os.environ.setdefault("DJANGO_SETTINGS_MODULE", "djtest.settings")

ENVIRONMENT_VARIABLE = "DJANGO_SETTINGS_MODULE"

class Settings(BaseSettings):
    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

        try:
            # 根据 manage.py 中的os.environ.setdefault("DJANGO_SETTINGS_MODULE", "djtest.settings")，设置的配置文件，import该配置文件
            mod = importlib.import_module(self.SETTINGS_MODULE)
        except ImportError as e:
            raise ImportError(
                "Could not import settings '%s' (Is it on sys.path? Is there an import error in the settings file?): %s"
                % (self.SETTINGS_MODULE, e)
            )


        tuple_settings = ("INSTALLED_APPS", "TEMPLATE_DIRS")
        # setting文件里面显示申明的属性集合，是个set类型的，是个集合
        self._explicit_settings = set()
        # dir（object）返回object的属性方法列表，因此，这里dir会返回setting文件里面的所有等号左边的属性
        for setting in dir(mod):
            if setting.isupper():
                setting_value = getattr(mod, setting)

                if (setting in tuple_settings and
                        isinstance(setting_value, six.string_types)):
                    raise ImproperlyConfigured("The %s setting must be a tuple. "
                            "Please fix your settings." % setting)
                # 将setting文件里面的所有属性提出来，放到这个Setting类里面来
                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