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The publisher/subscriber tutorial will get you started interacting with ROS topics, and the LaserScan documentation indicates the message structure. The rest is more of a Python question, but here's one way:

condition_met = false  # Set outside scan_callback scope, so it's shared between callbacks

scan_callback(msg):
    if not condition_met:  # Don't need to do anything after condition met
        for value in msg.ranges:
            if value < 0.5:
                condition_met = true
                comm_pub.publish('hello')
                return  # Don't need to check the rest

This isn't a working example, but it should give you an idea. If you're stuck on a specific part, post your code, and I'll be happy to help you figure things out.