[email protected]:~$ rostopic echo /ultrasonic_type
Traceback (most recent call last):
File “/opt/ros/indigo/bin/rostopic”, line 35, in
rostopic.rostopicmain()
File “/opt/ros/indigo/lib/python2.7/dist-packages/rostopic/init.py”, line 1916, in rostopicmain
_rostopic_cmd_echo(argv)
File “/opt/ros/indigo/lib/python2.7/dist-packages/rostopic/init

.py”, line 1257, in _rostopic_cmd_echo
_rostopic_echo(topic, callback_echo, bag_file=options.bag)
File “/opt/ros/indigo/lib/python2.7/dist-packages/rostopic/init.py”, line 915, in _rostopic_echo
sub = rospy.Subscriber(real_topic, msg_class, callback_echo.callback, {‘topic’: topic, ‘type_information’: type_information})
File “/opt/ros/indigo/lib/python2.7/dist-packages/rospy/topics.py”, line 533, in init
super(Subscriber, self).init(name, data_class, Registration.SUB)
File “/opt/ros/indigo/lib/python2.7/dist-packages/rospy/topics.py”, line 138, in init
raise ValueError(“data_class [%s] is not a class”%data_class)
ValueError: data_class [<module ‘ultrasonic_test’ from ‘/home/local_ws/devel/lib/python2.7/dist-packages/ultrasonic_test/init.pyc’>] is not a class

（1）上述報警是先定義了一個message，然後根據該message定義了一個topic，併發布該topic
（2）通過rostopic echo /topic 可以正常輸出
（3）修改message中的資料，再進行編譯正常，沒有報警
（4）通過rostopic echo /topic 則出現傷處報警

處理措施：
重新建立一個msg檔案，msg檔案中的資料格式和上述錯誤報警時的格式一樣，則新生成的msg訊息格式定義的topic則可以正常輸出。