Using python for automatically writing data in bag to csv file

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A non generic approach is rosbag_analysis (see http://www.ros.org/wiki/rosbag_analysis). I just check for some attribute names and I "guess" the type from the attribute name for a very narrow set of message types.

This is bad, but it works... :)

edit: rostopic -p does not do what I want as it works on topics "live" so it may drop messages... rosbag_analysis processes bag files offline using the bag Python API.

I was trying to figure out the same thing. Last few commands on this page worked perfectly for converting .bag to .csv.

http://wiki.fieldrobot.dk/index.php/FroboMind:Cheat_Sheet

You can introspect on a message using _slots and _slot_types. Here are their values for Header.

  __slots__ = ['seq','stamp','frame_id']
  _slot_types = ['uint32','time','string']

They have different underscores because __slots__ is a Python builtin and has special meaning.

from std_msgs.msg import Header
for name, type_ in zip(Header.__slots__, Header._slot_types):
     print name, type_

prints:

seq uint32
stamp time
frame_id string

You might find the info on the Interoperability wiki page helpful (even if not, please consider adding to it if you find something later on that isn't there). In particular it mentions the BagLoader class from starmac_tools, which was written mainly for the plotting use case you have in mind. Good Luck!