Get information about topic message at runtime

edit

I am trying to build an interface that universally bridges ROS topics to another system. I am importing the message type at runtime:

importcom = (  # import type
    "from " +
    topic_class +
    ".msg import " +
    topic_type
)
exec(importcom, globals())

This allows me to later subscribe to this topic dynamically at runtime.

My question is now: How can I gain insight in the structure of this message at runtime? Ideally I need a json object that includes the types of all the parameters that i expect in a message. e.g.:

{
   "angular" :
      {
           "x": float
           "y": float 
   "linear" :
      {
           "x": float
           "y": float 
....

(or something like this)