[django admin]name 'list_display' is not defined求解释！

#!/usr/bin/python
from blog.models import UserForm
from django.contrib import admin

class UserFormAdmin(admin.ModelAdmin):
    list_display('username','gender', 'datetime')
    #pass
admin.site.register(UserForm, UserFormAdmin)

环境：Python2.7.5+djanggo1.5.4 django python admin
[解决办法]
list_display = ('username','gender', 'datetime')