So it appears the Rosbag Code API does not handle the topic names like the other ROS tools regarding the first '/'.
<launch><group ns="my_ns"><node pkg="rosbag" type="record" name="record" args="-O odom.bag odom /odom"/></group></launch>
$ rostopic pub /my_ns/odom std_msgs/String foo$ rostopic pub /odom std_msgs/String bar
$ rosbag info odom.bagpath: odom.bagversion: 2.0duration: 5.9sstart: Dec 02 2010 21:46:02.39 (1291355162.39)end: Dec 02 2010 21:46:08.30 (1291355168.30)size: 5.3 KBmessages: 2compression: none [1/1 chunks]types: std_msgs/String [992ce8a1687cec8c8bd883ec73ca41d1]topics: /odom 1 msg : std_msgs/Stringodom 1 msg : std_msgs/String
$ rospython rosbag>>> import rosbag>>> with rosbag.Bag('~/.ros/odom.bag') as b:... print b.read_messages(topics=['odom']).next()[1]... print b.read_messages(topics=['/odom']).next()[1]...data: foodata: bar