Python 3.5.1 (default, Dec 2015, 13:05:11)
[GCC 4.8.2] on linux
   a=set()
=> None
   a.add(8)
=> None
   b=set([4])
=> None
   a
=> {8}
   b
=> {4}
   b.add(9)
=> None
   b.add(94)
=> None
   b
=> {9, 4, 94}
   a
=> {8}
   a.update(b)
=> None
   a
=> {8, 9, 4, 94}
   b
=> {9, 4, 94}