What is ugly about allowing strings?  CPython certainly has a way to to make sum(x, '') at least as efficient as y='';for in in x; y+= x is now.  What is ugly about making sum([a, b, ..]) be equivalent to a + b + .. so that non-empty lists of arbitrary types can be "summed"?  What is ugly about harmonizing sum(x) and reduce(operator.add, x) behaviors?