code: def create_uniq_field_dis_func(*classes):
      # type: (*Type) -> Callable
      
      if len(classes) < 2:
          raise ValueError("At least two classes required.")
      cls_and_attrs = [(cl, set(at.name for at in fields(cl))) for cl in classes]
      if len([attrs for _, attrs in cls_and_attrs if len(attrs) == 0]) > 1:
          raise ValueError("At least two classes have no attributes.")
      # TODO: Deal with a single class having no required attrs.
      # For each class, attempt to generate a single unique required field.
      uniq_attrs_dict = OrderedDict()  # type: Dict[str, Type]
      cls_and_attrs.sort(key=lambda c_a: -len(c_a[1]))
  
      fallback = None  # If none match, try this.
  
      for i, (cl, cl_reqs) in enumerate(cls_and_attrs):
          other_classes = cls_and_attrs[i + 1 :]
          if other_classes:
              other_reqs = reduce(or_, (c_a[1] for c_a in other_classes))
              uniq = cl_reqs - other_reqs
              if not uniq:
                  m = "{} has no usable unique attributes.".format(cl)
                  raise ValueError(m)
              uniq_attrs_dict[next(iter(uniq))] = cl
          else:
              fallback = cl
  
      def dis_func(data):
          # type: (Mapping) -> Optional[Type]
          if not isinstance(data, Mapping):
              raise ValueError("Only input mappings are supported.")
          for k, v in uniq_attrs_dict.items():
              if k in data:
                  return v
          return fallback
  
      return dis_func
docs: Given attr classes, generate a disambiguation function.
  
      The function is based on unique fields.