Python package anomoly

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from std_msgs.msg import String

when ROS encounters a statement like the one above, where does it search, to obtain String class? I check $PYTHONPATH and it doesn't seem like the class exists in a file located in that path. I searched the ROS package std_msgs..but it only shows a folder msg that has the String.msg file in it. I don't see any module called 'msg' in which I expect to find the 'String' class..

Could someone please tell me whats going on? Am I missing something fundamental or are things really twisted a little bit.. ?

Thanks