The union operation fails in the following use case

Python 2.6.4 (r264:75706, Jan 30 2010, 22:50:05) 
[GCC 4.3.1 20080507 (prerelease) [gcc-4_3-branch revision 135036]] on linux2
>>> def func():
...     a = set([0])
...     a.pop()
...     b = set([1, 2])
...     a.union(b)
...     return a
... 
>>> func()
set([])

Note, that is does *not* fail if it is *not* inside a function, i.e.

>>> a = set([0])
>>> a.pop()
0
>>> b = set([1, 2])
>>> a.union(b)
set([1, 2])
>>>

set.union doesn't change the first set (i.e. a), but returns a new sets (i.e. c):
>>> def func():
...   a = set([0])
...   a.pop()
...   print 'a:', a
...   b = set([1, 2])
...   c = a.union(b)
...   print 'a:', a
...   return c
...
>>> func()
a: set([])
a: set([])
set([1, 2])

Frank, try using "update" instead of "union".

Thanks, for the comments. I got it. 

In German we would say 'I was standing on the water hose' ... 
That is, something was choking the flow of my thoughts.

Best Regards

Frank