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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def validate(self, data):
 """Validate data. Raise NotValid error for invalid data."""

        validated = self._validated(data)
        errors = []
        for validator in self.additional_validators:
            if not validator(validated):
                errors.append(
                    "%s invalidated by '%s'" % (
                        validated, _get_repr(validator)))
        if errors:
            raise NotValid(*errors)

        if self.default is UNSPECIFIED:
            return validated

        if self.null_values is not UNSPECIFIED\
                and validated in self.null_values:
            return self.default

        if validated is None:
            return self.default

        return validated