How to obtain the msg format of a topic on runtime?

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Hi,

I need to implement a dynamic subscription mechanism, that monitors the available topics. Firstly it takes a view to the msg format of new topics and subscribes to this one depending on a special data format. The first step, to know the current available topics works fine with the following code:

available_topics_old=[]
def monitor():
   while not rospy.is_shutdown():
      master = rosgraph.masterapi.Master('/rostopic')
      available_topics = master.getPublishedTopics('/') 
      new_topics=[]
      for topic in available_topics:
         if not topic in available_topics_old:
            new_topics.append(topic)
      if new_topics:
          # here I need to know the format(file) of the assigned msg
   rospy.sleep(1.0)

How can I get the msg format of the topic?

Best wishes from Germany

Poseidonius