[Python-Dev] Strange locale problem with Python 3

[M.-A. Lemburg]

On 2009-02-01 19:44, Reto Schüttel wrote:
> 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.

This is probably due to the unconditional call to setlocale() in
pythonrun.c:

        /* Set up the LC_CTYPE locale, so we can obtain
           the locale's charset without having to switch
           locales. */
        setlocale(LC_CTYPE, "");

In Python 2, no such call is made and as a result the C lib defaults
to the "C" locale.

Calling setlocale() in an application is always dangerous due to the
many side-effects this can have on the C lib parsing and formatting
APIs.

If this is done just to figure the environment's locale settings,
then it's better to reset the locale to the one that was active
before the setlocale() call. Python 2 uses this approach. Python 3
does not.

> 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/

-- 
Marc-Andre Lemburg
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