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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 _read_preference_for(self, session):
 """Read only access to the read preference of this instance or session. """

        # Override this operation's read preference with the transaction's.
        if session:
            return session._txn_read_preference() or self.__read_preference
        return self.__read_preference