code: def validate_type_only(self, val):
          
          if not issubclass(self.definition, type(val)):
              raise ValidationError('expected type %s or subtype, got %s' %
                  (self.definition.__name__, generic_type_name(val)))
docs: Use this when you only want to validate that the type of an object
          is correct, but not yet validate each field.
  
          We check whether val is a Python parent class of the definition. This
          is because Union subtyping works in the opposite direction of Python
          inheritance. For example, if a union U2 extends U1 in Python, this
          validator will accept U1 in places where U2 is expected.