rospy types of message subfields

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Hi,

I generate an empty instance of a message type, e.g via genpy.message.get_message_class('sensor_msgs/JointState') and convert it to a string, which results in:

header: 
  seq: 0
  stamp: 
    secs: 0
    nsecs:         0
  frame_id: ''
name: []
position: []
velocity: []
effort: []

I would like to fill the arrays with default values, similar to what a "rostopic pub .." call on the command line would give me:

header:
  seq: 0
  stamp: {secs: 0, nsecs: 0}
  frame_id: ''
name: ['']
position: [0]
velocity: [0]
effort: [0]

However, on the python side I don't know how to find out what the types of the lists are, since a type() call on the individual subfields only yields 'list'. Does anyone know how to find out the type of specific fields in messages?