5, so long as the result
runs on my wife’s laptop, which doesn’t have any development packages

PyAtl Presentations

Last night’s Atlanta Python Meetup included several interesting
presentations. Living so far outside of Atlanta, it isn’t easy to make
it down for as many of the meetings as I would like, but it was
definitely worth the effort last night. We had a larger than usual
crowd, due to the fact that Google was sponsoring pizza and providing
speakers, all apparently part of their current recruiting drive.

Cary Hull of Google talked about twisted. I hadn’t ever really
looked at twisted closely, so the overview and examples he provided were
new material for me. It seems to be very informative and something worth
looking into, particularly since we are looking at redoing the
architecture of part of our system at work, and we will want to handle a
lot of sub-processes which might block on I/O from a number of sources.
We already know we want to use processes to take advantage of
multi-processor systems, but twisted seems to offer some nice tools for
managing those processes.

Sandwiched between Cary and the final presenter, Luis Caamano gave a
presentation on DynaCenter, our use of Pyro, and the event manager we
have built with it. Luis’ talk was our first “public” technical
presentation from Racemi, and it seemed to be well received.

After Luis, Dan Morrill of Google spoke on cross-site scripting
vulnerabilities. Lots of food for thought there, especially regarding
the trustworthiness of data coming from your own database. Dan is on the
web toolkit team at Google. He has obviously given similar presentations
before, and it was clear that he knew what he was talking about. Due to
a miscommunication about network access, the live demo he had planned
wasn’t possible. He wasn’t phased a bit, and proceeded to work up sample
code on the fly in front of the audience.

Noah also announced the formation of the PyAtl Book Club,
membership in which comes with a discount code for O’Reilly books. I’m
looking forward to participating, since it will be something I can do
without making that 90 minute drive. :-) If you are interested, even if
you aren’t in the Atlanta area, join our Google group.

S(a|i)mple Python Programs

Via Jesse Noller, I came across Steve Howell’s SimplePrograms
page in the Python wiki. What an excellent idea!

It’s a perfectly concise reference for program structure and common
usage patterns. It isn’t anywhere close to as exhaustive as the Python
Quick Reference
(can anything 59 pages long qualify as a “quick”
reference?) or the Python Phrasebook (another recent discovery, I
need to pick up a copy) – but that’s the point. It is a lot more
challenging to write concise examples than an exhaustive reference.
Thanks to Steve for a getting the ball rolling.

I do not see an example of raising exceptions. I’ll have to spend some
time thinking of the simplest program that would benefit from using