code: def validate(self):
          
          if not isinstance(self.fields, dict):
              raise TypeError(u'Expected dict fields, got: {} {}'.format(
                  type(self.fields).__name__, self.fields))
  
          for key, value in six.iteritems(self.fields):
              validate_safe_string(key)
              if not isinstance(value, Expression):
                  raise TypeError(
                      u'Expected Expression values in the fields dict, got: '
                      u'{} -> {}'.format(key, value))
docs: Ensure that the ConstructResult block is valid.