sortset.py

>>> a = [ 9, 5, 4, 2, 1]
>>> b = [ 2, 4, 7, 8, 10]
>>> def sortset(s, a):
...   return sorted(s, key=lambda e: a.index(e))
... 
>>> sortset(set(a) - set(b), a + b)
[9, 5, 1]
>>> sortset(set(b) - set(a), a + b)
[7, 8, 10]

setを、元の配列のindexでソート