Use Gtk.TreeViewSelection. You can get it from Gtk.TreeView (Assuming you have id column in your GtkTreeModel (GtkListStore or GtkTreeStore) constructor).
from gi.repository import Gtk
def yourcallback ( selection ):
model,list_iter = selection.get_selected ()
if list_iter != None:
# Debugging mode, uncomment to activate
# print "row index = " + model[list_iter][column_number_that_you_want]
# alternate
# def yourcallback (path, column, data):
# ........
..........
yourtvselection = yourtv.get_selection ()
yourtvselection.connect ( "changed" , yourcallback)
# alternative signal
# yourtv.connect ("row-activated", yourcallback)