string.join(list(A) listofstrings) takes a separator and a listofstrings and forms a new string by appending all strings in listofstrings. Between any two adjacent strings separator will be put. separator is allowed to be empty, in which case all strings are simply concatenated. If listofstrings is empty, the resulting string will be the empty string -- regardless of the separator.
string.join(set(A) setofstrings) behaves exactly like join on lists but takes its components from a set. Sind a set does not provide a certain ordering, the ordering of the part strings is random.