Parses the Accept-Language HTTP header and returns the list of locales by order of preference.

parse_accept_language.py

def parse_http_accept_language_header(langs_str):
    """Returns the list of languages from a string formatted
    according to the Accept-Language HTTP header, by order of
    preference.
    """

    def get_locale_q_pairs(lang_list):
        for language in lang_list:
            locale_q_pair = language.split(";")
            if len(locale_q_pair) == 1:
                # no q => q = 1
                yield language.strip(), 1.0
            else:
                locale, q = locale_q_pair
                yield locale.strip(), float(q.split("=")[1])

    try:
        locale_q_pairs = get_locale_q_pairs(langs_str.split(","))
        locale_q_pairs = sorted(locale_q_pairs, key=lambda kv: kv[1])
        return [locale for locale, _q in locale_q_pairs]
    except (ValueError, IndexError):  # format error or empty list
        return None


# Example of use with Bottle :
from bottle import route, run

@route('/')
def hello():
    lang_header_str = request.headers.get('Accept-Language', '')
    langs = parse_http_accept_language_header(lang_header_str)
    favorite_locale = langs[0] if langs else None

    return "User locale: {}".format(favorite_locale)

run(host='localhost', port=8080, debug=True)

I think the returned order is reversed. Higher 'q' should be higher priority. Therefore, you should add reverse=True to the sorted, ligne 19.
(or use favorite_locale = langs[-1] if langs else None)