What's New in SQLAlchemy 0.9?
About this Document
This document describes changes between SQLAlchemy version 0.8, undergoing maintenance releases as of May, 2013, and SQLAlchemy version 0.9, which is expected for release in late 2013.
Document last updated: July 26, 2013
This guide introduces what's new in SQLAlchemy version 0.9, and also documents changes which affect users migrating their applications from the 0.8 series of SQLAlchemy to 0.9.
Version 0.9 is a faster-than-usual push from version 0.8, featuring a more versatile codebase with regards to modern Python versions. See :ref:`behavioral_changes_09` for potentially backwards-incompatible changes.
Targeting Python 2.6 and Up Now, Python 3 without 2to3
The first achievement of the 0.9 release is to remove the dependency on the 2to3 tool for Python 3 compatibility. To make this more straightforward, the lowest Python release targeted now is 2.6, which features a wide degree of cross-compatibility with Python 3. All SQLAlchemy modules and unit tests are now interpreted equally well with any Python interpreter from 2.6 forward, including the 3.1 and 3.2 interpreters.
C Extensions Supported on Python 3
The C extensions have been ported to support Python 3 and now build in both Python 2 and Python 3 environments.
:meth:`.Query.select_from` no longer applies the clause to corresponding entities
The :meth:`.Query.select_from` method has been popularized in recent versions as a means of controlling the first thing that a :class:`.Query` object "selects from", typically for the purposes of controlling how a JOIN will render.
Consider the following example against the usual User mapping:
select_stmt = select([User]).where(User.id == 7).alias() q = session.query(User).\ join(select_stmt, User.id == select_stmt.c.id).\ filter(User.name == 'ed')
The above statement predictably renders SQL like the following:
SELECT "user".id AS user_id, "user".name AS user_name FROM "user" JOIN (SELECT "user".id AS id, "user".name AS name FROM "user" WHERE "user".id = :id_1) AS anon_1 ON "user".id = anon_1.id WHERE "user".name = :name_1
If we wanted to reverse the order of the left and right elements of the JOIN, the documentation would lead us to believe we could use :meth:`.Query.select_from` to do so:
q = session.query(User).\ select_from(select_stmt).\ join(User, User.id == select_stmt.c.id).\ filter(User.name == 'ed')
However, in version 0.8 and earlier, the above use of :meth:`.Query.select_from` would apply the select_stmt to replace the User entity, as it selects from the user table which is compatible with User:
-- SQLAlchemy 0.8 and earlier... SELECT anon_1.id AS anon_1_id, anon_1.name AS anon_1_name FROM (SELECT "user".id AS id, "user".name AS name FROM "user" WHERE "user".id = :id_1) AS anon_1 JOIN "user" ON anon_1.id = anon_1.id WHERE anon_1.name = :name_1
The above statement is a mess, the ON clause refers anon_1.id = anon_1.id, our WHERE clause has been replaced with anon_1 as well.
This behavior is quite intentional, but has a different use case from that which has become popular for :meth:`.Query.select_from`. The above behavior is now available by a new method known as :meth:`.Query.select_entity_from`. This is a lesser used behavior that in modern SQLAlchemy is roughly equivalent to selecting from a customized :func:`.aliased` construct:
select_stmt = select([User]).where(User.id == 7) user_from_stmt = aliased(User, select_stmt.alias()) q = session.query(user_from_stmt).filter(user_from_stmt.name == 'ed')
So with SQLAlchemy 0.9, our query that selects from select_stmt produces the SQL we expect:
-- SQLAlchemy 0.9 SELECT "user".id AS user_id, "user".name AS user_name FROM (SELECT "user".id AS id, "user".name AS name FROM "user" WHERE "user".id = :id_1) AS anon_1 JOIN "user" ON "user".id = id WHERE "user".name = :name_1
The :meth:`.Query.select_entity_from` method will be available in SQLAlchemy 0.8.2, so applications which rely on the old behavior can transition to this method first, ensure all tests continue to function, then upgrade to 0.9 without issue.
Backref handlers can now propagate more than one level deep
The mechanism by which attribute events pass along their "initiator", that is the object associated with the start of the event, has been changed; instead of a :class:`.AttributeImpl` being passed, a new object :class:`.attributes.Event` is passed instead; this object refers to the :class:`.AttributeImpl` as well as to an "operation token", representing if the operation is an append, remove, or replace operation.
The attribute event system no longer looks at this "initiator" object in order to halt a recursive series of attribute events. Instead, the system of preventing endless recursion due to mutually-dependent backref handlers has been moved to the ORM backref event handlers specifically, which now take over the role of ensuring that a chain of mutually-dependent events (such as append to collection A.bs, set many-to-one attribute B.a in response) doesn't go into an endless recursion stream. The rationale here is that the backref system, given more detail and control over event propagation, can finally allow operations more than one level deep to occur; the typical scenario is when a collection append results in a many-to-one replacement operation, which in turn should cause the item to be removed from a previous collection:
class Parent(Base): __tablename__ = 'parent' id = Column(Integer, primary_key=True) children = relationship("Child", backref="parent") class Child(Base): __tablename__ = 'child' id = Column(Integer, primary_key=True) parent_id = Column(ForeignKey('parent.id')) p1 = Parent() p2 = Parent() c1 = Child() p1.children.append(c1) assert c1.parent is p1 # backref event establishes c1.parent as p1 p2.children.append(c1) assert c1.parent is p2 # backref event establishes c1.parent as p2 assert c1 not in p1.children # second backref event removes c1 from p1.children
Above, prior to this change, the c1 object would still have been present in p1.children, even though it is also present in p2.children at the same time; the backref handlers would have stopped at replacing c1.parent with p2 instead of p1. In 0.9, using the more detailed :class:`.Event` object as well as letting the backref handlers make more detailed decisions about these objects, the propagation can continue onto removing c1 from p1.children while maintaining a check against the propagation from going into an endless recursive loop.
End-user code which a. makes use of the :meth:`.AttributeEvents.set`, :meth:`.AttributeEvents.append`, or :meth:`.AttributeEvents.remove` events, and b. initiates further attribute modification operations as a result of these events may need to be modified to prevent recursive loops, as the attribute system no longer stops a chain of events from propagating endlessly in the absense of the backref event handlers. Additionally, code which depends upon the value of the initiator will need to be adjusted to the new API, and furthermore must be ready for the value of initiator to change from its original value within a string of backref-initiated events, as the backref handlers may now swap in a new initiator value for some operations.
Association Proxy SQL Expression Improvements and Fixes
The == and != operators as implemented by an association proxy that refers to a scalar value on a scalar relationship now produces a more complete SQL expression, intended to take into account the "association" row being present or not when the comparison is against None.
Consider this mapping:
class A(Base): __tablename__ = 'a' id = Column(Integer, primary_key=True) b_id = Column(Integer, ForeignKey('b.id'), primary_key=True) b = relationship("B") b_value = association_proxy("b", "value") class B(Base): __tablename__ = 'b' id = Column(Integer, primary_key=True) value = Column(String)
Up through 0.8, a query like the following:
s.query(A).filter(A.b_value == None).all()
SELECT a.id AS a_id, a.b_id AS a_b_id FROM a WHERE EXISTS (SELECT 1 FROM b WHERE b.id = a.b_id AND b.value IS NULL)
In 0.9, it now produces:
SELECT a.id AS a_id, a.b_id AS a_b_id FROM a WHERE (EXISTS (SELECT 1 FROM b WHERE b.id = a.b_id AND b.value IS NULL)) OR a.b_id IS NULL
The difference being, it not only checks b.value, it also checks if a refers to no b row at all. This will return different results versus prior versions, for a system that uses this type of comparison where some parent rows have no association row.
More critically, a correct expression is emitted for A.b_value != None. In 0.8, this would return True for A rows that had no b:
SELECT a.id AS a_id, a.b_id AS a_b_id FROM a WHERE NOT (EXISTS (SELECT 1 FROM b WHERE b.id = a.b_id AND b.value IS NULL))
Now in 0.9, the check has been reworked so that it ensures the A.b_id row is present, in addition to B.value being non-NULL:
SELECT a.id AS a_id, a.b_id AS a_b_id FROM a WHERE EXISTS (SELECT 1 FROM b WHERE b.id = a.b_id AND b.value IS NOT NULL)
In addition, the has() operator is enhanced such that you can call it against a scalar column value with no criterion only, and it will produce criteria that checks for the association row being present or not:
SELECT a.id AS a_id, a.b_id AS a_b_id FROM a WHERE EXISTS (SELECT 1 FROM b WHERE b.id = a.b_id)
This is equivalent to A.b.has(), but allows one to query against b_value directly.
Schema identifiers now carry along their own quoting information
This change simplifies the Core's usage of so-called "quote" flags, such as the quote flag passed to :class:`.Table` and :class:`.Column`. The flag is now internalized within the string name itself, which is now represented as an instance of :class:`.quoted_name`, a string subclass. The :class:`.IdentifierPreparer` now relies solely on the quoting preferences reported by the :class:`.quoted_name` object rather than checking for any explicit quote flags in most cases. The issue resolved here includes that various case-sensitive methods such as :meth:`.Engine.has_table` as well as similar methods within dialects now function with explicitly quoted names, without the need to complicate or introduce backwards-incompatible changes to those APIs (many of which are 3rd party) with the details of quoting flags - in particular, a wider range of identifiers now function correctly with the so-called "uppercase" backends like Oracle, Firebird, and DB2 (backends that store and report upon table and column names using all uppercase for case insensitive names).
The :class:`.quoted_name` object is used internally as needed; however if other keywords require fixed quoting preferences, the class is available publically.
Event Removal API
Events established using :func:`.event.listen` or :func:`.event.listens_for` can now be removed using the new :func:`.event.remove` function. The target, identifier and fn arguments sent to :func:`.event.remove` need to match exactly those which were sent for listening, and the event will be removed from all locations in which it had been established:
@event.listens_for(MyClass, "before_insert", propagate=True) def my_before_insert(mapper, connection, target): """listen for before_insert""" # ... event.remove(MyClass, "before_insert", my_before_insert)
In the example above, the propagate=True flag is set. This means my_before_insert() is established as a listener for MyClass as well as all subclasses of MyClass. The system tracks everywhere that the my_before_insert() listener function had been placed as a result of this call and removes it as a result of calling :func:`.event.remove`.
The removal system uses a registry to associate arguments passed to :func:`.event.listen` with collections of event listeners, which are in many cases wrapped versions of the original user-supplied function. This registry makes heavy use of weak references in order to allow all the contained contents, such as listener targets, to be garbage collected when they go out of scope.
INSERT from SELECT
After literally years of pointless procrastination this relatively minor syntactical feature has been added, and is also backported to 0.8.3, so technically isn't "new" in 0.9. A :func:`.select` construct or other compatible construct can be passed to the new method :meth:`.Insert.from_select` where it will be used to render an INSERT .. SELECT construct:
>>> from sqlalchemy.sql import table, column >>> t1 = table('t1', column('a'), column('b')) >>> t2 = table('t2', column('x'), column('y')) >>> print(t1.insert().from_select(['a', 'b'], t2.select().where(t2.c.y == 5))) INSERT INTO t1 (a, b) SELECT t2.x, t2.y FROM t2 WHERE t2.y = :y_1
The construct is smart enough to also accommodate ORM objects such as classes and :class:`.Query` objects:
s = Session() q = s.query(User.id, User.name).filter_by(name='ed') ins = insert(Address).from_select((Address.id, Address.email_address), q)
INSERT INTO addresses (id, email_address) SELECT users.id AS users_id, users.name AS users_name FROM users WHERE users.name = :name_1
Server Side Version Counting
The versioning feature of the ORM (now also documented at :ref:`mapper_version_counter`) can now make use of server-side version counting schemes, such as those produced by triggers or database system columns, as well as conditional programmatic schemes outside of the version_id_counter function itself. By providing the value False to the version_id_generator parameter, the ORM will use the already-set version identifier, or alternatively fetch the version identifier from each row at the same time the INSERT or UPDATE is emitted. When using a server-generated version identifier, it is strongly recommended that this feature be used only on a backend where RETURNING can also be used, else the additional SELECT statements will add significant performance overhead. The example provided at :ref:`server_side_version_counter` illustrates the usage of the Postgresql xmin system column in order to integrate it with the ORM's versioning feature.
As a related feature, a new method :meth:`.ValuesBase.return_defaults` has been added to :class:`.Insert` and :class:`.Update` which in constrast to the :meth:`.UpdateBase.returning` method provides RETURNING support that integrates with the "implicit identifier returning" feature and is automatically utilized or de-utilized depending on backend.
Improvements that should produce no compatibility issues, but are good to be aware of in case there are unexpected issues.
Many JOIN and LEFT OUTER JOIN expressions will no longer be wrapped in (SELECT * FROM ..) AS ANON_1
For many years, the SQLAlchemy ORM has been held back from being able to nest a JOIN inside the right side of an existing JOIN (typically a LEFT OUTER JOIN, as INNER JOINs could always be flattened):
SELECT a.*, b.*, c.* FROM a LEFT OUTER JOIN (b JOIN c ON b.id = c.id) ON a.id
This was due to the fact that SQLite, even today, cannot parse a statement of the above format:
SQLite version 188.8.131.52 2013-01-09 11:53:05 Enter ".help" for instructions Enter SQL statements terminated with a ";" sqlite> create table a(id integer); sqlite> create table b(id integer); sqlite> create table c(id integer); sqlite> select a.id, b.id, c.id from a left outer join (b join c on b.id=c.id) on b.id=a.id; Error: no such column: b.id
Right-outer-joins are of course another way to work around right-side parenthesization; this would be significantly complicated and visually unpleasant to implement, but fortunately SQLite doesn't support RIGHT OUTER JOIN either :):
sqlite> select a.id, b.id, c.id from b join c on b.id=c.id ...> right outer join a on b.id=a.id; Error: RIGHT and FULL OUTER JOINs are not currently supported
Back in 2005, it wasn't clear if other databases had trouble with this form, but today it seems clear every database tested except SQLite now supports it (Oracle 8, a very old database, doesn't support the JOIN keyword at all, but SQLAlchemy has always had a simple rewriting scheme in place for Oracle's syntax). To make matters worse, SQLAlchemy's usual workaround of applying a SELECT often degrades performance on platforms like Postgresql and MySQL:
SELECT a.*, anon_1.* FROM a LEFT OUTER JOIN ( SELECT b.id AS b_id, c.id AS c_id FROM b JOIN c ON b.id = c.id ) AS anon_1 ON a.id=anon_1.b_id
A JOIN like the above form is commonplace when working with joined-table inheritance structures; any time :meth:`.Query.join` is used to join from some parent to a joined-table subclass, or when :func:`.joinedload` is used similarly, SQLAlchemy's ORM would always make sure a nested JOIN was never rendered, lest the query wouldn't be able to run on SQLite. Even though the Core has always supported a JOIN of the more compact form, the ORM had to avoid it.
An additional issue would arise when producing joins across many-to-many relationships where special criteria is present in the ON clause. Consider an eager load join like the following:
Assuming a many-to-many from Order to Item which actually refers to a subclass like Subitem, the SQL for the above would look like:
SELECT order.id, order.name FROM order LEFT OUTER JOIN order_item ON order.id = order_item.order_id LEFT OUTER JOIN item ON order_item.item_id = item.id AND item.type = 'subitem'
What's wrong with the above query? Basically, that it will load many order / order_item rows where the criteria of item.type == 'subitem' is not true.
As of SQLAlchemy 0.9, an entirely new approach has been taken. The ORM no longer worries about nesting JOINs in the right side of an enclosing JOIN, and it now will render these as often as possible while still returning the correct results. When the SQL statement is passed to be compiled, the dialect compiler will rewrite the join to suit the target backend, if that backend is known to not support a right-nested JOIN (which currently is only SQLite - if other backends have this issue please let us know!).
So a regular query(Parent).join(Subclass) will now usually produce a simpler expression:
SELECT parent.id AS parent_id FROM parent JOIN ( base_table JOIN subclass_table ON base_table.id = subclass_table.id) ON parent.id = base_table.parent_id
Joined eager loads like query(Parent).options(joinedload(Parent.subclasses)) will alias the individual tables instead of wrapping in an ANON_1:
SELECT parent.*, base_table_1.*, subclass_table_1.* FROM parent LEFT OUTER JOIN ( base_table AS base_table_1 JOIN subclass_table AS subclass_table_1 ON base_table_1.id = subclass_table_1.id) ON parent.id = base_table_1.parent_id
Many-to-many joins and eagerloads will right nest the "secondary" and "right" tables:
SELECT order.id, order.name FROM order LEFT OUTER JOIN (order_item JOIN item ON order_item.item_id = item.id AND item.type = 'subitem') ON order_item.order_id = order.id
All of these joins, when rendered with a :class:`.Select` statement that specifically specifies use_labels=True, which is true for all the queries the ORM emits, are candidates for "join rewriting", which is the process of rewriting all those right-nested joins into nested SELECT statements, while maintaining the identical labeling used by the :class:`.Select`. So SQLite, the one database that won't support this very common SQL syntax even in 2013, shoulders the extra complexity itself, with the above queries rewritten as:
-- sqlite only! SELECT parent.id AS parent_id FROM parent JOIN ( SELECT base_table.id AS base_table_id, base_table.parent_id AS base_table_parent_id, subclass_table.id AS subclass_table_id FROM base_table JOIN subclass_table ON base_table.id = subclass_table.id ) AS anon_1 ON parent.id = anon_1.base_table_parent_id -- sqlite only! SELECT parent.id AS parent_id, anon_1.subclass_table_1_id AS subclass_table_1_id, anon_1.base_table_1_id AS base_table_1_id, anon_1.base_table_1_parent_id AS base_table_1_parent_id FROM parent LEFT OUTER JOIN ( SELECT base_table_1.id AS base_table_1_id, base_table_1.parent_id AS base_table_1_parent_id, subclass_table_1.id AS subclass_table_1_id FROM base_table AS base_table_1 JOIN subclass_table AS subclass_table_1 ON base_table_1.id = subclass_table_1.id ) AS anon_1 ON parent.id = anon_1.base_table_1_parent_id -- sqlite only! SELECT "order".id AS order_id FROM "order" LEFT OUTER JOIN ( SELECT order_item_1.order_id AS order_item_1_order_id, order_item_1.item_id AS order_item_1_item_id, item.id AS item_id, item.type AS item_type FROM order_item AS order_item_1 JOIN item ON item.id = order_item_1.item_id AND item.type IN (?) ) AS anon_1 ON "order".id = anon_1.order_item_1_order_id
The :meth:`.Join.alias`, :func:`.aliased` and :func:`.with_polymorphic` functions now support a new argument, flat=True, which is used to construct aliases of joined-table entities without embedding into a SELECT. This flag is not on by default, to help with backwards compatibility - but now a "polymorhpic" selectable can be joined as a target without any subqueries generated:
employee_alias = with_polymorphic(Person, [Engineer, Manager], flat=True) session.query(Company).join( Company.employees.of_type(employee_alias) ).filter( or_( Engineer.primary_language == 'python', Manager.manager_name == 'dilbert' ) )
Generates (everywhere except SQLite):
SELECT companies.company_id AS companies_company_id, companies.name AS companies_name FROM companies JOIN ( people AS people_1 LEFT OUTER JOIN engineers AS engineers_1 ON people_1.person_id = engineers_1.person_id LEFT OUTER JOIN managers AS managers_1 ON people_1.person_id = managers_1.person_id ) ON companies.company_id = people_1.company_id WHERE engineers.primary_language = %(primary_language_1)s OR managers.manager_name = %(manager_name_1)s
ORM can efficiently fetch just-generated INSERT/UPDATE defaults using RETURNING
The :class:`.Mapper` has long supported an undocumented flag known as eager_defaults=True. The effect of this flag is that when an INSERT or UPDATE proceeds, and the row is known to have server-generated default values, a SELECT would immediately follow it in order to "eagerly" load those new values. Normally, the server-generated columns are marked as "expired" on the object, so that no overhead is incurred unless the application actually accesses these columns soon after the flush. The eager_defaults flag was therefore not of much use as it could only decrease performance, and was present only to support exotic event schemes where users needed default values to be available immediately within the flush process.
In 0.9, as a result of the version id enhancements, eager_defaults can now emit a RETURNING clause for these values, so on a backend with strong RETURNING support in particular Postgresql, the ORM can fetch newly generated default and SQL expression values inline with the INSERT or UPDATE. The feature takes place automatically when the target backend and :class:`.Table` supports "implicit returning".
Label constructs can now render as their name alone in an ORDER BY
For the case where a :class:`.Label` is used in both the columns clause as well as the ORDER BY clause of a SELECT, the label will render as just it's name in the ORDER BY clause, assuming the underlying dialect reports support of this feature.
E.g. an example like:
from sqlalchemy.sql import table, column, select, func t = table('t', column('c1'), column('c2')) expr = (func.foo(t.c.c1) + t.c.c2).label("expr") stmt = select([expr]).order_by(expr) print stmt
Prior to 0.9 would render as:
SELECT foo(t.c1) + t.c2 AS expr FROM t ORDER BY foo(t.c1) + t.c2
And now renders as:
SELECT foo(t.c1) + t.c2 AS expr FROM t ORDER BY expr
The ORDER BY only renders the label if the label isn't further embedded into an expression within the ORDER BY, other than a simple ASC or DESC.
The above format works on all databases tested, but might have compatibility issues with older database versions (MySQL 4? Oracle 8? etc.). Based on user reports we can add rules that will disable the feature based on database version detection.
Columns can reliably get their type from a column referred to via ForeignKey
There's a long standing behavior which says that a :class:`.Column` can be declared without a type, as long as that :class:`.Column` is referred to by a :class:`.ForeignKeyConstraint`, and the type from the referenced column will be copied into this one. The problem has been that this feature never worked very well and wasn't maintained. The core issue was that the :class:`.ForeignKey` object doesn't know what target :class:`.Column` it refers to until it is asked, typically the first time the foreign key is used to construct a :class:`.Join`. So until that time, the parent :class:`.Column` would not have a type, or more specifically, it would have a default type of :class:`.NullType`.
While it's taken a long time, the work to reorganize the initialization of :class:`.ForeignKey` objects has been completed such that this feature can finally work acceptably. At the core of the change is that the :attr:`.ForeignKey.column` attribute no longer lazily initializes the location of the target :class:`.Column`; the issue with this system was that the owning :class:`.Column` would be stuck with :class:`.NullType` as its type until the :class:`.ForeignKey` happened to be used.
In the new version, the :class:`.ForeignKey` coordinates with the eventual :class:`.Column` it will refer to using internal attachment events, so that the moment the referencing :class:`.Column` is associated with the :class:`.MetaData`, all :class:`.ForeignKey` objects that refer to it will be sent a message that they need to initialize their parent column. This system is more complicated but works more solidly; as a bonus, there are now tests in place for a wide variety of :class:`.Column` / :class:`.ForeignKey` configuration scenarios and error messages have been improved to be very specific to no less than seven different error conditions.
Scenarios which now work correctly include:
>>> from sqlalchemy import Table, MetaData, Column, Integer, ForeignKey >>> metadata = MetaData() >>> t2 = Table('t2', metadata, Column('t1id', ForeignKey('t1.id'))) >>> t2.c.t1id.type NullType() >>> t1 = Table('t1', metadata, Column('id', Integer, primary_key=True)) >>> t2.c.t1id.type Integer()
The system now works with :class:`.ForeignKeyConstraint` as well:
>>> from sqlalchemy import Table, MetaData, Column, Integer, ForeignKeyConstraint >>> metadata = MetaData() >>> t2 = Table('t2', metadata, ... Column('t1a'), Column('t1b'), ... ForeignKeyConstraint(['t1a', 't1b'], ['t1.a', 't1.b'])) >>> t2.c.t1a.type NullType() >>> t2.c.t1b.type NullType() >>> t1 = Table('t1', metadata, ... Column('a', Integer, primary_key=True), ... Column('b', Integer, primary_key=True)) >>> t2.c.t1a.type Integer() >>> t2.c.t1b.type Integer()
>>> from sqlalchemy import Table, MetaData, Column, Integer, ForeignKey >>> metadata = MetaData() >>> t2 = Table('t2', metadata, Column('t1id', ForeignKey('t1.id'))) >>> t3 = Table('t3', metadata, Column('t2t1id', ForeignKey('t2.t1id'))) >>> t2.c.t1id.type NullType() >>> t3.c.t2t1id.type NullType() >>> t1 = Table('t1', metadata, Column('id', Integer, primary_key=True)) >>> t2.c.t1id.type Integer() >>> t3.c.t2t1id.type Integer()
Firebird fdb is now the default Firebird dialect.
The fdb dialect is now used if an engine is created without a dialect specifier, i.e. firebird://. fdb is a kinterbasdb compatible DBAPI which per the Firebird project is now their official Python driver.
Firebird fdb and kinterbasdb set retaining=False by default
Both the fdb and kinterbasdb DBAPIs support a flag retaining=True which can be passed to the commit() and rollback() methods of its connection. The documented rationale for this flag is so that the DBAPI can re-use internal transaction state for subsequent transactions, for the purposes of improving performance. However, newer documentation refers to analyses of Firebird's "garbage collection" which expresses that this flag can have a negative effect on the database's ability to process cleanup tasks, and has been reported as lowering performance as a result.
It's not clear how this flag is actually usable given this information, and as it appears to be only a performance enhancing feature, it now defaults to False. The value can be controlled by passing the flag retaining=True to the :func:`.create_engine` call. This is a new flag which is added as of 0.8.2, so applications on 0.8.2 can begin setting this to True or False as desired.