list problem

[placid]

Hi all,

I have two lists that contain strings in the form string + number for
example

>>> list1 = [ ' XXX1', 'XXX2', 'XXX3', 'XXX5']

the second list contains strings that are identical to the first list,
so lets say the second list contains the following

>>> list1 = [ ' XXX1', 'XXX2', 'XXX3', 'XXX6']

and now what ive been trying to do is find the first string that is
available,
i.e a string that is in neither of the two lists so the following code
should only print XXX4 then return.

for i in xrange(1,10):
    numpart = str(1) + str("%04i" %i)
    str = "XXX" + numpart

      for list1_elm in list1:
          if list1_elm == str:
               break
          else:
               for list2_elm in list2:
                   if list2_elm == str:
                      break
                   else:
                      print str
                      return

Cheer