docs: def _validate(self, value):
          """Perform validation on ``value``. Raise a :exc:`ValidationError` if validation
          does not succeed.
          """
          errors = []
          kwargs = {}
          for validator in self.validators:
              try:
                  r = validator(value)
                  if not isinstance(validator, Validator) and r is False:
                      self.fail('validator_failed')
              except ValidationError as err:
                  kwargs.update(err.kwargs)
                  if isinstance(err.messages, dict):
                      errors.append(err.messages)
                  else:
                      errors.extend(err.messages)
          if errors:
              raise ValidationError(errors, **kwargs)