"""
Settings for REST framework are all namespaced in the REST_FRAMEWORK setting.
For example your project's `settings.py` file might look like this:

REST_FRAMEWORK = {
    'DEFAULT_RENDERER_CLASSES': (
        'rest_framework.renderers.JSONRenderer',
        'rest_framework.renderers.YAMLRenderer',
    )
    'DEFAULT_PARSER_CLASSES': (
        'rest_framework.parsers.JSONParser',
        'rest_framework.parsers.YAMLParser',
    )
}

This module provides the `api_setting` object, that is used to access
REST framework settings, checking for user settings first, then falling
back to the defaults.
"""
from django.conf import settings
from django.utils import importlib
from six import string_types



USER_SETTINGS = getattr(settings, 'REST_FRAMEWORK', None)

DEFAULTS = {
    'DEFAULT_RENDERER_CLASSES': (
        'rest_framework.renderers.JSONRenderer',
        'rest_framework.renderers.BrowsableAPIRenderer',
    ),
    'DEFAULT_PARSER_CLASSES': (
        'rest_framework.parsers.JSONParser',
        'rest_framework.parsers.FormParser',
        'rest_framework.parsers.MultiPartParser'
    ),
    'DEFAULT_AUTHENTICATION_CLASSES': (
        'rest_framework.authentication.SessionAuthentication',
        'rest_framework.authentication.BasicAuthentication'
    ),
    'DEFAULT_PERMISSION_CLASSES': (
        'rest_framework.permissions.AllowAny',
    ),
    'DEFAULT_THROTTLE_CLASSES': (
    ),

    'DEFAULT_CONTENT_NEGOTIATION_CLASS':
        'rest_framework.negotiation.DefaultContentNegotiation',
    'DEFAULT_MODEL_SERIALIZER_CLASS':
        'rest_framework.serializers.ModelSerializer',
    'DEFAULT_PAGINATION_SERIALIZER_CLASS':
        'rest_framework.pagination.PaginationSerializer',

    'DEFAULT_THROTTLE_RATES': {
        'user': None,
        'anon': None,
    },

    # Pagination
    'PAGINATE_BY': None,
    'PAGINATE_BY_PARAM': None,

    # Filtering
    'FILTER_BACKEND': None,

    # Authentication
    'UNAUTHENTICATED_USER': 'django.contrib.auth.models.AnonymousUser',
    'UNAUTHENTICATED_TOKEN': None,

    # Browser enhancements
    'FORM_METHOD_OVERRIDE': '_method',
    'FORM_CONTENT_OVERRIDE': '_content',
    'FORM_CONTENTTYPE_OVERRIDE': '_content_type',
    'URL_ACCEPT_OVERRIDE': 'accept',
    'URL_FORMAT_OVERRIDE': 'format',

    'FORMAT_SUFFIX_KWARG': 'format',
}


# List of settings that may be in string import notation.
IMPORT_STRINGS = (
    'DEFAULT_RENDERER_CLASSES',
    'DEFAULT_PARSER_CLASSES',
    'DEFAULT_AUTHENTICATION_CLASSES',
    'DEFAULT_PERMISSION_CLASSES',
    'DEFAULT_THROTTLE_CLASSES',
    'DEFAULT_CONTENT_NEGOTIATION_CLASS',
    'DEFAULT_MODEL_SERIALIZER_CLASS',
    'DEFAULT_PAGINATION_SERIALIZER_CLASS',
    'FILTER_BACKEND',
    'UNAUTHENTICATED_USER',
    'UNAUTHENTICATED_TOKEN',
)


def perform_import(val, setting_name):
    """
    If the given setting is a string import notation,
    then perform the necessary import or imports.
    """
    if isinstance(val, string_types):
        return import_from_string(val, setting_name)
    elif isinstance(val, (list, tuple)):
        return [import_from_string(item, setting_name) for item in val]
    return val


def import_from_string(val, setting_name):
    """
    Attempt to import a class from a string representation.
    """
    try:
        # Nod to tastypie's use of importlib.
        parts = val.split('.')
        module_path, class_name = '.'.join(parts[:-1]), parts[-1]
        module = importlib.import_module(module_path)
        return getattr(module, class_name)
    except:
        msg = "Could not import '%s' for API setting '%s'" % (val, setting_name)
        raise ImportError(msg)


class APISettings(object):
    """
    A settings object, that allows API settings to be accessed as properties.
    For example:

        from rest_framework.settings import api_settings
        print api_settings.DEFAULT_RENDERER_CLASSES

    Any setting with string import paths will be automatically resolved
    and return the class, rather than the string literal.
    """
    def __init__(self, user_settings=None, defaults=None, import_strings=None):
        self.user_settings = user_settings or {}
        self.defaults = defaults or {}
        self.import_strings = import_strings or ()

    def __getattr__(self, attr):
        if attr not in self.defaults.keys():
            raise AttributeError("Invalid API setting: '%s'" % attr)

        try:
            # Check if present in user settings
            val = self.user_settings[attr]
        except KeyError:
            # Fall back to defaults
            val = self.defaults[attr]

        # Coerce import strings into classes
        if val and attr in self.import_strings:
            val = perform_import(val, attr)

        self.validate_setting(attr, val)

        # Cache the result
        setattr(self, attr, val)
        return val

    def validate_setting(self, attr, val):
        if attr == 'FILTER_BACKEND' and val is not None:
            # Make sure we can initialize the class
            val()

api_settings = APISettings(USER_SETTINGS, DEFAULTS, IMPORT_STRINGS)