Specifying a namespace in includ

解决方法：
1:在对应的子应用下，添加app_name属性

from django.conf.urls import url
from . import views
app_name = 'users'
urlpatterns = [
    url(r'^index', views.index, name='index')
]

2: 在项目的URL目录下，修改include的写法

from django.contrib import admin
from django.conf.urls import url, include
from django.urls import path
import users.urls 
urlpatterns = [
    path('admin/', admin.site.urls),
    url(r'^users/', include(('users.urls','users'), namespace='users')),
]
注意其写法：include(('users.urls','users'), namespace='users'))