Rosbag Python API: why do messages from a rosbag have mangled __class__?

edit

I find that messages loaded from a rosbag via the Python API, have a weird __class__ attribute:

import rosbag
from sensor_msgs.msg import PointCloud2

# Round-trip a PointCloud2 message to a rosbag and back
bag = rosbag.Bag('test.bag', 'w')
try:
    scan = PointCloud2()
    print("__class__should be: \n{}".format(scan.__class__))
    bag.write('scan', msg)
finally:
    bag.close()

bag = rosbag.Bag('test.bag')
for topic, msg, t in bag.read_messages(topics=['scan']):
    print("But it comes out as: \n{}".format(msg.__class__))
bag.close()

The output:

__class__should be: 
<class 'sensor_msgs.msg._PointCloud2.PointCloud2'>
But it comes out as: 
<class 'tmpf1u1_e._sensor_msgs__PointCloud2'>

Is this a bug, or a feature?

ROS Indigo, Ubuntu 14, Python 2.7