Python sqlalchemy.event.listens_for() Examples

def before_compile(self, query):
        """Receive the :class:`.Query` object before it is composed into a
        core :class:`.Select` object.

        This event is intended to allow changes to the query given::

            @event.listens_for(Query, "before_compile", retval=True)
            def no_deleted(query):
                for desc in query.column_descriptions:
                    if desc['type'] is User:
                        entity = desc['entity']
                        query = query.filter(entity.deleted == False)
                return query

        The event should normally be listened with the ``retval=True``
        parameter set, so that the modified query may be returned.


        """ 

def before_compile(self, query):
        """Receive the :class:`.Query` object before it is composed into a
        core :class:`.Select` object.

        This event is intended to allow changes to the query given::

            @event.listens_for(Query, "before_compile", retval=True)
            def no_deleted(query):
                for desc in query.column_descriptions:
                    if desc['type'] is User:
                        entity = desc['entity']
                        query = query.filter(entity.deleted == False)
                return query

        The event should normally be listened with the ``retval=True``
        parameter set, so that the modified query may be returned.


        """ 

def capture_engine_context_buffer(**kw):
    from .env import _sqlite_file_db
    from sqlalchemy import event

    buf = compat.StringIO()

    eng = _sqlite_file_db()

    conn = eng.connect()

    @event.listens_for(conn, "before_cursor_execute")
    def bce(conn, cursor, statement, parameters, context, executemany):
        buf.write(statement + "\n")

    kw.update({"connection": conn})
    conf = EnvironmentContext.configure

    def configure(*arg, **opt):
        opt.update(**kw)
        return conf(*arg, **opt)

    with mock.patch.object(EnvironmentContext, "configure", configure):
        yield buf 

def before_compile(self, query):
        """Receive the :class:`.Query` object before it is composed into a
        core :class:`.Select` object.

        This event is intended to allow changes to the query given::

            @event.listens_for(Query, "before_compile", retval=True)
            def no_deleted(query):
                for desc in query.column_descriptions:
                    if desc['type'] is User:
                        entity = desc['entity']
                        query = query.filter(entity.deleted == False)
                return query

        The event should normally be listened with the ``retval=True``
        parameter set, so that the modified query may be returned.


        """ 

def init_session(request, configuration_loaded):
    # Init registry
    additional_setting = {'unittest': True}

    if len(BlokManager.list()) == 0:
        BlokManager.load()
    registry = RegistryManager.get(Configuration.get('db_name'),
                                   **additional_setting)
    registry.commit()
    # Add savepoint
    registry.begin_nested()

    @event.listens_for(registry.session, 'after_transaction_end')
    def restart_savepoint(session, transaction):
        if transaction.nested and not transaction._parent.nested:
            session.expire_all()
            session.begin_nested()

    logger.info('Creating registry')
    yield registry

    request.addfinalizer(registry.session.close) 

def instrument_class(self, mapper, class_):
        """Receive a class when the mapper is first constructed,
        before instrumentation is applied to the mapped class.

        This event is the earliest phase of mapper construction.
        Most attributes of the mapper are not yet initialized.

        This listener can either be applied to the :class:`.Mapper`
        class overall, or to any un-mapped class which serves as a base
        for classes that will be mapped (using the ``propagate=True`` flag)::

            Base = declarative_base()

            @event.listens_for(Base, "instrument_class", propagate=True)
            def on_new_class(mapper, cls_):
                " ... "

        :param mapper: the :class:`.Mapper` which is the target
         of this event.
        :param class\_: the mapped class.

        """ 

def instrument_class(self, mapper, class_):
        """Receive a class when the mapper is first constructed,
        before instrumentation is applied to the mapped class.

        This event is the earliest phase of mapper construction.
        Most attributes of the mapper are not yet initialized.

        This listener can either be applied to the :class:`.Mapper`
        class overall, or to any un-mapped class which serves as a base
        for classes that will be mapped (using the ``propagate=True`` flag)::

            Base = declarative_base()

            @event.listens_for(Base, "instrument_class", propagate=True)
            def on_new_class(mapper, cls_):
                " ... "

        :param mapper: the :class:`.Mapper` which is the target
         of this event.
        :param class\_: the mapped class.

        """ 

def instrument_class(self, mapper, class_):
        r"""Receive a class when the mapper is first constructed,
        before instrumentation is applied to the mapped class.

        This event is the earliest phase of mapper construction.
        Most attributes of the mapper are not yet initialized.

        This listener can either be applied to the :class:`.Mapper`
        class overall, or to any un-mapped class which serves as a base
        for classes that will be mapped (using the ``propagate=True`` flag)::

            Base = declarative_base()

            @event.listens_for(Base, "instrument_class", propagate=True)
            def on_new_class(mapper, cls_):
                " ... "

        :param mapper: the :class:`.Mapper` which is the target
         of this event.
        :param class\_: the mapped class.

        """ 

def setup_listener(mapper, class_):
    name = class_.__name__
    discriminator = name.lower()
    class_.addresses = relationship(
        Address,
        primaryjoin=and_(
            class_.id == foreign(remote(Address.parent_id)),
            Address.discriminator == discriminator,
        ),
        backref=backref(
            "parent_%s" % discriminator,
            primaryjoin=remote(class_.id) == foreign(Address.parent_id),
        ),
    )

    @event.listens_for(class_.addresses, "append")
    def append_address(target, value, initiator):
        value.discriminator = discriminator 

def default_listener(col_attr, default):
    """Establish a default-setting listener.

    Given a class attribute and a :class:`.DefaultGenerator` instance.
    The default generator should be a :class:`.ColumnDefault` object with a
    plain Python value or callable default; otherwise, the appropriate behavior
    for SQL functions and defaults should be determined here by the
    user integrating this feature.

    """

    @event.listens_for(col_attr, "init_scalar", retval=True, propagate=True)
    def init_scalar(target, value, dict_):

        if default.is_callable:
            # the callable of ColumnDefault always accepts a context
            # argument; we can pass it as None here.
            value = default.arg(None)
        elif default.is_scalar:
            value = default.arg
        else:
            # default is a Sequence, a SQL expression, server
            # side default generator, or other non-Python-evaluable
            # object.  The feature here can't easily support this.   This
            # can be made to return None, rather than raising,
            # or can procure a connection from an Engine
            # or Session and actually run the SQL, if desired.
            raise NotImplementedError(
                "Can't invoke pre-default for a SQL-level column default"
            )

        # set the value in the given dict_; this won't emit any further
        # attribute set events or create attribute "history", but the value
        # will be used in the INSERT statement
        dict_[col_attr.key] = value

        # return the value as well
        return value 

def versioned_session(session):
    @event.listens_for(session, "before_flush")
    def before_flush(session, flush_context, instances):
        for obj in versioned_objects(session.dirty):
            create_version(obj, session)
        for obj in versioned_objects(session.deleted):
            create_version(obj, session, deleted=True) 

def remove(target, identifier, fn):
    """Remove an event listener.

    The arguments here should match exactly those which were sent to
    :func:`.listen`; all the event registration which proceeded as a result
    of this call will be reverted by calling :func:`.remove` with the same
    arguments.

    e.g.::

        # if a function was registered like this...
        @event.listens_for(SomeMappedClass, "before_insert", propagate=True)
        def my_listener_function(*arg):
            pass

        # ... it's removed like this
        event.remove(SomeMappedClass, "before_insert", my_listener_function)

    Above, the listener function associated with ``SomeMappedClass`` was also
    propagated to subclasses of ``SomeMappedClass``; the :func:`.remove`
    function will revert all of these operations.

    .. versionadded:: 0.9.0

    """
    _event_key(target, identifier, fn).remove() 

def before_execute(
        self, conn, clauseelement, multiparams, params, execution_options
    ):
        """Intercept high level execute() events, receiving uncompiled
        SQL constructs and other objects prior to rendering into SQL.

        This event is good for debugging SQL compilation issues as well
        as early manipulation of the parameters being sent to the database,
        as the parameter lists will be in a consistent format here.

        This event can be optionally established with the ``retval=True``
        flag.  The ``clauseelement``, ``multiparams``, and ``params``
        arguments should be returned as a three-tuple in this case::

            @event.listens_for(Engine, "before_execute", retval=True)
            def before_execute(conn, clauseelement, multiparams, params):
                # do something with clauseelement, multiparams, params
                return clauseelement, multiparams, params

        :param conn: :class:`_engine.Connection` object
        :param clauseelement: SQL expression construct, :class:`.Compiled`
         instance, or string statement passed to
         :meth:`_engine.Connection.execute`.
        :param multiparams: Multiple parameter sets, a list of dictionaries.
        :param params: Single parameter set, a single dictionary.
        :param execution_options: dictionary of execution
         options passed along with the statement, if any.  This is a merge
         of all options that will be used, including those of the statement,
         the connection, and those passed in to the method itself for
         the 2.0 style of execution.

         .. versionadded: 1.4

        .. seealso::

            :meth:`.before_cursor_execute`

        """ 

def before_cursor_execute(
        self, conn, cursor, statement, parameters, context, executemany
    ):
        """Intercept low-level cursor execute() events before execution,
        receiving the string SQL statement and DBAPI-specific parameter list to
        be invoked against a cursor.

        This event is a good choice for logging as well as late modifications
        to the SQL string.  It's less ideal for parameter modifications except
        for those which are specific to a target backend.

        This event can be optionally established with the ``retval=True``
        flag.  The ``statement`` and ``parameters`` arguments should be
        returned as a two-tuple in this case::

            @event.listens_for(Engine, "before_cursor_execute", retval=True)
            def before_cursor_execute(conn, cursor, statement,
                            parameters, context, executemany):
                # do something with statement, parameters
                return statement, parameters

        See the example at :class:`_events.ConnectionEvents`.

        :param conn: :class:`_engine.Connection` object
        :param cursor: DBAPI cursor object
        :param statement: string SQL statement, as to be passed to the DBAPI
        :param parameters: Dictionary, tuple, or list of parameters being
         passed to the ``execute()`` or ``executemany()`` method of the
         DBAPI ``cursor``.  In some cases may be ``None``.
        :param context: :class:`.ExecutionContext` object in use.  May
         be ``None``.
        :param executemany: boolean, if ``True``, this is an ``executemany()``
         call, if ``False``, this is an ``execute()`` call.

        .. seealso::

            :meth:`.before_execute`

            :meth:`.after_cursor_execute`

        """ 

def instrument_class(self, mapper, class_):
        r"""Receive a class when the mapper is first constructed,
        before instrumentation is applied to the mapped class.

        This event is the earliest phase of mapper construction.
        Most attributes of the mapper are not yet initialized.

        This listener can either be applied to the :class:`_orm.Mapper`
        class overall, or to any un-mapped class which serves as a base
        for classes that will be mapped (using the ``propagate=True`` flag)::

            Base = declarative_base()

            @event.listens_for(Base, "instrument_class", propagate=True)
            def on_new_class(mapper, cls_):
                " ... "

        :param mapper: the :class:`_orm.Mapper` which is the target
         of this event.
        :param class\_: the mapped class.

        """ 

def init_ndb_events(engine):
    """Initialize NDB Events.

    Function starts NDB specific events.
    """
    @event.listens_for(engine, "before_cursor_execute", retval=True)
    def before_cursor_execute(conn, cursor, statement, parameters, context,
                              executemany):
        """Listen for specific SQL strings and replace automatically.

        Function will intercept any raw execute calls and automatically
        convert InnoDB to NDBCLUSTER, drop SAVEPOINT requests, drop
        ROLLBACK requests, and drop RELEASE SAVEPOINT requests.
        """
        if ndb_status(engine):
            statement = engine_regex.sub("ENGINE=NDBCLUSTER", statement)
            if re.match(trans_regex, statement):
                statement = "SET @oslo_db_ndb_savepoint_rollback_disabled = 0;"

        return statement, parameters 

def register_engine(engine):
    event.listen(engine, "handle_error", handler, retval=True)

    @event.listens_for(engine, "rollback_savepoint")
    def rollback_savepoint(conn, name, context):
        exc_info = sys.exc_info()
        if exc_info[1]:
            # NOTE(zzzeek) accessing conn.info on an invalidated
            # connection causes it to reconnect, which we don't
            # want to do inside a rollback handler
            if not conn.invalidated:
                conn.info[ROLLBACK_CAUSE_KEY] = exc_info[1]
        # NOTE(zzzeek) this eliminates a reference cycle between tracebacks
        # that would occur in Python 3 only, which has been shown to occur if
        # this function were in fact part of the traceback.  That's not the
        # case here however this is left as a defensive measure.
        del exc_info

    # try to clear the "cause" ASAP outside of savepoints,
    # by grabbing the end of transaction events...
    @event.listens_for(engine, "rollback")
    @event.listens_for(engine, "commit")
    def pop_exc_tx(conn):
        # NOTE(zzzeek) accessing conn.info on an invalidated
        # connection causes it to reconnect, which we don't
        # want to do inside a rollback handler
        if not conn.invalidated:
            conn.info.pop(ROLLBACK_CAUSE_KEY, None)

    # .. as well as connection pool checkin (just in case).
    # the .info dictionary lasts as long as the DBAPI connection itself
    # and is cleared out when the connection is recycled or closed
    # due to invalidate etc.
    @event.listens_for(engine, "checkin")
    def pop_exc_checkin(dbapi_conn, connection_record):
        connection_record.info.pop(ROLLBACK_CAUSE_KEY, None) 

def before_cursor_execute(self, conn, cursor, statement,
                              parameters, context, executemany):
        """Intercept low-level cursor execute() events before execution,
        receiving the string SQL statement and DBAPI-specific parameter list to
        be invoked against a cursor.

        This event is a good choice for logging as well as late modifications
        to the SQL string.  It's less ideal for parameter modifications except
        for those which are specific to a target backend.

        This event can be optionally established with the ``retval=True``
        flag.  The ``statement`` and ``parameters`` arguments should be
        returned as a two-tuple in this case::

            @event.listens_for(Engine, "before_cursor_execute", retval=True)
            def before_cursor_execute(conn, cursor, statement,
                            parameters, context, executemany):
                # do something with statement, parameters
                return statement, parameters

        See the example at :class:`.ConnectionEvents`.

        :param conn: :class:`.Connection` object
        :param cursor: DBAPI cursor object
        :param statement: string SQL statement, as to be passed to the DBAPI
        :param parameters: Dictionary, tuple, or list of parameters being
         passed to the ``execute()`` or ``executemany()`` method of the
         DBAPI ``cursor``.  In some cases may be ``None``.
        :param context: :class:`.ExecutionContext` object in use.  May
         be ``None``.
        :param executemany: boolean, if ``True``, this is an ``executemany()``
         call, if ``False``, this is an ``execute()`` call.

        See also:

        :meth:`.before_execute`

        :meth:`.after_cursor_execute`

        """ 

def _sqlite_post_configure_engine(url, engine, follower_ident):
    from sqlalchemy import event

    @event.listens_for(engine, "connect")
    def connect(dbapi_connection, connection_record):
        # use file DBs in all cases, memory acts kind of strangely
        # as an attached
        if not follower_ident:
            dbapi_connection.execute(
                'ATTACH DATABASE "test_schema.db" AS test_schema')
        else:
            dbapi_connection.execute(
                'ATTACH DATABASE "%s_test_schema.db" AS test_schema'
                % follower_ident) 

def after_soft_rollback(self, session, previous_transaction):
        """Execute after any rollback has occurred, including "soft"
        rollbacks that don't actually emit at the DBAPI level.

        This corresponds to both nested and outer rollbacks, i.e.
        the innermost rollback that calls the DBAPI's
        rollback() method, as well as the enclosing rollback
        calls that only pop themselves from the transaction stack.

        The given :class:`.Session` can be used to invoke SQL and
        :meth:`.Session.query` operations after an outermost rollback
        by first checking the :attr:`.Session.is_active` flag::

            @event.listens_for(Session, "after_soft_rollback")
            def do_something(session, previous_transaction):
                if session.is_active:
                    session.execute("select * from some_table")

        :param session: The target :class:`.Session`.
        :param previous_transaction: The :class:`.SessionTransaction`
         transactional marker object which was just closed.   The current
         :class:`.SessionTransaction` for the given :class:`.Session` is
         available via the :attr:`.Session.transaction` attribute.

        .. versionadded:: 0.7.3

        """ 

def after_transaction_end(self, session, transaction):
        """Execute when the span of a :class:`.SessionTransaction` ends.

        This event differs from :meth:`~.SessionEvents.after_commit`
        in that it corresponds to all :class:`.SessionTransaction`
        objects in use, including those for nested transactions
        and subtransactions, and is always matched by a corresponding
        :meth:`~.SessionEvents.after_transaction_create` event.

        :param session: the target :class:`.Session`.
        :param transaction: the target :class:`.SessionTransaction`.

         To detect if this is the outermost
         :class:`.SessionTransaction`, as opposed to a "subtransaction" or a
         SAVEPOINT, test that the :attr:`.SessionTransaction.parent` attribute
         is ``None``::

                @event.listens_for(session, "after_transaction_create")
                def after_transaction_end(session, transaction):
                    if transaction.parent is None:
                        # work with top-level transaction

         To detect if the :class:`.SessionTransaction` is a SAVEPOINT, use the
         :attr:`.SessionTransaction.nested` attribute::

                @event.listens_for(session, "after_transaction_create")
                def after_transaction_end(session, transaction):
                    if transaction.nested:
                        # work with SAVEPOINT transaction


        .. seealso::

            :class:`.SessionTransaction`

            :meth:`~.SessionEvents.after_transaction_create`

        """ 

def after_transaction_create(self, session, transaction):
        """Execute when a new :class:`.SessionTransaction` is created.

        This event differs from :meth:`~.SessionEvents.after_begin`
        in that it occurs for each :class:`.SessionTransaction`
        overall, as opposed to when transactions are begun
        on individual database connections.  It is also invoked
        for nested transactions and subtransactions, and is always
        matched by a corresponding
        :meth:`~.SessionEvents.after_transaction_end` event
        (assuming normal operation of the :class:`.Session`).

        :param session: the target :class:`.Session`.
        :param transaction: the target :class:`.SessionTransaction`.

         To detect if this is the outermost
         :class:`.SessionTransaction`, as opposed to a "subtransaction" or a
         SAVEPOINT, test that the :attr:`.SessionTransaction.parent` attribute
         is ``None``::

                @event.listens_for(session, "after_transaction_create")
                def after_transaction_create(session, transaction):
                    if transaction.parent is None:
                        # work with top-level transaction

         To detect if the :class:`.SessionTransaction` is a SAVEPOINT, use the
         :attr:`.SessionTransaction.nested` attribute::

                @event.listens_for(session, "after_transaction_create")
                def after_transaction_create(session, transaction):
                    if transaction.nested:
                        # work with SAVEPOINT transaction


        .. seealso::

            :class:`.SessionTransaction`

            :meth:`~.SessionEvents.after_transaction_end`

        """ 

def instrument_class(self, mapper, class_):
        r"""Receive a class when the mapper is first constructed,
        before instrumentation is applied to the mapped class.

        This event is the earliest phase of mapper construction.
        Most attributes of the mapper are not yet initialized.

        This listener can either be applied to the :class:`.Mapper`
        class overall, or to any un-mapped class which serves as a base
        for classes that will be mapped (using the ``propagate=True`` flag)::

            Base = declarative_base()

            @event.listens_for(Base, "instrument_class", propagate=True)
            def on_new_class(mapper, cls_):
                " ... "

        :param mapper: the :class:`.Mapper` which is the target
         of this event.
        :param class\_: the mapped class.

        """ 

def _sqlite_post_configure_engine(url, engine, follower_ident):
    from sqlalchemy import event

    @event.listens_for(engine, "connect")
    def connect(dbapi_connection, connection_record):
        # use file DBs in all cases, memory acts kind of strangely
        # as an attached
        if not follower_ident:
            dbapi_connection.execute(
                'ATTACH DATABASE "test_schema.db" AS test_schema')
        else:
            dbapi_connection.execute(
                'ATTACH DATABASE "%s_test_schema.db" AS test_schema'
                % follower_ident) 

def listens_for(target, identifier, *args, **kw):
    """Decorate a function as a listener for the given target + identifier.

    e.g.::

        from sqlalchemy import event
        from sqlalchemy.schema import UniqueConstraint

        @event.listens_for(UniqueConstraint, "after_parent_attach")
        def unique_constraint_name(const, table):
            const.name = "uq_%s_%s" % (
                table.name,
                list(const.columns)[0].name
            )

    A given function can also be invoked for only the first invocation
    of the event using the ``once`` argument::

        @event.listens_for(Mapper, "before_configure", once=True)
        def on_config():
            do_config()


    .. versionadded:: 0.9.4 Added ``once=True`` to :func:`.event.listen`
       and :func:`.event.listens_for`.

    .. seealso::

        :func:`.listen` - general description of event listening

    """
    def decorate(fn):
        listen(target, identifier, fn, *args, **kw)
        return fn
    return decorate 

def before_cursor_execute(self, conn, cursor, statement,
                              parameters, context, executemany):
        """Intercept low-level cursor execute() events before execution,
        receiving the string SQL statement and DBAPI-specific parameter list to
        be invoked against a cursor.

        This event is a good choice for logging as well as late modifications
        to the SQL string.  It's less ideal for parameter modifications except
        for those which are specific to a target backend.

        This event can be optionally established with the ``retval=True``
        flag.  The ``statement`` and ``parameters`` arguments should be
        returned as a two-tuple in this case::

            @event.listens_for(Engine, "before_cursor_execute", retval=True)
            def before_cursor_execute(conn, cursor, statement,
                            parameters, context, executemany):
                # do something with statement, parameters
                return statement, parameters

        See the example at :class:`.ConnectionEvents`.

        :param conn: :class:`.Connection` object
        :param cursor: DBAPI cursor object
        :param statement: string SQL statement, as to be passed to the DBAPI
        :param parameters: Dictionary, tuple, or list of parameters being
         passed to the ``execute()`` or ``executemany()`` method of the
         DBAPI ``cursor``.  In some cases may be ``None``.
        :param context: :class:`.ExecutionContext` object in use.  May
         be ``None``.
        :param executemany: boolean, if ``True``, this is an ``executemany()``
         call, if ``False``, this is an ``execute()`` call.

        See also:

        :meth:`.before_execute`

        :meth:`.after_cursor_execute`

        """ 

def before_execute(self, conn, clauseelement, multiparams, params):
        """Intercept high level execute() events, receiving uncompiled
        SQL constructs and other objects prior to rendering into SQL.

        This event is good for debugging SQL compilation issues as well
        as early manipulation of the parameters being sent to the database,
        as the parameter lists will be in a consistent format here.

        This event can be optionally established with the ``retval=True``
        flag.  The ``clauseelement``, ``multiparams``, and ``params``
        arguments should be returned as a three-tuple in this case::

            @event.listens_for(Engine, "before_execute", retval=True)
            def before_execute(conn, clauseelement, multiparams, params):
                # do something with clauseelement, multiparams, params
                return clauseelement, multiparams, params

        :param conn: :class:`.Connection` object
        :param clauseelement: SQL expression construct, :class:`.Compiled`
         instance, or string statement passed to :meth:`.Connection.execute`.
        :param multiparams: Multiple parameter sets, a list of dictionaries.
        :param params: Single parameter set, a single dictionary.

        See also:

        :meth:`.before_cursor_execute`

        """ 

def after_soft_rollback(self, session, previous_transaction):
        """Execute after any rollback has occurred, including "soft"
        rollbacks that don't actually emit at the DBAPI level.

        This corresponds to both nested and outer rollbacks, i.e.
        the innermost rollback that calls the DBAPI's
        rollback() method, as well as the enclosing rollback
        calls that only pop themselves from the transaction stack.

        The given :class:`.Session` can be used to invoke SQL and
        :meth:`.Session.query` operations after an outermost rollback
        by first checking the :attr:`.Session.is_active` flag::

            @event.listens_for(Session, "after_soft_rollback")
            def do_something(session, previous_transaction):
                if session.is_active:
                    session.execute("select * from some_table")

        :param session: The target :class:`.Session`.
        :param previous_transaction: The :class:`.SessionTransaction`
         transactional marker object which was just closed.   The current
         :class:`.SessionTransaction` for the given :class:`.Session` is
         available via the :attr:`.Session.transaction` attribute.

        .. versionadded:: 0.7.3

        """ 

def after_transaction_end(self, session, transaction):
        """Execute when the span of a :class:`.SessionTransaction` ends.

        This event differs from :meth:`~.SessionEvents.after_commit`
        in that it corresponds to all :class:`.SessionTransaction`
        objects in use, including those for nested transactions
        and subtransactions, and is always matched by a corresponding
        :meth:`~.SessionEvents.after_transaction_create` event.

        :param session: the target :class:`.Session`.
        :param transaction: the target :class:`.SessionTransaction`.

         To detect if this is the outermost
         :class:`.SessionTransaction`, as opposed to a "subtransaction" or a
         SAVEPOINT, test that the :attr:`.SessionTransaction.parent` attribute
         is ``None``::

                @event.listens_for(session, "after_transaction_create")
                def after_transaction_end(session, transaction):
                    if transaction.parent is None:
                        # work with top-level transaction

         To detect if the :class:`.SessionTransaction` is a SAVEPOINT, use the
         :attr:`.SessionTransaction.nested` attribute::

                @event.listens_for(session, "after_transaction_create")
                def after_transaction_end(session, transaction):
                    if transaction.nested:
                        # work with SAVEPOINT transaction


        .. seealso::

            :class:`.SessionTransaction`

            :meth:`~.SessionEvents.after_transaction_create`

        """ 

def after_transaction_create(self, session, transaction):
        """Execute when a new :class:`.SessionTransaction` is created.

        This event differs from :meth:`~.SessionEvents.after_begin`
        in that it occurs for each :class:`.SessionTransaction`
        overall, as opposed to when transactions are begun
        on individual database connections.  It is also invoked
        for nested transactions and subtransactions, and is always
        matched by a corresponding
        :meth:`~.SessionEvents.after_transaction_end` event
        (assuming normal operation of the :class:`.Session`).

        :param session: the target :class:`.Session`.
        :param transaction: the target :class:`.SessionTransaction`.

         To detect if this is the outermost
         :class:`.SessionTransaction`, as opposed to a "subtransaction" or a
         SAVEPOINT, test that the :attr:`.SessionTransaction.parent` attribute
         is ``None``::

                @event.listens_for(session, "after_transaction_create")
                def after_transaction_create(session, transaction):
                    if transaction.parent is None:
                        # work with top-level transaction

         To detect if the :class:`.SessionTransaction` is a SAVEPOINT, use the
         :attr:`.SessionTransaction.nested` attribute::

                @event.listens_for(session, "after_transaction_create")
                def after_transaction_create(session, transaction):
                    if transaction.nested:
                        # work with SAVEPOINT transaction


        .. seealso::

            :class:`.SessionTransaction`

            :meth:`~.SessionEvents.after_transaction_end`

        """