Release: 1.0.0 | Release Date: Not released

SQLAlchemy 1.0 Documentation

ORM Events

The ORM includes a wide variety of hooks available for subscription.

New in version 0.7: The event supersedes 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

Bases: sqlalchemy.event.base.Events

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass.some_attribute, 'append')
def receive_append(target, value, initiator):
    "listen for the 'append' event"

    # ... (event handling logic) ...
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

    An instance of attributes.Event representing the initiation of the event. May be modified from its original value by backref handlers in order to control chained event propagation.

    Changed in version 0.9.0: the initiator argument is now passed as a attributes.Event object, and may be modified by backref handlers within a chain of backref-linked events.

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass.some_attribute, 'remove')
def receive_remove(target, value, initiator):
    "listen for the 'remove' event"

    # ... (event handling logic) ...
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

    An instance of attributes.Event representing the initiation of the event. May be modified from its original value by backref handlers in order to control chained event propagation.

    Changed in version 0.9.0: the initiator argument is now passed as a attributes.Event object, and may be modified by backref handlers within a chain of backref-linked events.

Returns:

No return value is defined for this event.

set(target, value, oldvalue, initiator)

Receive a scalar set event.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass.some_attribute, 'set')
def receive_set(target, value, oldvalue, initiator):
    "listen for the 'set' event"

    # ... (event handling logic) ...

# named argument style (new in 0.9)
@event.listens_for(SomeClass.some_attribute, 'set', named=True)
def receive_set(**kw):
    "listen for the 'set' event"
    target = kw['target']
    value = kw['value']

    # ... (event handling logic) ...
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

    An instance of attributes.Event representing the initiation of the event. May be modified from its original value by backref handlers in order to control chained event propagation.

    Changed in version 0.9.0: the initiator argument is now passed as a attributes.Event object, and may be modified by backref handlers within a chain of backref-linked events.

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

Bases: sqlalchemy.event.base.Events

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 SomeClass,
# to execute during the "before_insert" hook
event.listen(
    SomeClass, 'before_insert', my_before_insert_listener)

Available targets include:

  • mapped classes
  • unmapped superclasses of mapped or to-be-mapped classes (using the propagate=True flag)
  • Mapper objects
  • the Mapper class itself and the mapper() function indicate listening for all mappers.

Changed in version 0.8.0: mapper events can be associated with unmapped superclasses of mapped classes.

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 and/or the mappers of inheriting classes, as well as any 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.
after_configured()

Called after a series of mappers have been configured.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'after_configured')
def receive_after_configured():
    "listen for the 'after_configured' event"

    # ... (event handling logic) ...

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

This event can only be applied to the Mapper class or mapper() function, and not to individual mappings or mapped classes. It is only invoked for all mappings as a whole:

from sqlalchemy.orm import mapper

@event.listens_for(mapper, "after_configured")
def go():
    # ...

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. To ensure that a particular event is only called once and no further, the once=True argument (new in 0.9.4) can be applied:

from sqlalchemy.orm import mapper

@event.listens_for(mapper, "after_configured", once=True)
def go():
    # ...
after_delete(mapper, connection, target)

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

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'after_delete')
def receive_after_delete(mapper, connection, target):
    "listen for the 'after_delete' event"

    # ... (event handling logic) ...

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.

Warning

Mapper-level flush events are designed to operate on attributes local to the immediate object being handled and via SQL operations with the given Connection only. Handlers here should not make alterations to the state of the Session overall, and in general should not affect any relationship() -mapped attributes, as session cascade rules will not function properly, nor is it always known if the related class has already been handled. Operations that are not supported in mapper events include:

  • Session.add()
  • Session.delete()
  • Mapped collection append, add, remove, delete, discard, etc.
  • Mapped relationship attribute set/del events, i.e. someobject.related = someotherobject

Operations which manipulate the state of the object relative to other objects are better handled:

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'after_insert')
def receive_after_insert(mapper, connection, target):
    "listen for the 'after_insert' event"

    # ... (event handling logic) ...

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.

Warning

Mapper-level flush events are designed to operate on attributes local to the immediate object being handled and via SQL operations with the given Connection only. Handlers here should not make alterations to the state of the Session overall, and in general should not affect any relationship() -mapped attributes, as session cascade rules will not function properly, nor is it always known if the related class has already been handled. Operations that are not supported in mapper events include:

  • Session.add()
  • Session.delete()
  • Mapped collection append, add, remove, delete, discard, etc.
  • Mapped relationship attribute set/del events, i.e. someobject.related = someotherobject

Operations which manipulate the state of the object relative to other objects are better handled:

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'after_update')
def receive_after_update(mapper, connection, target):
    "listen for the 'after_update' event"

    # ... (event handling logic) ...

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.

Warning

Mapper-level flush events are designed to operate on attributes local to the immediate object being handled and via SQL operations with the given Connection only. Handlers here should not make alterations to the state of the Session overall, and in general should not affect any relationship() -mapped attributes, as session cascade rules will not function properly, nor is it always known if the related class has already been handled. Operations that are not supported in mapper events include:

  • Session.add()
  • Session.delete()
  • Mapped collection append, add, remove, delete, discard, etc.
  • Mapped relationship attribute set/del events, i.e. someobject.related = someotherobject

Operations which manipulate the state of the object relative to other objects are better handled:

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'append_result')
def receive_append_result(mapper, context, row, target, result, **kw):
    "listen for the 'append_result' event"

    # ... (event handling logic) ...

# named argument style (new in 0.9)
@event.listens_for(SomeClass, 'append_result', named=True)
def receive_append_result(**kw):
    "listen for the 'append_result' event"
    mapper = kw['mapper']
    context = kw['context']

    # ... (event handling logic) ...

Deprecated since version 0.9: the append_result() event should be considered as legacy. It is a difficult to use method whose original purpose is better suited by custom collection classes.

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_configured()

Called before a series of mappers have been configured.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'before_configured')
def receive_before_configured():
    "listen for the 'before_configured' event"

    # ... (event handling logic) ...

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

This event can only be applied to the Mapper class or mapper() function, and not to individual mappings or mapped classes. It is only invoked for all mappings as a whole:

from sqlalchemy.orm import mapper

@event.listens_for(mapper, "before_configured")
def go():
    # ...

Theoretically this event is called once per application, but is actually called any time new mappers are to be 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. To ensure that a particular event is only called once and no further, the once=True argument (new in 0.9.4) can be applied:

from sqlalchemy.orm import mapper

@event.listens_for(mapper, "before_configured", once=True)
def go():
    # ...

New in version 0.9.3.

before_delete(mapper, connection, target)

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

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'before_delete')
def receive_before_delete(mapper, connection, target):
    "listen for the 'before_delete' event"

    # ... (event handling logic) ...

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.

Warning

Mapper-level flush events are designed to operate on attributes local to the immediate object being handled and via SQL operations with the given Connection only. Handlers here should not make alterations to the state of the Session overall, and in general should not affect any relationship() -mapped attributes, as session cascade rules will not function properly, nor is it always known if the related class has already been handled. Operations that are not supported in mapper events include:

  • Session.add()
  • Session.delete()
  • Mapped collection append, add, remove, delete, discard, etc.
  • Mapped relationship attribute set/del events, i.e. someobject.related = someotherobject

Operations which manipulate the state of the object relative to other objects are better handled:

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'before_insert')
def receive_before_insert(mapper, connection, target):
    "listen for the 'before_insert' event"

    # ... (event handling logic) ...

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.

Warning

Mapper-level flush events are designed to operate on attributes local to the immediate object being handled and via SQL operations with the given Connection only. Handlers here should not make alterations to the state of the Session overall, and in general should not affect any relationship() -mapped attributes, as session cascade rules will not function properly, nor is it always known if the related class has already been handled. Operations that are not supported in mapper events include:

  • Session.add()
  • Session.delete()
  • Mapped collection append, add, remove, delete, discard, etc.
  • Mapped relationship attribute set/del events, i.e. someobject.related = someotherobject

Operations which manipulate the state of the object relative to other objects are better handled:

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'before_update')
def receive_before_update(mapper, connection, target):
    "listen for the 'before_update' event"

    # ... (event handling logic) ...

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.

Warning

Mapper-level flush events are designed to operate on attributes local to the immediate object being handled and via SQL operations with the given Connection only. Handlers here should not make alterations to the state of the Session overall, and in general should not affect any relationship() -mapped attributes, as session cascade rules will not function properly, nor is it always known if the related class has already been handled. Operations that are not supported in mapper events include:

  • Session.add()
  • Session.delete()
  • Mapped collection append, add, remove, delete, discard, etc.
  • Mapped relationship attribute set/del events, i.e. someobject.related = someotherobject

Operations which manipulate the state of the object relative to other objects are better handled:

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'create_instance')
def receive_create_instance(mapper, context, row, class_):
    "listen for the 'create_instance' event"

    # ... (event handling logic) ...

# named argument style (new in 0.9)
@event.listens_for(SomeClass, 'create_instance', named=True)
def receive_create_instance(**kw):
    "listen for the 'create_instance' event"
    mapper = kw['mapper']
    context = kw['context']

    # ... (event handling logic) ...

Deprecated since version 0.9: the create_instance() event should be considered as legacy. Manipulation of the object construction mechanics during a load should not be necessary.

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'instrument_class')
def receive_instrument_class(mapper, class_):
    "listen for the 'instrument_class' event"

    # ... (event handling logic) ...

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 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_):
    " ... "
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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'mapper_configured')
def receive_mapper_configured(mapper, class_):
    "listen for the 'mapper_configured' event"

    # ... (event handling logic) ...

This event is the latest phase of mapper construction, and is invoked when the mapped classes are first used, so that relationships between mappers can be resolved. When the event is called, the mapper should be in its final state.

While the configuration event normally occurs automatically, it can be forced to occur ahead of time, in the case where the event is needed before any actual mapper usage, by using the configure_mappers() function.

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'populate_instance')
def receive_populate_instance(mapper, context, row, target, **kw):
    "listen for the 'populate_instance' event"

    # ... (event handling logic) ...

# named argument style (new in 0.9)
@event.listens_for(SomeClass, 'populate_instance', named=True)
def receive_populate_instance(**kw):
    "listen for the 'populate_instance' event"
    mapper = kw['mapper']
    context = kw['context']

    # ... (event handling logic) ...

Deprecated since version 0.9: the populate_instance() event should be considered as legacy. The mechanics of instance population should not need modification; special “on load” rules can as always be accommodated by the InstanceEvents.load event.

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'translate_row')
def receive_translate_row(mapper, context, row):
    "listen for the 'translate_row' event"

    # ... (event handling logic) ...

Deprecated since version 0.9: the translate_row() event should be considered as legacy. The row as delivered in a mapper load operation typically requires that highly technical details be accommodated in order to identity the correct column keys are present in the row, rendering this particular event hook as difficult to use and unreliable.

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

Bases: sqlalchemy.event.base.Events

Define events specific to object lifecycle.

e.g.:

from sqlalchemy import event

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

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

Available targets include:

  • mapped classes
  • unmapped superclasses of mapped or to-be-mapped classes (using the propagate=True flag)
  • Mapper objects
  • the Mapper class itself and the mapper() function indicate listening for all mappers.

Changed in version 0.8.0: instance events can be associated with unmapped superclasses of mapped classes.

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 classes as well as the class 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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'expire')
def receive_expire(target, attrs):
    "listen for the 'expire' event"

    # ... (event handling logic) ...

‘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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'first_init')
def receive_first_init(manager, cls):
    "listen for the 'first_init' event"

    # ... (event handling logic) ...
init(target, args, kwargs)

Receive an instance when its constructor is called.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'init')
def receive_init(target, args, kwargs):
    "listen for the 'init' event"

    # ... (event handling logic) ...

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 its constructor has been called, and raised an exception.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'init_failure')
def receive_init_failure(target, args, kwargs):
    "listen for the 'init_failure' event"

    # ... (event handling logic) ...

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'load')
def receive_load(target, context):
    "listen for the 'load' event"

    # ... (event handling logic) ...

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'pickle')
def receive_pickle(target, state_dict):
    "listen for the 'pickle' event"

    # ... (event handling logic) ...
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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'refresh')
def receive_refresh(target, context, attrs):
    "listen for the 'refresh' event"

    # ... (event handling logic) ...
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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'resurrect')
def receive_resurrect(target):
    "listen for the 'resurrect' event"

    # ... (event handling logic) ...
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 its associated state has been unpickled.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeClass, 'unpickle')
def receive_unpickle(target, state_dict):
    "listen for the 'unpickle' event"

    # ... (event handling logic) ...
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

Bases: sqlalchemy.event.base.Events

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeSessionOrFactory, 'after_attach')
def receive_after_attach(session, instance):
    "listen for the 'after_attach' event"

    # ... (event handling logic) ...

This is called after an add, delete or merge.

Note

As of 0.8, this event fires off after the item has been fully associated with the session, which is different than previous releases. For event handlers that require the object not yet be part of session state (such as handlers which may autoflush while the target object is not yet complete) consider the new before_attach() event.

See also

before_attach()

after_begin(session, transaction, connection)

Execute after a transaction is begun on a connection

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeSessionOrFactory, 'after_begin')
def receive_after_begin(session, transaction, connection):
    "listen for the 'after_begin' event"

    # ... (event handling logic) ...
Parameters:
after_bulk_delete(delete_context)

Execute after a bulk delete operation to the session.

Example argument forms:

from sqlalchemy import event

# standard decorator style (arguments as of 0.9)
@event.listens_for(SomeSessionOrFactory, 'after_bulk_delete')
def receive_after_bulk_delete(delete_context):
    "listen for the 'after_bulk_delete' event"

    # ... (event handling logic) ...

# legacy calling style (pre-0.9)
@event.listens_for(SomeSessionOrFactory, 'after_bulk_delete')
def receive_after_bulk_delete(session, query, query_context, result):
    "listen for the 'after_bulk_delete' event"

    # ... (event handling logic) ...

Changed in version 0.9: The after_bulk_delete event now accepts the arguments delete_context. Listener functions which accept the previous argument signature(s) listed above will be automatically adapted to the new signature.

This is called as a result of the Query.delete() method.

Parameters:delete_context

a “delete context” object which contains details about the update, including these attributes:

  • session - the Session involved
  • query -the Query object that this update operation was called upon.
  • context The QueryContext object, corresponding to the invocation of an ORM query.
  • result the ResultProxy returned as a result of the bulk DELETE operation.
after_bulk_update(update_context)

Execute after a bulk update operation to the session.

Example argument forms:

from sqlalchemy import event

# standard decorator style (arguments as of 0.9)
@event.listens_for(SomeSessionOrFactory, 'after_bulk_update')
def receive_after_bulk_update(update_context):
    "listen for the 'after_bulk_update' event"

    # ... (event handling logic) ...

# legacy calling style (pre-0.9)
@event.listens_for(SomeSessionOrFactory, 'after_bulk_update')
def receive_after_bulk_update(session, query, query_context, result):
    "listen for the 'after_bulk_update' event"

    # ... (event handling logic) ...

Changed in version 0.9: The after_bulk_update event now accepts the arguments update_context. Listener functions which accept the previous argument signature(s) listed above will be automatically adapted to the new signature.

This is called as a result of the Query.update() method.

Parameters:update_context

an “update context” object which contains details about the update, including these attributes:

  • session - the Session involved
  • query -the Query object that this update operation was called upon.
  • context The QueryContext object, corresponding to the invocation of an ORM query.
  • result the ResultProxy returned as a result of the bulk UPDATE operation.
after_commit(session)

Execute after a commit has occurred.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeSessionOrFactory, 'after_commit')
def receive_after_commit(session):
    "listen for the 'after_commit' event"

    # ... (event handling logic) ...

Note

The after_commit() hook is not per-flush, that is, the Session can emit SQL to the database many times within the scope of a transaction. For interception of these events, use the before_flush(), after_flush(), or after_flush_postexec() events.

Note

The Session is not in an active transaction when the after_commit() event is invoked, and therefore can not emit SQL. To emit SQL corresponding to every transaction, use the before_commit() event.

Parameters:session – The target Session.
after_flush(session, flush_context)

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

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeSessionOrFactory, 'after_flush')
def receive_after_flush(session, flush_context):
    "listen for the 'after_flush' event"

    # ... (event handling logic) ...

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeSessionOrFactory, 'after_flush_postexec')
def receive_after_flush_postexec(session, flush_context):
    "listen for the 'after_flush_postexec' event"

    # ... (event handling logic) ...

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeSessionOrFactory, 'after_rollback')
def receive_after_rollback(session):
    "listen for the 'after_rollback' event"

    # ... (event handling logic) ...

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.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeSessionOrFactory, 'after_soft_rollback')
def receive_after_soft_rollback(session, previous_transaction):
    "listen for the 'after_soft_rollback' event"

    # ... (event handling logic) ...

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:

New in version 0.7.3.

after_transaction_create(session, transaction)

Execute when a new SessionTransaction is created.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeSessionOrFactory, 'after_transaction_create')
def receive_after_transaction_create(session, transaction):
    "listen for the 'after_transaction_create' event"

    # ... (event handling logic) ...

This event differs from after_begin() in that it occurs for each 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 after_transaction_end() event (assuming normal operation of the Session).

Parameters:

New in version 0.8.

after_transaction_end(session, transaction)

Execute when the span of a SessionTransaction ends.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeSessionOrFactory, 'after_transaction_end')
def receive_after_transaction_end(session, transaction):
    "listen for the 'after_transaction_end' event"

    # ... (event handling logic) ...

This event differs from after_commit() in that it corresponds to all SessionTransaction objects in use, including those for nested transactions and subtransactions, and is always matched by a corresponding after_transaction_create() event.

Parameters:

New in version 0.8.

before_attach(session, instance)

Execute before an instance is attached to a session.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeSessionOrFactory, 'before_attach')
def receive_before_attach(session, instance):
    "listen for the 'before_attach' event"

    # ... (event handling logic) ...

This is called before an add, delete or merge causes the object to be part of the session.

New in version 0.8.: Note that after_attach() now fires off after the item is part of the session. before_attach() is provided for those cases where the item should not yet be part of the session state.

See also

after_attach()

before_commit(session)

Execute before commit is called.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeSessionOrFactory, 'before_commit')
def receive_before_commit(session):
    "listen for the 'before_commit' event"

    # ... (event handling logic) ...

Note

The before_commit() hook is not per-flush, that is, the Session can emit SQL to the database many times within the scope of a transaction. For interception of these events, use the before_flush(), after_flush(), or after_flush_postexec() events.

Parameters:session – The target Session.
before_flush(session, flush_context, instances)

Execute before flush process has started.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeSessionOrFactory, 'before_flush')
def receive_before_flush(session, flush_context, instances):
    "listen for the 'before_flush' event"

    # ... (event handling logic) ...
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

Defines SQLAlchemy’s system of class instrumentation.

This module is usually not directly visible to user applications, but defines a large part of the ORM’s interactivity.

instrumentation.py deals with registration of end-user classes for state tracking. It interacts closely with state.py and attributes.py which establish per-instance and per-class-attribute instrumentation, respectively.

The class instrumentation system can be customized on a per-class or global basis using the sqlalchemy.ext.instrumentation module, which provides the means to build and specify alternate instrumentation forms.

class sqlalchemy.orm.events.InstrumentationEvents

Bases: sqlalchemy.event.base.Events

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. If the “propagate=True” flag is passed to event.listen(), the event will fire off for subclasses of that class as well.

The Python type builtin is also accepted as a target, which when used has the effect of events being emitted for all classes.

Note the “propagate” flag here is defaulted to True, unlike the other class level events where it defaults to False. This means that new subclasses will also be the subject of these events, when a listener is established on a superclass.

Changed in version 0.8: - events here will emit based on comparing the incoming class to the type of class passed to event.listen(). Previously, the event would fire for any class unconditionally regardless of what class was sent for listening, despite documentation which stated the contrary.

attribute_instrument(cls, key, inst)

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeBaseClass, 'attribute_instrument')
def receive_attribute_instrument(cls, key, inst):
    "listen for the 'attribute_instrument' event"

    # ... (event handling logic) ...

Called when an attribute is instrumented.

class_instrument(cls)

Called after the given class is instrumented.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeBaseClass, 'class_instrument')
def receive_class_instrument(cls):
    "listen for the 'class_instrument' event"

    # ... (event handling logic) ...

To get at the ClassManager, use manager_of_class().

class_uninstrument(cls)

Called before the given class is uninstrumented.

Example argument forms:

from sqlalchemy import event

# standard decorator style
@event.listens_for(SomeBaseClass, 'class_uninstrument')
def receive_class_uninstrument(cls):
    "listen for the 'class_uninstrument' event"

    # ... (event handling logic) ...

To get at the ClassManager, use manager_of_class().