def __getitem__(oid_tid):
    """
        Given an (oid, tid) pair, return the cache data (state_bytes,
        tid_int) for that object.

        The returned *tid_int* must match the requested tid.

        If the ``(oid, tid)`` pair isn't in the cache, return None.

        A special tid value of None means that the client doesn't know the TID to ask
        for and wants the best available; they are then required to verify
        that the returned object is visible (that is, within the ranges of TIDs
        the MVCC state permits).
        """
