** python session
   :PROPERTIES:
   :DATE:     2011-06-19
   :END:
#+begin_src python :results output :session mypy 
x = 1
for i in range(1,5):
  x = x + i
  print x

print "I throw an error"
#+end_src

#+name:
: 
: ... ... ... 2
: 4
: 7
: 11
: I throw an error

#+begin_src python :results output :session
  print y
#+end_src

#+name:
: Traceback (most recent call last):
:   File "<stdin>", line 1, in <module>
: NameError: name 'y' is not defined