Reset locale setting in qtconsole

[phillipberndt]

The setlocale() man-page states that

On startup of the main program, the portable "C" locale is selected as default.

The QT backend (or PyQT or QT itself) apparently makes a call to setlocale() according to my computer's settings:

In [1]: from ctypes import CDLL, c_char_p
In [2]: c = CDLL("libc.so.6")
In [3]: c.setlocale.restype = c_char_p
In [4]: c.setlocale(0)
Out[4]: 'de_DE.UTF-8'

I noticed this when I tried to use Cantera (a chemical kinetics library with a Python interface) from IPython and got different results depending on whether I used the QTConsole or not. Turns out they make a call to atof()/strtod() without overriding the locale setting beforehand, which they probably do because they rely on the statement from the man-page and figure that a user willingly overriding this value would know what he is doing.

In my opinion, IPython's state after startup should not depend on the choice of the frontend. I therefore suggest that you either reset the locale setting (must be done using setlocale(), updating the environment is not enough) after initializing QT or, alternatively, call setlocale(0, "") when the QT-backend is not used.

(I will also file a bug against Cantera, proposing to issue a warning when an unexpected locale is set.)

[takluyver]

The kernel isn't actually loading Qt, though - the frontend runs in a separate process. And indeed, when I try that code in the Qt console:

In [1]: from ctypes import CDLL, c_char_p
In [2]: c = CDLL("libc.so.6")
In [3]: c.setlocale.restype = c_char_p
In [4]: c.setlocale(0)

Out[5]: 'C'

[takluyver]

(To avoid doubt: the Out[] number is 5 because I copied and pasted the whole block into input cell 5 - I didn't sneakily add in another line to get that output)

[phillipberndt]

Ah, thanks. This narrows it down. Whether or not the locale is set must depend on --pylab then:

$ ipython --pylab=qt test.py

Welcome to pylab, a matplotlib-based Python environment [backend: Qt4Agg].
For more information, type 'help(pylab)'.
de_DE.UTF-8
$ ipython --pylab=tk test.py

Welcome to pylab, a matplotlib-based Python environment [backend: TkAgg].
For more information, type 'help(pylab)'.
C

If I use python directly, I can also reproduce the problem, but only after opening at least one window: (test.py is a script with the commands from above)

$ python
Python 2.6.6 (r266:84292, Dec 26 2010, 22:31:48) 
[GCC 4.4.5] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import matplotlib
>>> matplotlib.use("Qt4Agg")
>>> import pylab
>>> matplotlib.interactive(True)
>>> execfile("test.py")
C
>>> pylab.plot(1,2)
[<matplotlib.lines.Line2D object at 0x28bd190>]
>>> execfile("test.py")
de_DE.UTF-8

So there must be some initialization in ipython causing QT to invoke setlocale() before actually opening a plot window. However, I guess the place to resolve this is upstream. :-) I will report the problem to @matplotlib.

[takluyver]

OK, thanks. I'll close this issue then.