import ar_track_alvar

edit

You can run ar_track_alvar as a stand alone node and then subscribe to the ar_pose_marker topic it produces or the TF frames that it broadcasts from your python script. This way you don't have to interact directly with the code of ar_track_alvar at all.

You'll need to add a package dependency on the ar_track_alvar package because it uses a custom message type to publish these pose estimates, but apart from that it should be as simple as adding a subscriber to your python node.

Hope this helps.