How to use the aiopg.sa.exc function in aiopg

processor, obj, index = self._result_proxy._key_fallback(key)
        # Do we need slicing at all? RowProxy now is Mapping not Sequence
        # except TypeError:
        #     if isinstance(key, slice):
        #         l = []
        #         for processor, value in zip(self._processors[key],
        #                                     self._row[key]):
        #             if processor is None:
        #                 l.append(value)
        #             else:
        #                 l.append(processor(value))
        #         return tuple(l)
        #     else:
        #         raise
        if index is None:
            raise exc.InvalidRequestError(
                "Ambiguous column name '%s' in result set! "
                "try 'use_labels' option on select statement." % key)
        if processor is not None:
            return processor(self._row[index])
        else:
            return self._row[index]

async def begin_twophase(self, xid=None):
        """Begin a two-phase or XA transaction and return a transaction
        handle.

        The returned object is an instance of
        TwoPhaseTransaction, which in addition to the
        methods provided by Transaction, also provides a
        TwoPhaseTransaction.prepare() method.

        xid - the two phase transaction id.  If not supplied, a
        random id will be generated.
        """

        if self._transaction is not None:
            raise exc.InvalidRequestError(
                "Cannot start a two phase transaction when a transaction "
                "is already in progress.")
        if xid is None:
            xid = self._dialect.create_xid()
        self._transaction = TwoPhaseTransaction(self, xid)
        await self._begin_impl()
        return self._transaction

async def prepare(self):
        """Prepare this TwoPhaseTransaction.

        After a PREPARE, the transaction can be committed.
        """

        if not self._parent.is_active:
            raise exc.InvalidRequestError("This transaction is inactive")
        await self._connection._prepare_twophase_impl(self._xid)
        self._is_prepared = True

# match is only on name.
                result = map[key.key]
            # search extra hard to make sure this
            # isn't a column/label name overlap.
            # this check isn't currently available if the row
            # was unpickled.
            if (result is not None and
                    result[1] is not None):
                for obj in result[1]:
                    if key._compare_name_for_result(obj):
                        break
                else:
                    result = None
        if result is None:
            if raiseerr:
                raise exc.NoSuchColumnError(
                    "Could not locate column in row for column '%s'" %
                    expression._string_or_unprintable(key))
            else:
                return None
        else:
            map[key] = result
        return result