"""
Settings and configuration for Django.

Values will be read from the module specified by the DJANGO_SETTINGS_MODULE environment
variable, and then from django.conf.global_settings; see the global settings file for
a list of all possible variables.
"""

import importlib
import os
import time

from django.conf import global_settings
from django.core.exceptions import ImproperlyConfigured
from django.utils.functional import LazyObject, empty

ENVIRONMENT_VARIABLE = "DJANGO_SETTINGS_MODULE"


class LazySettings(LazyObject):
    """
    A lazy proxy for either global Django settings or a custom settings object.
    The user can manually configure settings prior to using them. Otherwise,
    Django uses the settings module pointed to by DJANGO_SETTINGS_MODULE.
    """
    def _setup(self, name=None):
        """
        Load the settings module pointed to by the environment variable. This
        is used the first time we need any settings at all, if the user has not
        previously configured the settings manually.
        """
        settings_module = os.environ.get(ENVIRONMENT_VARIABLE)
        if not settings_module:
            desc = ("setting %s" % name) if name else "settings"
            raise ImproperlyConfigured(
                "Requested %s, but settings are not configured. "
                "You must either define the environment variable %s "
                "or call settings.configure() before accessing settings."
                % (desc, ENVIRONMENT_VARIABLE))

        self._wrapped = Settings(settings_module)

    def __repr__(self):
        # Hardcode the class name as otherwise it yields 'Settings'.
        if self._wrapped is empty:
            return '<LazySettings [Unevaluated]>'
        return '<LazySettings "%(settings_module)s">' % {
            'settings_module': self._wrapped.SETTINGS_MODULE,
        }

    def __getattr__(self, name):
        """
        Return the value of a setting and cache it in self.__dict__.
        """
        if self._wrapped is empty:
            self._setup(name)
        val = getattr(self._wrapped, name)
        self.__dict__[name] = val
        return val

    def __setattr__(self, name, value):
        """
        Set the value of setting. Clear all cached values if _wrapped changes
        (@override_settings does this) or clear single values when set.
        """
        if name == '_wrapped':
            self.__dict__.clear()
        else:
            self.__dict__.pop(name, None)
        super(LazySettings, self).__setattr__(name, value)

    def __delattr__(self, name):
        """
        Delete a setting and clear it from cache if needed.
        """
        super(LazySettings, self).__delattr__(name)
        self.__dict__.pop(name, None)

    def configure(self, default_settings=global_settings, **options):
        """
        Called to manually configure the settings. The 'default_settings'
        parameter sets where to retrieve any unspecified values from (its
        argument must support attribute access (__getattr__)).
        """
        if self._wrapped is not empty:
            raise RuntimeError('Settings already configured.')
        holder = UserSettingsHolder(default_settings)
        for name, value in options.items():
            setattr(holder, name, value)
        self._wrapped = holder

    @property
    def configured(self):
        """
        Returns True if the settings have already been configured.
        """
        return self._wrapped is not empty


class Settings:
    def __init__(self, settings_module):
        # update this dict from global settings (but only for ALL_CAPS settings)
        for setting in dir(global_settings):
            if setting.isupper():
                setattr(self, setting, getattr(global_settings, setting))

        # store the settings module in case someone later cares
        self.SETTINGS_MODULE = settings_module

        mod = importlib.import_module(self.SETTINGS_MODULE)

        tuple_settings = (
            "INSTALLED_APPS",
            "TEMPLATE_DIRS",
            "LOCALE_PATHS",
        )
        self._explicit_settings = set()
        for setting in dir(mod):
            if setting.isupper():
                setting_value = getattr(mod, setting)

                if (setting in tuple_settings and
                        not isinstance(setting_value, (list, tuple))):
                    raise ImproperlyConfigured("The %s setting must be a list or a tuple. " % setting)
                setattr(self, setting, setting_value)
                self._explicit_settings.add(setting)

        if not self.SECRET_KEY:
            raise ImproperlyConfigured("The SECRET_KEY setting must not be empty.")

        if hasattr(time, 'tzset') and self.TIME_ZONE:
            # When we can, attempt to validate the timezone. If we can't find
            # this file, no check happens and it's harmless.
            zoneinfo_root = '/usr/share/zoneinfo'
            if (os.path.exists(zoneinfo_root) and not
                    os.path.exists(os.path.join(zoneinfo_root, *(self.TIME_ZONE.split('/'))))):
                raise ValueError("Incorrect timezone setting: %s" % self.TIME_ZONE)
            # Move the time zone info into os.environ. See ticket #2315 for why
            # we don't do this unconditionally (breaks Windows).
            os.environ['TZ'] = self.TIME_ZONE
            time.tzset()

    def is_overridden(self, setting):
        return setting in self._explicit_settings

    def __repr__(self):
        return '<%(cls)s "%(settings_module)s">' % {
            'cls': self.__class__.__name__,
            'settings_module': self.SETTINGS_MODULE,
        }


class UserSettingsHolder:
    """
    Holder for user configured settings.
    """
    # SETTINGS_MODULE doesn't make much sense in the manually configured
    # (standalone) case.
    SETTINGS_MODULE = None

    def __init__(self, default_settings):
        """
        Requests for configuration variables not in this class are satisfied
        from the module specified in default_settings (if possible).
        """
        self.__dict__['_deleted'] = set()
        self.default_settings = default_settings

    def __getattr__(self, name):
        if name in self._deleted:
            raise AttributeError
        return getattr(self.default_settings, name)

    def __setattr__(self, name, value):
        self._deleted.discard(name)
        super(UserSettingsHolder, self).__setattr__(name, value)

    def __delattr__(self, name):
        self._deleted.add(name)
        if hasattr(self, name):
            super(UserSettingsHolder, self).__delattr__(name)

    def __dir__(self):
        return sorted(
            s for s in list(self.__dict__) + dir(self.default_settings)
            if s not in self._deleted
        )

    def is_overridden(self, setting):
        deleted = (setting in self._deleted)
        set_locally = (setting in self.__dict__)
        set_on_default = getattr(self.default_settings, 'is_overridden', lambda s: False)(setting)
        return (deleted or set_locally or set_on_default)

    def __repr__(self):
        return '<%(cls)s>' % {
            'cls': self.__class__.__name__,
        }


settings = LazySettings()
