docs: def convert(self, args, handler=None):
          """Prepare filters."""
          name = args
          field = attr = None
          opts = ()
          if isinstance(args, (list, tuple)):
              name, *opts = args
              if opts:
                  attr = opts.pop()
              if opts:
                  field = opts.pop()
  
          if not field and handler and handler.Schema:
              field = handler.Schema._declared_fields.get(attr or name) or \
                  self.FILTER_CLASS.field_cls()
              field.attribute = field.attribute or attr or name
          return self.FILTER_CLASS(name, attr=attr, field=field, *opts)