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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 _is_authenticated(self):
 """Checks if credentials allow for authenticated carto access"""

        if not self.auth_api_client.is_valid_api_key():
            raise CartoException(
                'Cannot authenticate user `{}`. Check credentials.'.format(
                    self.creds.username()))