[Python-Dev] Strange locale problem with Python 3 (original) (raw)

M.-A. Lemburg mal at egenix.com
Mon Feb 2 14:40:55 CET 2009

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 LCALL 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 "deCH.UTF-8". When manually setting LCALL=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 py3klocaleproblem.py) ... for 3.0 ... $ (cd build/lib*-3.0; python3.0 py3klocaleproblem.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 eGenix.com

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