rospy service question, how to specify service type

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You are correct that AddTwoInts is an object. You imported it with the from beginner_tutorials.srv import * command.

Please note that the following explanation applies to rosbuild. I haven't tried catkin just yet, and things are different there.

Anyway, in rosbuild, the generation of the python module happens when you run the command rosmake. There's a macro you put in the CMakeLists.txt file, rosbuild_gensrv() that does it for you. This is why you still have to occasionally run rosmake even if you're using python.

(If I understand correctly, the same thing is happening in catkin, just without macros and using python or built in cmake macros instead. But like I said, I haven't actually tried it...)

If you'd like to look at the generated code, it should be in beginner_tutorials/src/beginner_tutorials/srv/_AddTwoInts.py