# SQLAlchemy 0.7 Documentation

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Release: 0.7.3 | Release Date: October 16, 2011

# ORM Events

The ORM includes a wide variety of hooks available for subscription. The event system in 0.7 is all new and supercedes the previous system of “extension” classes. For an introduction to the event API, see Events. Non-ORM events such as those regarding connections and low-level statement execution are described in Core Events.

## Attribute Events

class `sqlalchemy.orm.events.``AttributeEvents`

  Define events for object attributes.

  These are typically defined on the class-bound descriptor for the target class.

  e.g.:

      from sqlalchemy import event

      def my_append_listener(target, value, initiator):
          print "received append event for target: %s" % target

      event.listen(MyClass.collection, 'append', my_append_listener)

  Listeners have the option to return a possibly modified version of the value, when the `retval=True` flag is passed to `listen()`:

      def validate_phone(target, value, oldvalue, initiator):
          "Strip non-numeric characters from a phone number"

          return re.sub(r'(?![0-9])', '', value)

      # setup listener on UserContact.phone attribute, instructing
      # it to use the return value
      listen(UserContact.phone, 'set', validate_phone, retval=True)

  A validation function like the above can also raise an exception such as `ValueError` to halt the operation.

  Several modifiers are available to the `listen()` function.

  Parameters:

  - active_history=False – When True, indicates that the “set” event would like to receive the “old” value being replaced unconditionally, even if this requires firing off database loads. Note that `active_history` can also be set directly via `column_property()` and `relationship()`.
  - propagate=False – When True, the listener function will be established not just for the class attribute given, but for attributes of the same name on all current subclasses of that class, as well as all future subclasses of that class, using an additional listener that listens for instrumentation events.
  - raw=False – When True, the “target” argument to the event will be the `InstanceState` management object, rather than the mapped instance itself.
  - retval=False – when True, the user-defined event listening must return the “value” argument from the function. This gives the listening function the opportunity to change the value that is ultimately used for a “set” or “append” event.

  `append`(target, value, initiator)

    Receive a collection append event.

    Parameters:

    - target – the object instance receiving the event. If the listener is registered with `raw=True`, this will be the `InstanceState` object.
    - value – the value being appended. If this listener is registered with `retval=True`, the listener function must return this value, or a new value which replaces it.
    - initiator – the attribute implementation object which initiated this event.

    Returns: | if the event was registered with `retval=True`, the given value, or a new effective value, should be returned.

  `remove`(target, value, initiator)

    Receive a collection remove event.

    Parameters:

    - target – the object instance receiving the event. If the listener is registered with `raw=True`, this will be the `InstanceState` object.
    - value – the value being removed.
    - initiator – the attribute implementation object which initiated this event.

    Returns: | No return value is defined for this event.

  `set`(target, value, oldvalue, initiator)

    Receive a scalar set event.

    Parameters:

    - target – the object instance receiving the event. If the listener is registered with `raw=True`, this will be the `InstanceState` object.
    - value – the value being set. If this listener is registered with `retval=True`, the listener function must return this value, or a new value which replaces it.
    - oldvalue – the previous value being replaced. This may also be the symbol `NEVER_SET` or `NO_VALUE`. If the listener is registered with `active_history=True`, the previous value of the attribute will be loaded from the database if the existing value is currently unloaded or expired.
    - initiator – the attribute implementation object which initiated this event.

    Returns: | if the event was registered with `retval=True`, the given value, or a new effective value, should be returned.

## Mapper Events

class `sqlalchemy.orm.events.``MapperEvents`

  Define events specific to mappings.

  e.g.:

      from sqlalchemy import event

      def my_before_insert_listener(mapper, connection, target):
          # execute a stored procedure upon INSERT,
          # apply the value to the row to be inserted
          target.calculated_value = connection.scalar(
                                      "select my_special_function(%d)"
                                      % target.special_number)

      # associate the listener function with SomeMappedClass,
      # to execute during the "before_insert" hook
      event.listen(SomeMappedClass, 'before_insert', my_before_insert_listener)

  Available targets include mapped classes, instances of `Mapper` (i.e. returned by `mapper()`, `class_mapper()` and similar), as well as the `Mapper` class and `mapper()` function itself for global event reception:

      from sqlalchemy.orm import mapper

      def some_listener(mapper, connection, target):
          log.debug("Instance %s being inserted" % target)

      # attach to all mappers
      event.listen(mapper, 'before_insert', some_listener)

  Mapper events provide hooks into critical sections of the mapper, including those related to object instrumentation, object loading, and object persistence. In particular, the persistence methods `before_insert()`, and `before_update()` are popular places to augment the state being persisted - however, these methods operate with several significant restrictions. The user is encouraged to evaluate the `SessionEvents.before_flush()` and `SessionEvents.after_flush()` methods as more flexible and user-friendly hooks in which to apply additional database state during a flush.

  When using `MapperEvents`, several modifiers are available to the `event.listen()` function.

  Parameters:

  - propagate=False – When True, the event listener should be applied to all inheriting mappers as well as the mapper which is the target of this listener.
  - raw=False – When True, the “target” argument passed to applicable event listener functions will be the instance’s `InstanceState` management object, rather than the mapped instance itself.
  - retval=False –

    when True, the user-defined event function must have a return value, the purpose of which is either to control subsequent event propagation, or to otherwise alter the operation in progress by the mapper. Possible return values are:

    - `sqlalchemy.orm.interfaces.EXT_CONTINUE` - continue event processing normally.
    - `sqlalchemy.orm.interfaces.EXT_STOP` - cancel all subsequent event handlers in the chain.
    - other values - the return value specified by specific listeners, such as `translate_row()` or `create_instance()`.

  `after_configured`()

    Called after a series of mappers have been configured.

    This corresponds to the `orm.configure_mappers()` call, which note is usually called automatically as mappings are first used.

    Theoretically this event is called once per application, but is actually called any time new mappers have been affected by a `orm.configure_mappers()` call. If new mappings are constructed after existing ones have already been used, this event can be called again.

  `after_delete`(mapper, connection, target)

    Receive an object instance after a DELETE statement has been emitted corresponding to that instance.

    This event is used to emit additional SQL statements on the given connection as well as to perform application specific bookkeeping related to a deletion event.

    The event is often called for a batch of objects of the same class after their DELETE statements have been emitted at once in a previous step.

    Handlers should not alter mapped attributes on the objects just flushed or on other objects of the same class, nor should any other ORM-based operation such as `Session.add` take place here. Attribute changes on objects that were already flushed will be discarded, and changes to the flush plan will also not take place. Use `SessionEvents.before_flush()` to change the flush plan on flush.

    Parameters:

    - mapper – the `Mapper` which is the target of this event.
    - connection – the `Connection` being used to emit DELETE statements for this instance. This provides a handle into the current transaction on the target database specific to this instance.
    - target – the mapped instance being deleted. If the event is configured with `raw=True`, this will instead be the `InstanceState` state-management object associated with the instance.

    Returns: | No return value is supported by this event.

  `after_insert`(mapper, connection, target)

    Receive an object instance after an INSERT statement is emitted corresponding to that instance.

    This event is used to modify in-Python-only state on the instance after an INSERT occurs, as well as to emit additional SQL statements on the given connection.

    The event is often called for a batch of objects of the same class after their INSERT statements have been emitted at once in a previous step. In the extremely rare case that this is not desirable, the `mapper()` can be configured with `batch=False`, which will cause batches of instances to be broken up into individual (and more poorly performing) event->persist->event steps.

    Handlers should not alter mapped attributes on the objects just flushed or on other objects of the same class, nor should any other ORM-based operation such as `Session.add` take place here. Attribute changes on objects that were already flushed will be discarded, and changes to the flush plan will also not take place. Use `SessionEvents.before_flush()` to change the flush plan on flush.

    Parameters:

    - mapper – the `Mapper` which is the target of this event.
    - connection – the `Connection` being used to emit INSERT statements for this instance. This provides a handle into the current transaction on the target database specific to this instance.
    - target – the mapped instance being persisted. If the event is configured with `raw=True`, this will instead be the `InstanceState` state-management object associated with the instance.

    Returns: | No return value is supported by this event.

  `after_update`(mapper, connection, target)

    Receive an object instance after an UPDATE statement is emitted corresponding to that instance.

    This event is used to modify in-Python-only state on the instance after an UPDATE occurs, as well as to emit additional SQL statements on the given connection.

    This method is called for all instances that are marked as “dirty”, even those which have no net changes to their column-based attributes, and for which no UPDATE statement has proceeded. An object is marked as dirty when any of its column-based attributes have a “set attribute” operation called or when any of its collections are modified. If, at update time, no column-based attributes have any net changes, no UPDATE statement will be issued. This means that an instance being sent to `after_update()` is not a guarantee that an UPDATE statement has been issued.

    To detect if the column-based attributes on the object have net changes, and therefore resulted in an UPDATE statement, use `object_session(instance).is_modified(instance, include_collections=False)`.

    The event is often called for a batch of objects of the same class after their UPDATE statements have been emitted at once in a previous step. In the extremely rare case that this is not desirable, the `mapper()` can be configured with `batch=False`, which will cause batches of instances to be broken up into individual (and more poorly performing) event->persist->event steps.

    Handlers should not alter mapped attributes on the objects just flushed or on other objects of the same class, nor should any other ORM-based operation such as `Session.add` take place here. Attribute changes on objects that were already flushed will be discarded, and changes to the flush plan will also not take place. Use `SessionEvents.before_flush()` to change the flush plan on flush.

    Parameters:

    - mapper – the `Mapper` which is the target of this event.
    - connection – the `Connection` being used to emit UPDATE statements for this instance. This provides a handle into the current transaction on the target database specific to this instance.
    - target – the mapped instance being persisted. If the event is configured with `raw=True`, this will instead be the `InstanceState` state-management object associated with the instance.

    Returns: | No return value is supported by this event.

  `append_result`(mapper, context, row, target, result, **flags)

    Receive an object instance before that instance is appended to a result list.

    This is a rarely used hook which can be used to alter the construction of a result list returned by `Query`.

    Parameters:

    - mapper – the `Mapper` which is the target of this event.
    - context – the `QueryContext`, which includes a handle to the current `Query` in progress as well as additional state information.
    - row – the result row being handled. This may be an actual `RowProxy` or may be a dictionary containing `Column` objects as keys.
    - target – the mapped instance being populated. If the event is configured with `raw=True`, this will instead be the `InstanceState` state-management object associated with the instance.
    - result – a list-like object where results are being appended.
    - **flags – Additional state information about the current handling of the row.

    Returns: | If this method is registered with `retval=True`, a return value of `EXT_STOP` will prevent the instance from being appended to the given result list, whereas a return value of `EXT_CONTINUE` will result in the default behavior of appending the value to the result list.

  `before_delete`(mapper, connection, target)

    Receive an object instance before a DELETE statement is emitted corresponding to that instance.

    This event is used to emit additional SQL statements on the given connection as well as to perform application specific bookkeeping related to a deletion event.

    The event is often called for a batch of objects of the same class before their DELETE statements are emitted at once in a later step.

    Handlers should not modify any attributes which are mapped by `relationship()`, nor should they attempt to make any modifications to the `Session` in this hook (including `Session.add()`, `Session.delete()`, etc.) - such changes will not take effect. For overall changes to the “flush plan”, use `SessionEvents.before_flush()`.

    Parameters:

    - mapper – the `Mapper` which is the target of this event.
    - connection – the `Connection` being used to emit DELETE statements for this instance. This provides a handle into the current transaction on the target database specific to this instance.
    - target – the mapped instance being deleted. If the event is configured with `raw=True`, this will instead be the `InstanceState` state-management object associated with the instance.

    Returns: | No return value is supported by this event.

  `before_insert`(mapper, connection, target)

    Receive an object instance before an INSERT statement is emitted corresponding to that instance.

    This event is used to modify local, non-object related attributes on the instance before an INSERT occurs, as well as to emit additional SQL statements on the given connection.

    The event is often called for a batch of objects of the same class before their INSERT statements are emitted at once in a later step. In the extremely rare case that this is not desirable, the `mapper()` can be configured with `batch=False`, which will cause batches of instances to be broken up into individual (and more poorly performing) event->persist->event steps.

    Handlers should not modify any attributes which are mapped by `relationship()`, nor should they attempt to make any modifications to the `Session` in this hook (including `Session.add()`, `Session.delete()`, etc.) - such changes will not take effect. For overall changes to the “flush plan”, use `SessionEvents.before_flush()`.

    Parameters:

    - mapper – the `Mapper` which is the target of this event.
    - connection – the `Connection` being used to emit INSERT statements for this instance. This provides a handle into the current transaction on the target database specific to this instance.
    - target – the mapped instance being persisted. If the event is configured with `raw=True`, this will instead be the `InstanceState` state-management object associated with the instance.

    Returns: | No return value is supported by this event.

  `before_update`(mapper, connection, target)

    Receive an object instance before an UPDATE statement is emitted corresponding to that instance.

    This event is used to modify local, non-object related attributes on the instance before an UPDATE occurs, as well as to emit additional SQL statements on the given connection.

    This method is called for all instances that are marked as “dirty”, even those which have no net changes to their column-based attributes. An object is marked as dirty when any of its column-based attributes have a “set attribute” operation called or when any of its collections are modified. If, at update time, no column-based attributes have any net changes, no UPDATE statement will be issued. This means that an instance being sent to `before_update()` is not a guarantee that an UPDATE statement will be issued, although you can affect the outcome here by modifying attributes so that a net change in value does exist.

    To detect if the column-based attributes on the object have net changes, and will therefore generate an UPDATE statement, use `object_session(instance).is_modified(instance, include_collections=False)`.

    The event is often called for a batch of objects of the same class before their UPDATE statements are emitted at once in a later step. In the extremely rare case that this is not desirable, the `mapper()` can be configured with `batch=False`, which will cause batches of instances to be broken up into individual (and more poorly performing) event->persist->event steps.

    Handlers should not modify any attributes which are mapped by `relationship()`, nor should they attempt to make any modifications to the `Session` in this hook (including `Session.add()`, `Session.delete()`, etc.) - such changes will not take effect. For overall changes to the “flush plan”, use `SessionEvents.before_flush()`.

    Parameters:

    - mapper – the `Mapper` which is the target of this event.
    - connection – the `Connection` being used to emit UPDATE statements for this instance. This provides a handle into the current transaction on the target database specific to this instance.
    - target – the mapped instance being persisted. If the event is configured with `raw=True`, this will instead be the `InstanceState` state-management object associated with the instance.

    Returns: | No return value is supported by this event.

  `create_instance`(mapper, context, row, class_)

    Receive a row when a new object instance is about to be created from that row.

    The method can choose to create the instance itself, or it can return EXT_CONTINUE to indicate normal object creation should take place. This listener is typically registered with `retval=True`.

    Parameters:

    - mapper – the `Mapper` which is the target of this event.
    - context – the `QueryContext`, which includes a handle to the current `Query` in progress as well as additional state information.
    - row – the result row being handled. This may be an actual `RowProxy` or may be a dictionary containing `Column` objects as keys.
    - class_ – the mapped class.

    Returns: | When configured with `retval=True`, the return value should be a newly created instance of the mapped class, or `EXT_CONTINUE` indicating that default object construction should take place.

  `instrument_class`(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 generally only be applied to the `Mapper` class overall.

    Parameters:

    - mapper – the `Mapper` which is the target of this event.
    - class_ – the mapped class.

  `mapper_configured`(mapper, class_)

    Called when the mapper for the class is fully configured.

    This event is the latest phase of mapper construction. The mapper should be in its final state.

    Parameters:

    - mapper – the `Mapper` which is the target of this event.
    - class_ – the mapped class.

  `populate_instance`(mapper, context, row, target, **flags)

    Receive an instance before that instance has its attributes populated.

    This usually corresponds to a newly loaded instance but may also correspond to an already-loaded instance which has unloaded attributes to be populated. The method may be called many times for a single instance, as multiple result rows are used to populate eagerly loaded collections.

    Most usages of this hook are obsolete. For a generic “object has been newly created from a row” hook, use `InstanceEvents.load()`.

    Parameters:

    - mapper – the `Mapper` which is the target of this event.
    - context – the `QueryContext`, which includes a handle to the current `Query` in progress as well as additional state information.
    - row – the result row being handled. This may be an actual `RowProxy` or may be a dictionary containing `Column` objects as keys.
    - target – the mapped instance. If the event is configured with `raw=True`, this will instead be the `InstanceState` state-management object associated with the instance.

    Returns: | When configured with `retval=True`, a return value of `EXT_STOP` will bypass instance population by the mapper. A value of `EXT_CONTINUE` indicates that default instance population should take place.

  `translate_row`(mapper, context, row)

    Perform pre-processing on the given result row and return a new row instance.

    This listener is typically registered with `retval=True`. It is called when the mapper first receives a row, before the object identity or the instance itself has been derived from that row. The given row may or may not be a `RowProxy` object - it will always be a dictionary-like object which contains mapped columns as keys. The returned object should also be a dictionary-like object which recognizes mapped columns as keys.

    Parameters:

    - mapper – the `Mapper` which is the target of this event.
    - context – the `QueryContext`, which includes a handle to the current `Query` in progress as well as additional state information.
    - row – the result row being handled. This may be an actual `RowProxy` or may be a dictionary containing `Column` objects as keys.

    Returns: | When configured with `retval=True`, the function should return a dictionary-like row object, or `EXT_CONTINUE`, indicating the original row should be used.

## Instance Events

class `sqlalchemy.orm.events.``InstanceEvents`

  Define events specific to object lifecycle.

  e.g.:

      from sqlalchemy import event

      def my_load_listener(target, context):
          print "on load!"

      event.listen(SomeMappedClass, 'load', my_load_listener)

  Available targets include mapped classes, instances of `Mapper` (i.e. returned by `mapper()`, `class_mapper()` and similar), as well as the `Mapper` class and `mapper()` function itself for global event reception:

      from sqlalchemy.orm import mapper

      def some_listener(target, context):
          log.debug("Instance %s being loaded" % target)

      # attach to all mappers
      event.listen(mapper, 'load', some_listener)

  Instance events are closely related to mapper events, but are more specific to the instance and its instrumentation, rather than its system of persistence.

  When using `InstanceEvents`, several modifiers are available to the `event.listen()` function.

  Parameters:

  - propagate=False – When True, the event listener should be applied to all inheriting mappers as well as the mapper which is the target of this listener.
  - raw=False – When True, the “target” argument passed to applicable event listener functions will be the instance’s `InstanceState` management object, rather than the mapped instance itself.

  `expire`(target, attrs)

    Receive an object instance after its attributes or some subset have been expired.

    ‘keys’ is a list of attribute names. If None, the entire state was expired.

    Parameters:

    - target – the mapped instance. If the event is configured with `raw=True`, this will instead be the `InstanceState` state-management object associated with the instance.
    - attrs – iterable collection of attribute names which were expired, or None if all attributes were expired.

  `first_init`(manager, cls)

    Called when the first instance of a particular mapping is called.

  `init`(target, args, kwargs)

    Receive an instance when it’s constructor is called.

    This method is only called during a userland construction of an object. It is not called when an object is loaded from the database.

  `init_failure`(target, args, kwargs)

    Receive an instance when it’s constructor has been called, and raised an exception.

    This method is only called during a userland construction of an object. It is not called when an object is loaded from the database.

  `load`(target, context)

    Receive an object instance after it has been created via `__new__`, and after initial attribute population has occurred.

    This typically occurs when the instance is created based on incoming result rows, and is only called once for that instance’s lifetime.

    Note that during a result-row load, this method is called upon the first row received for this instance. Note that some attributes and collections may or may not be loaded or even initialized, depending on what’s present in the result rows.

    Parameters:

    - target – the mapped instance. If the event is configured with `raw=True`, this will instead be the `InstanceState` state-management object associated with the instance.
    - context – the `QueryContext` corresponding to the current `Query` in progress. This argument may be `None` if the load does not correspond to a `Query`, such as during `Session.merge()`.

  `pickle`(target, state_dict)

    Receive an object instance when its associated state is being pickled.

    Parameters:

    - target – the mapped instance. If the event is configured with `raw=True`, this will instead be the `InstanceState` state-management object associated with the instance.
    - state_dict – the dictionary returned by `InstanceState.__getstate__`, containing the state to be pickled.

  `refresh`(target, context, attrs)

    Receive an object instance after one or more attributes have been refreshed from a query.

    Parameters:

    - target – the mapped instance. If the event is configured with `raw=True`, this will instead be the `InstanceState` state-management object associated with the instance.
    - context – the `QueryContext` corresponding to the current `Query` in progress.
    - attrs – iterable collection of attribute names which were populated, or None if all column-mapped, non-deferred attributes were populated.

  `resurrect`(target)

    Receive an object instance as it is ‘resurrected’ from garbage collection, which occurs when a “dirty” state falls out of scope.

    Parameters: | target – the mapped instance. If the event is configured with `raw=True`, this will instead be the `InstanceState` state-management object associated with the instance.

  `unpickle`(target, state_dict)

    Receive an object instance after it’s associated state has been unpickled.

    Parameters:

    - target – the mapped instance. If the event is configured with `raw=True`, this will instead be the `InstanceState` state-management object associated with the instance.
    - state_dict – the dictionary sent to `InstanceState.__setstate__`, containing the state dictionary which was pickled.

## Session Events

class `sqlalchemy.orm.events.``SessionEvents`

  Define events specific to `Session` lifecycle.

  e.g.:

      from sqlalchemy import event
      from sqlalchemy.orm import sessionmaker

      def my_before_commit(session):
          print "before commit!"

      Session = sessionmaker()

      event.listen(Session, "before_commit", my_before_commit)

  The `listen()` function will accept `Session` objects as well as the return result of `sessionmaker()` and `scoped_session()`.

  Additionally, it accepts the `Session` class which will apply listeners to all `Session` instances globally.

  `after_attach`(session, instance)

    Execute after an instance is attached to a session.

    This is called after an add, delete or merge.

  `after_begin`(session, transaction, connection)

    Execute after a transaction is begun on a connection

    transaction is the SessionTransaction. This method is called after an engine level transaction is begun on a connection.

    Parameters:

    - session – The target `Session`.
    - transaction – The `SessionTransaction`.
    - connection – The `Connection` object which will be used for SQL statements.

  `after_bulk_delete`(session, query, query_context, result)

    Execute after a bulk delete operation to the session.

    This is called after a session.query(...).delete()

    query is the query object that this delete operation was called on. query_context was the query context object. result is the result object returned from the bulk operation.

  `after_bulk_update`(session, query, query_context, result)

    Execute after a bulk update operation to the session.

    This is called after a session.query(...).update()

    query is the query object that this update operation was called on. query_context was the query context object. result is the result object returned from the bulk operation.

  `after_commit`(session)

    Execute after a commit has occurred.

    Note that this may not be per-flush if a longer running transaction is ongoing.

    Parameters: | session – The target `Session`.

  `after_flush`(session, flush_context)

    Execute after flush has completed, but before commit has been called.

    Note that the session’s state is still in pre-flush, i.e. ‘new’, ‘dirty’, and ‘deleted’ lists still show pre-flush state as well as the history settings on instance attributes.

    Parameters:

    - session – The target `Session`.
    - flush_context – Internal `UOWTransaction` object which handles the details of the flush.

  `after_flush_postexec`(session, flush_context)

    Execute after flush has completed, and after the post-exec state occurs.

    This will be when the ‘new’, ‘dirty’, and ‘deleted’ lists are in their final state. An actual commit() may or may not have occurred, depending on whether or not the flush started its own transaction or participated in a larger transaction.

    Parameters:

    - session – The target `Session`.
    - flush_context – Internal `UOWTransaction` object which handles the details of the flush.

  `after_rollback`(session)

    Execute after a real DBAPI rollback has occurred.

    Note that this event only fires when the actual rollback against the database occurs - it does not fire each time the `Session.rollback()` method is called, if the underlying DBAPI transaction has already been rolled back. In many cases, the `Session` will not be in an “active” state during this event, as the current transaction is not valid. To acquire a `Session` which is active after the outermost rollback has proceeded, use the `SessionEvents.after_soft_rollback()` event, checking the `Session.is_active` flag.

    Parameters: | session – The target `Session`.

  `after_soft_rollback`(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 `Session` can be used to invoke SQL and `Session.query()` operations after an outermost rollback by first checking the `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")

    Parameters:

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

    New in 0.7.3.

  `before_commit`(session)

    Execute before commit is called.

    Note that this may not be per-flush if a longer running transaction is ongoing.

    Parameters: | session – The target `Session`.

  `before_flush`(session, flush_context, instances)

    Execute before flush process has started.

    instances is an optional list of objects which were passed to the `flush()` method.

    Parameters:

    - session – The target `Session`.
    - flush_context – Internal `UOWTransaction` object which handles the details of the flush.
    - instances – Usually `None`, this is the collection of objects which can be passed to the `Session.flush()` method (note this usage is deprecated).

## Instrumentation Events

class `sqlalchemy.orm.events.``InstrumentationEvents`

  Events related to class instrumentation events.

  The listeners here support being established against any new style class, that is any object that is a subclass of ‘type’. Events will then be fired off for events against that class as well as all subclasses. ‘type’ itself is also accepted as a target in which case the events fire for all classes.

  `attribute_instrument`(cls, key, inst)

    Called when an attribute is instrumented.

  `class_instrument`(cls)

    Called after the given class is instrumented.

    To get at the `ClassManager`, use `manager_of_class()`.

  `class_uninstrument`(cls)

    Called before the given class is uninstrumented.

    To get at the `ClassManager`, use `manager_of_class()`.

## Alternate Class Instrumentation

class `sqlalchemy.orm.interfaces.``InstrumentationManager`(class_)

  User-defined class instrumentation extension.

  `InstrumentationManager` can be subclassed in order to change how class instrumentation proceeds. This class exists for the purposes of integration with other object management frameworks which would like to entirely modify the instrumentation methodology of the ORM, and is not intended for regular usage. For interception of class instrumentation events, see `InstrumentationEvents`.

  For an example of `InstrumentationManager`, see the example Attribute Instrumentation.

  The API for this class should be considered as semi-stable, and may change slightly with new releases.

  `dict_getter`(class_)
  `dispose`(class_, manager)
  `get_instance_dict`(class_, instance)
  `initialize_instance_dict`(class_, instance)
  `install_descriptor`(class_, key, inst)
  `install_member`(class_, key, implementation)
  `install_state`(class_, instance, state)
  `instrument_attribute`(class_, key, inst)
  `instrument_collection_class`(class_, key, collection_class)
  `manage`(class_, manager)
  `manager_getter`(class_)
  `post_configure_attribute`(class_, key, inst)
  `remove_state`(class_, instance)
  `state_getter`(class_)
  `uninstall_descriptor`(class_, key)
  `uninstall_member`(class_, key)