code: def assert_valid(self, instance, value=None):
          
          if value is None:
              value = instance._get(self.name)
          if (
                  value is not None and
                  not self.equal(value, self.validate(instance, value))
          ):
              message = 'Invalid value for property: {}: {}'.format(
                  self.name, value
              )
              raise ValidationError(message, 'invalid', self.name, instance)
          return True
docs: Returns True if the Property is valid on a HasProperties instance
  
          Raises a ValueError if the value is invalid.