in 3rd line, list comprehension tries to access class_attribute1 as a
global variable (code is valid in python 2.5)

>>> class Foo(object):
...   class_attribute1 = 1
...   class_attribute2 = [class_attribute1 for x in range(8)]
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in Foo
  File "<stdin>", line 3, in <listcomp>
NameError: global name 'class_attribute1' is not defined