I get an extra blank item plus some strange formatting on my output when I use the :session key in a python source block. Here's a simple example showing the problem:
--------------- * Examples #+BEGIN_SRC python :results output print "hello" print "bye" print 2 #+END_SRC #+RESULTS: : hello : bye : 2 #+BEGIN_SRC python :results output :session print "hello" print "bye" print 2 #+END_SRC #+RESULTS: : : hello : >>> bye : >>> 2