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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 _authenticate_client(self, client):
 """Authenticate the client if necessary."""

        if self.login and not self.restart_required:
            try:
                db = client[self.auth_source]
                if self.x509_extra_user:
                    db.authenticate(
                        DEFAULT_SUBJECT,
                        mechanism='MONGODB-X509'
                    )
                else:
                    db.authenticate(
                        self.login, self.password)
            except Exception:
                logger.exception(
                    "Could not authenticate to %r as %s/%s"
                    % (client, self.login, self.password))
                raise