Empty sets are pickled as set([]).  The pickle contains serialization of an empty list that is passed to set constructor during unpickling:

>>> dis(dumps(set()))
    0: \x80 PROTO      3
    2: c    GLOBAL     'builtins set'
   16: q    BINPUT     0
   18: ]    EMPTY_LIST
   19: q    BINPUT     1
   21: \x85 TUPLE1
   22: q    BINPUT     2
   24: R    REDUCE
   25: q    BINPUT     3
   27: .    STOP

The proposed patch pickles empty set as set() instead:


>>> dis(dumps(set()))
    0: \x80 PROTO      3
    2: c    GLOBAL     'builtins set'
   16: q    BINPUT     0
   18: )    EMPTY_TUPLE
   19: R    REDUCE
   20: q    BINPUT     1
   22: .    STOP