Python - default locale is not working

Question

This is the situation:

In [1]: import datetime

In [2]: import locale

In [3]: locale.getlocale()
Out[3]: ('es_ES', 'UTF-8')

In [4]: locale.getdefaultlocale()
Out[4]: ('es_ES', 'UTF-8')

In [5]: datetime.datetime.strftime(datetime.datetime.today(), '%B')
Out[5]: 'July'

But the output should be julio

If I set the locale, then it works

In [6]: locale.setlocale(locale.LC_ALL, 'es_ES.UTF8')
Out[6]: 'es_ES.UTF8'

In [7]: datetime.datetime.strftime(datetime.datetime.today(), '%B')
Out[7]: 'julio'

It happens both in Python2 (2.7.10) and Python3 (3.5.0+)

The system locale is set to Spanish

LANG=es_ES.UTF-8
LANGUAGE=
LC_CTYPE="es_ES.UTF-8"
LC_NUMERIC=es_ES.UTF-8
LC_TIME=es_ES.UTF-8
LC_COLLATE="es_ES.UTF-8"
LC_MONETARY=es_ES.UTF-8
LC_MESSAGES="es_ES.UTF-8"
LC_PAPER=es_ES.UTF-8
LC_NAME=es_ES.UTF-8
LC_ADDRESS=es_ES.UTF-8
LC_TELEPHONE=es_ES.UTF-8
LC_MEASUREMENT=es_ES.UTF-8
LC_IDENTIFICATION=es_ES.UTF-8
LC_ALL=

EDIT:

Seeing the locale output, I realised that LC_ALL was not set. I checked /etc/default/locale as it is suggested here and I found a mixture of locales.

LANG="es_ES.UTF-8"
LC_ALL=
LC_NUMERIC="nl_NL.UTF-8"
LC_TIME="nl_NL.UTF-8"
LC_MONETARY="nl_NL.UTF-8"
LC_PAPER="nl_NL.UTF-8"
LC_NAME="nl_NL.UTF-8"
LC_ADDRESS="nl_NL.UTF-8"
LC_TELEPHONE="nl_NL.UTF-8"
LC_MEASUREMENT="nl_NL.UTF-8"
LC_IDENTIFICATION="nl_NL.UTF-8"

Maybe because I updated from 15.04 to 15.10?. Anyway, I filled LC_ALL and changed the rest of variables to es_ES, executing again locale-gen as root. However, even after reboot the system, the situation is the same.

Solution

Normally you should not set LC_ALL persistently as a global environment variable. If you do that on a desktop, you make the GUIs for setting languages/locales useless.

As regards your Python program, you need this line:

locale.setlocale(locale.LC_ALL, '')

It makes the locale, i.e. the output of the locale command, effective within the Python program.

Comments

[Hatshepsut]

Here's an issue in the bug tracker: bugs.python.org/issue29457

[Manuel]

The thing is that I am using a program (not written by me) that uses the locale module. However, when it prints information, it does not use the appropriate locale. Checking what could been happening, I discovered the issue I am describing.

[Gunnar Hjalmarsson]

@Manuel: Well, whoever wrote the program, it probably misses the line I suggested.