Allow overriding how the locale is set

[aceisace]

Analog to the flask-babel locale_selector, these small, but effective changes provide a much higher user-comfort as they are not restricted to only the integrated locale-selection method.

For instance, some users may want to save the locale of a logged-in-user which has a preferred locale in the request session or even as a cookie. Currently, it is not possible except to override the locale inside a view function like this:

from app.main import babel
async def some_function(abc: str):
    babel.locale = "de"

Which is fairly un-pythonic and adds a lot of repetitive code when you have a lot of view functions. Instead of this, one can, optionally override this behaviour like this:

In main.py:

# code omitted before
...

def get_locale_from_session(request: Request) -> str:
    # If user logged in, we stored locale in session:
    session_locale = request.session.get("locale")
    print(f"session_locale: {session_locale}")
    if session_locale and session_locale in settings.SUPPORTED_LANGUAGES:
        return session_locale

    # Fallback to Accept-Language header, or Babel default locale
    accept_language = request.headers.get("accept-language", "")

    if accept_language:
        languages = [lang.split(";")[0] for lang in accept_language.split(",")]

        # Check if any of the user's preferred languages match supported ones
        for lang in languages:
            if lang in settings.SUPPORTED_LANGUAGES:
                print("get_locale: ", lang)
                return lang

    return babel.default_locale  # Fallback if no match is found

# before 
app.add_middleware(BabelMiddleware, babel_configs=babel_configs, jinja2_templates=templates)
# after
app.add_middleware(BabelMiddleware, babel_configs=babel_configs, jinja2_templates=templates, locale_selector=get_locale_from_session)

Which does the following; if a user is logged in, take their preferred locale to set the babel locale. If no user was found (i.e. not logged in), use the accept-language header instead. Of course, the user is free to set the locale in whichever way they want, e.g. database queries, cookies, sessions etc.

On my end, I have done some testing with some fairly good results. I kindly ask to merge the changes into main after doing some tests on your end too and having these changes available in the next release in the near future.