Source code for core.util.accept_language

"""A package to parse Accept-Language headers.

This is based on accept_language.py. Here is the original licensing
information for accept_language.py:

Copyright [2017] [Chatbot Developers]

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at:

     http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
"""

import logging
import re

from collections import namedtuple
from operator import attrgetter

VALIDATE_LANG_REGEX = re.compile('^[a-z]+$', flags=re.IGNORECASE)
QUALITY_VAL_SUB_REGEX = re.compile('^q=', flags=re.IGNORECASE)
DEFAULT_QUALITY_VALUE = 1.0
MAX_HEADER_LEN = 8192
Lang = namedtuple('Lang', ('language', 'locale', 'quality'))

logger = logging.getLogger(__name__)


def parse_accept_language(accept_language_str, default_quality=None):
    """
    Parse a RFC 2616 Accept-Language string.
    https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14

    :param accept_language_str: A string in RFC 2616 format.
    :type accept_language_str: str
    :returns: List of `Lang` namedtuples.
    :rtype: list

    :Example:

        >>> parse_accept_language('en-US,el;q=0.8')
        [
            Lang(locale='en_US', language='en', quality=1.0),
            Lang(locale=None, language='el', quality=0.8),
        ]

    """
    if not accept_language_str:
        return []

    if len(accept_language_str) > MAX_HEADER_LEN:
        raise ValueError('Accept-Language too long, max length is 8192')

    parsed_langs = []
    for accept_lang_segment in accept_language_str.split(','):
        quality_value = default_quality or DEFAULT_QUALITY_VALUE
        lang_code = accept_lang_segment.strip()
        if ';' in accept_lang_segment:
            lang_code, quality_value = accept_lang_segment.split(';')
            quality_value = float(QUALITY_VAL_SUB_REGEX.sub('', quality_value))

        lang_code_components = re.split('-|_', lang_code)

        if not all(VALIDATE_LANG_REGEX.match(c.strip()) for c in lang_code_components):
            continue

        if len(lang_code_components) == 1:
            # language code 2/3 letters, e.g. fr
            language = lang_code_components[0].lower()
            locale = None
        else:
            # full language tag, e.g. en-US
            language = lang_code_components[0].lower()
            locale = '{}_{}'.format(
                language, lang_code_components[1].upper(),
            )
        parsed_langs.append(
            Lang(locale=locale, language=language.strip(), quality=quality_value)
        )
    return sorted(parsed_langs, key=attrgetter('quality'), reverse=True)