Hi
While helping Brandon Rhodes to port PyEphem[1] to Python 3 we
struggled over a
strange locale-related problem on OS X. PyEphem is a library which can
do
astronomical computations like tracking the position of stars, planets
and
earth satellites relative to the earth's position. When testing out
the Python
3 release of PyEphem I noticed that on my OS X laptop a lot of
calculations
were wrong (not completely wrong, but clearly not accurate) compared
to Python
2.5. We (well mostly Brandon) were able to track down the problem to
the TLE
parser (TLE are data file containing the orbital elements of an
object) which
appears to read most values wrong with python 3. In fact it cut of the
decimal
parts of all floats (1.123232 got 1, etc). Manually setting LANG and
LC_ALL to
C solved the problem.
It turns out that some parts of Python 3 got more locale dependent on
some
platforms. The only platform I am aware of is OS X, on Linux Python 3
appears
to behave like Python 2.x did.
In case of PyEphem the problem was in the C extension which got more
locale
dependent, for example atof() or scanf() with Python 3 now expected
the german
decimal-delimiter ',' instead of the '.' in floats (3.14 vs. 3,14). On
the
other hand the constructor of float still expects '.' all the time.
But the
built-in function strptime() honors locales with Python 3 and expects
german
week day.
I've written a simple script and a simple C extension which
illustrates the
problem. Both the extension and the script run python 2.x and python
3, so you
can easily compare the result while executing the script in different
environments.
I was only able to reproduce the problem on OS X (10.5) and using a
german
locale like "de_CH.UTF-8". When manually setting LC_ALL=C, the
differences
disappears.
I can't imagine that his behavior was really intended, and I hope the
test case
helps you guys to identify/fix this problem.
Download the test case from:
http://github.com/retoo/py3k-locale-problem/tarball/master
or get it using git:
git://github.com/retoo/py3k-locale-problem.git
You can use the following steps to build it:
$ python2.5 setup.py build
$ python3.0 setup.py build
To run the tests with python 2.5, enter:
$ (cd build/lib*-2.5; python2.5 py3k_locale_problem.py)
... for 3.0 ...
$ (cd build/lib*-3.0; python3.0 py3k_locale_problem.py)
In the file 'results.txt' you can see the output from my OS X system.
Cheers,
Reto Schüttel
[1] http://rhodesmill.org/pyephem/
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