How to properly union with set

I understand that any python set union with empty set would result in itself. But some strange behave I detect when union is inside of a for loop.

looks good

num= set([2,3,4])
emp= set()
print num|emp
>>>set([2, 3, 4])

confused

s = set()
inp = ["dr101-mr99","mr99-out00","dr101-out00","scout1-scout2","scout3-    scout1","scout1-scout4","scout4-sscout","sscout-super"]
for ele in inp:
  r = set(ele.split("-"))
  print r
  s.union(r)
print s
 >>>set(['mr99', 'dr101'])
    set(['out00', 'mr99'])
    set(['out00', 'dr101'])
    set(['scout1', 'scout2'])
    set(['scout1', 'scout3'])
    set(['scout4', 'scout1'])
    set(['scout4', 'sscout'])
    set(['super', 'sscout'])
    set([])

anyone could tell me why the last set s is empty?
is the output supposed to be every unique element in the set?