docs: def safe_type(self, data, tree):
          """
          Make sure that the incoming data complies with the class type we
          are expecting it to be. In this case, classes that inherit from this
          base class expect data to be of type ``list``.
          """
          if not isinstance(data, list):
              name = self.__class__.__name__
              msg = "did not pass validation against callable: %s" % name
              reason = 'expected a list but got %s' % safe_repr(data)
              raise Invalid(self.schema, tree, reason=reason, pair='value', msg=msg)