There is a rather silly "workaround":

  In [17]: class Start(object): 
      ...:     def __add__(self, other): 
      ...:         return other 
      ...:                                                                                                                                                                                                                                     
  In [18]: sum(["a", "b", "c"], start=Start())                                                                                                                                                                                                 
  Out[18]: 'abc'