code: def _next_lowest_integer(group_keys):
      
      try: #TODO Replace with max default value when dropping compatibility with Python < 3.4
          largest_int= max([ int(val) for val in group_keys if _is_int(val)])
      except:
          largest_int= 0
      return largest_int + 1
docs: returns the lowest available integer in a set of dict keys