From the aptitude help:
aptitude 0.6.8.2 Usage: aptitude [-S fname] [-u|-i] .. snip .. markauto - Mark packages as having been automatically installed. unmarkauto - Mark packages as having been manually installed. .. snip .. So why when I mark my package as being automatically installed, does aptitude attempt to uninstall it?!
# aptitude -s -y markauto python-pip The following packages will be REMOVED: python-pip{u} 0 packages upgraded, 0 newly installed, 1 to remove and 0 not upgraded. Need to get 0 B of archives. After unpacking 468 kB will be freed. Would download/install/remove packages.