Python: django 报错 'Specifying a namespace in include() without providing an app_name ’

在根目录下的urls.py中使用了include方法，并且使用了namespace参数：

urlpatterns = [
    path('admin/', admin.site.urls),
    path('', include('book.urls', namespace='book')),
]

在启动项目时，会报错：

  File "/home/vubuntu/PycharmProjects/pythonProject/bookmanager/urls.py", line 21, in 
    path('', include('book.urls', namespace='book')),
  File "/home/vubuntu/PycharmProjects/pythonProject/venv/lib/python3.8/site-packages/django/urls/conf.py", line 42, in include
    raise ImproperlyConfigured(
django.core.exceptions.ImproperlyConfigured: Specifying a namespace in include() without providing an app_name is not supported. Set the app_name attribute in the included module, or pass a 2-tuple containing the list of patterns and app_name instead.

这是因为django2相对于django1做了改动，在include函数里增加了参数app_name，表示app的名字。

两种解决方法：

1. 在include中传入该app的名字（第二个参数），即

   path('', include(('book.urls','book'), namespace='book')),
   

2. 在每一个app的urls.py文件中添加app_name变量，即

   from django.contrib import admin
   from django.urls import re_path, include
   from book import views
   
   app_name = 'book'