GeezerVison

The problem with this is that it can change based on environment variables:

>>> locale.setlocale(locale.LC_TIME, '')
'ru_RU.utf8'
>>> os.environ['LC_ALL'] = 'en_US.utf8'
>>> locale.setlocale(locale.LC_TIME, '')
'en_US.utf8'

But the instance is supposed to capture the preferred locale when it's created.

Why is it so important for you to get the locale when Python is started? As you shown, there are many ways to change locales. And using an locale set before all the changes is irrelevant, so it's now outdated, no?

The previous code based on getdefaultlocale() was a one time check of LC_ALL, LC_CTYPE, and LANG. This was technically wrong, but in practice the default LC_TIME is usually the same as the default LC_CTYPE.

I think it's correct to use the current LC_TIME locale, if it was configured already. Not doing so was a bug. However, it should try to remain close to the previous default behavior when instantiating LocaleTextCalendar() when LC_TIME is the initial "C" value, which is usually the case if a script doesn't explicitly set LC_ALL or LC_TIME.

But changing it to re-evaluate the default via setlocale(LC_TIME, "") for every formatweekday() call is going too far. That depends on the current values of the environment variables LC_ALL, LC_TIME, and LANG, which is very different from the one-time getdefaultlocale() call in the previous code.

Suggestion:

if locale is None:
    locale = _locale.getlocale(_locale.LC_TIME)
    if locale == (None, None):
        with different_locale(""):
            locale = _locale.getlocale(_locale.LC_TIME)
self.locale = locale

I would prefer to not change the locale in the constructor. That's why I proposed this specific PR.

Why is not okay to temporarily modify LC_TIME in __init__(), but fine to do so in formatweekday() and formatmonthname()? The class is documented as not thread safe with respect to the process locale.

It is ok to call setlocale() in __init__(). But setlocale() affects all threads, so I prefer to avoid calling it if possible.

I think that change is the less of the two. I don't think it's worth changing the behavior from the previous code that looked up the default locale to use via getdefaultlocale() only once, instead of calling it on every entry of different_locale. But that's just me voicing one opinion. I'd prefer for other people to way in on exactly which default behaviors are important to preserve, or whether the entire default behavior of locale=None should change to use the LC_TIME locale even if it's "C".