Python编程从入门到实践 第18章Django入门 第361页“定义模型”出错，找不出原因。

使用“《Python编程》文件-更新” 的文件也出错
django.core.exceptions.ImproperlyConfigured: Requested setting INSTALLED_APPS, but settings are not configured. You must either define the environment variable DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.