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Have you tried rostopic echo-ing the topic? In the sensor_msgs/PointCloud message, there is a points object of geometry_msgs/Point32's which have an X, Y, and Z for each point found by the camera. I'd recommend looking there.

After that, look in the ROS subscriber python tutorials to get an idea of how to subscribe to that topic, and use the correct part of the message in the callback function that you define to populate the list of depths you need.

You will probably need to transform the points into whatever reference frame you need.