SQLAlchemy 1.1 Documentation
ORM Events¶
The ORM includes a wide variety of hooks available for subscription.
For an introduction to the most commonly used ORM events, see the section Tracking Object and Session Changes with Events. The event system in general is discussed at 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 tolisten()
: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 viacolumn_property()
andrelationship()
. - 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 theInstanceState
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 aattributes.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.- target¶ – the object instance receiving the event. If the listener is registered with
-
dispose_collection
(target, collection, collection_adpater)¶ Receive a ‘collection dispose’ event.
Example argument forms:
from sqlalchemy import event # standard decorator style @event.listens_for(SomeClass.some_attribute, 'dispose_collection') def receive_dispose_collection(target, collection, collection_adpater): "listen for the 'dispose_collection' event" # ... (event handling logic) ...
This event is triggered for a collection-based attribute when a collection is replaced, that is:
u1.addresses.append(a1) u1.addresses = [a2, a3] # <- old collection is disposed
The mechanics of the event will typically include that the given collection is empty, even if it stored objects while being replaced.
New in version 1.0.0: the
AttributeEvents.init_collection()
andAttributeEvents.dispose_collection()
events supersede thecollection.linker
hook.
-
init_collection
(target, collection, collection_adapter)¶ Receive a ‘collection init’ event.
Example argument forms:
from sqlalchemy import event # standard decorator style @event.listens_for(SomeClass.some_attribute, 'init_collection') def receive_init_collection(target, collection, collection_adapter): "listen for the 'init_collection' event" # ... (event handling logic) ...
This event is triggered for a collection-based attribute, when the initial “empty collection” is first generated for a blank attribute, as well as for when the collection is replaced with a new one, such as via a set event.
E.g., given that
User.addresses
is a relationship-based collection, the event is triggered here:u1 = User() u1.addresses.append(a1) # <- new collection
and also during replace operations:
u1.addresses = [a2, a3] # <- new collection
Parameters: - target¶ – the object instance receiving the event. If the listener is registered with
raw=True
, this will be theInstanceState
object. - collection¶ – the new collection. This will always be generated from what was specified as
RelationshipProperty.collection_class
, and will always be empty. - collection_adpater¶ – the
CollectionAdapter
that will mediate internal access to the collection.
New in version 1.0.0: the
AttributeEvents.init_collection()
andAttributeEvents.dispose_collection()
events supersede thecollection.linker
hook.- target¶ – the object instance receiving the event. If the listener is registered with
-
init_scalar
(target, value, dict_)¶ Receive a scalar “init” event.
Example argument forms:
from sqlalchemy import event # standard decorator style @event.listens_for(SomeClass.some_attribute, 'init_scalar') def receive_init_scalar(target, value, dict_): "listen for the 'init_scalar' event" # ... (event handling logic) ...
This event is invoked when an uninitialized, unpersisted scalar attribute is accessed. A value of
None
is typically returned in this case; no changes are made to the object’s state.The event handler can alter this behavior in two ways. One is that a value other than
None
may be returned. The other is that the value may be established as part of the object’s state, which will also have the effect that it is persisted.Typical use is to establish a specific default value of an attribute upon access:
SOME_CONSTANT = 3.1415926 @event.listens_for( MyClass.some_attribute, "init_scalar", retval=True, propagate=True) def _init_some_attribute(target, dict_, value): dict_['some_attribute'] = SOME_CONSTANT return SOME_CONSTANT
Above, we initialize the attribute
MyClass.some_attribute
to the value ofSOME_CONSTANT
. The above code includes the following features:- By setting the value
SOME_CONSTANT
in the givendict_
, we indicate that the value is to be persisted to the database. The given value is only persisted to the database if we explicitly associate it with the object. Thedict_
given is the__dict__
element of the mapped object, assuming the default attribute instrumentation system is in place. - By establishing the
retval=True
flag, the value we return from the function will be returned by the attribute getter. Without this flag, the event is assumed to be a passive observer and the return value of our function is ignored. - The
propagate=True
flag is significant if the mapped class includes inheriting subclasses, which would also make use of this event listener. Without this flag, an inheriting subclass will not use our event handler.
When we establish the value in the given dictionary, the value will be used in the INSERT statement established by the unit of work. Normally, the default returned value of
None
is not established as part of the object, to avoid the issue of mutations occurring to the object in response to a normally passive “get” operation, and also sidesteps the issue of whether or not theAttributeEvents.set()
event should be awkwardly fired off during an attribute access operation. This does not impact the INSERT operation since theNone
value matches the value ofNULL
that goes into the database in any case; note thatNone
is skipped during the INSERT to ensure that column and SQL-level default functions can fire off.The attribute set event
AttributeEvents.set()
as well as the related validation feature provided byorm.validates
is not invoked when we apply our value to the givendict_
. To have these events to invoke in response to our newly generated value, apply the value to the given object as a normal attribute set operation:SOME_CONSTANT = 3.1415926 @event.listens_for( MyClass.some_attribute, "init_scalar", retval=True, propagate=True) def _init_some_attribute(target, dict_, value): # will also fire off attribute set events target.some_attribute = SOME_CONSTANT return SOME_CONSTANT
When multiple listeners are set up, the generation of the value is “chained” from one listener to the next by passing the value returned by the previous listener that specifies
retval=True
as thevalue
argument of the next listener.The
AttributeEvents.init_scalar()
event may be used to extract values from the default values and/or callables established on mappedColumn
objects. See the “active column defaults” example in Attribute Instrumentation for an example of this.New in version 1.1.
Parameters: - target¶ – the object instance receiving the event. If the listener is registered with
raw=True
, this will be theInstanceState
object. - value¶ – the value that is to be returned before this event listener were invoked. This value begins as the value
None
, however will be the return value of the previous event handler function if multiple listeners are present. - dict_¶ – the attribute dictionary of this mapped object. This is normally the
__dict__
of the object, but in all cases represents the destination that the attribute system uses to get at the actual value of this attribute. Placing the value in this dictionary has the effect that the value will be used in the INSERT statement generated by the unit of work.
See also
Attribute Instrumentation - see the
active_column_defaults.py
example.- By setting the value
-
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 theInstanceState
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 aattributes.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.
- target¶ – the object instance receiving the event. If the listener is registered with
-
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 theInstanceState
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
orNO_VALUE
. If the listener is registered withactive_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 aattributes.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.- target¶ – the object instance receiving the event. If the listener is registered with
- 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
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 themapper()
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()
, andbefore_update()
are popular places to augment the state being persisted - however, these methods operate with several significant restrictions. The user is encouraged to evaluate theSessionEvents.before_flush()
andSessionEvents.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 theevent.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) ...
The
MapperEvents.after_configured()
event is invoked each time theorm.configure_mappers()
function is invoked, after the function has completed its work.orm.configure_mappers()
is typically invoked automatically as mappings are first used, as well as each time new mappers have been made available and new mapper use is detected.Contrast this event to the
MapperEvents.mapper_configured()
event, which is called on a per-mapper basis while the configuration operation proceeds; unlike that event, when this event is invoked, all cross-configurations (e.g. backrefs) will also have been made available for any mappers that were pending. Also constrast toMapperEvents.before_configured()
, which is invoked before the series of mappers has been configured.This event can only be applied to the
Mapper
class ormapper()
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 will likely be called again. To ensure that a particular event is only called once and no further, theonce=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 only allow very limited operations, on attributes local to the row being operated upon only, as well as allowing any SQL to be emitted on the given
Connection
. Please read fully the notes at Mapper-level Events for guidelines on using these methods; generally, theSessionEvents.before_flush()
method should be preferred for general on-flush changes.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 theInstanceState
state-management object associated with the instance.
Returns: No return value is supported by this event.
See also
- mapper¶ – the
-
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 withbatch=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 only allow very limited operations, on attributes local to the row being operated upon only, as well as allowing any SQL to be emitted on the given
Connection
. Please read fully the notes at Mapper-level Events for guidelines on using these methods; generally, theSessionEvents.before_flush()
method should be preferred for general on-flush changes.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 theInstanceState
state-management object associated with the instance.
Returns: No return value is supported by this event.
See also
- mapper¶ – the
-
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 withbatch=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 only allow very limited operations, on attributes local to the row being operated upon only, as well as allowing any SQL to be emitted on the given
Connection
. Please read fully the notes at Mapper-level Events for guidelines on using these methods; generally, theSessionEvents.before_flush()
method should be preferred for general on-flush changes.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 theInstanceState
state-management object associated with the instance.
Returns: No return value is supported by this event.
See also
- mapper¶ – the
-
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) ...
The
MapperEvents.before_configured()
event is invoked each time theorm.configure_mappers()
function is invoked, before the function has done any of its work.orm.configure_mappers()
is typically invoked automatically as mappings are first used, as well as each time new mappers have been made available and new mapper use is detected.This event can only be applied to the
Mapper
class ormapper()
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(): # ...
Constrast this event to
MapperEvents.after_configured()
, which is invoked after the series of mappers has been configured, as well asMapperEvents.mapper_configured()
, which is invoked on a per-mapper basis as each one is configured to the extent possible.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 will likely be called again. To ensure that a particular event is only called once and no further, theonce=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 only allow very limited operations, on attributes local to the row being operated upon only, as well as allowing any SQL to be emitted on the given
Connection
. Please read fully the notes at Mapper-level Events for guidelines on using these methods; generally, theSessionEvents.before_flush()
method should be preferred for general on-flush changes.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 theInstanceState
state-management object associated with the instance.
Returns: No return value is supported by this event.
See also
- mapper¶ – the
-
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 withbatch=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 only allow very limited operations, on attributes local to the row being operated upon only, as well as allowing any SQL to be emitted on the given
Connection
. Please read fully the notes at Mapper-level Events for guidelines on using these methods; generally, theSessionEvents.before_flush()
method should be preferred for general on-flush changes.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 theInstanceState
state-management object associated with the instance.
Returns: No return value is supported by this event.
See also
- mapper¶ – the
-
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 withbatch=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 only allow very limited operations, on attributes local to the row being operated upon only, as well as allowing any SQL to be emitted on the given
Connection
. Please read fully the notes at Mapper-level Events for guidelines on using these methods; generally, theSessionEvents.before_flush()
method should be preferred for general on-flush changes.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 theInstanceState
state-management object associated with the instance.
Returns: No return value is supported by this event.
See also
- mapper¶ – the
-
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 thepropagate=True
flag):Base = declarative_base() @event.listens_for(Base, "instrument_class", propagate=True) def on_new_class(mapper, cls_): " ... "
Parameters:
-
mapper_configured
(mapper, class_)¶ Called when a specific mapper has completed its own configuration within the scope of the
configure_mappers()
call.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) ...
The
MapperEvents.mapper_configured()
event is invoked for each mapper that is encountered when theorm.configure_mappers()
function proceeds through the current list of not-yet-configured mappers.orm.configure_mappers()
is typically invoked automatically as mappings are first used, as well as each time new mappers have been made available and new mapper use is detected.When the event is called, the mapper should be in its final state, but not including backrefs that may be invoked from other mappers; they might still be pending within the configuration operation. Bidirectional relationships that are instead configured via the
orm.relationship.back_populates
argument will be fully available, since this style of relationship does not rely upon other possibly-not-configured mappers to know that they exist.For an event that is guaranteed to have all mappers ready to go including backrefs that are defined only on other mappings, use the
MapperEvents.after_configured()
event; this event invokes only after all known mappings have been fully configured.The
MapperEvents.mapper_configured()
event, unlikeMapperEvents.before_configured()
orMapperEvents.after_configured()
, is called for each mapper/class individually, and the mapper is passed to the event itself. It also is called exactly once for a particular mapper. The event is therefore useful for configurational steps that benefit from being invoked just once on a specific mapper basis, which don’t require that “backref” configurations are necessarily ready yet.Parameters:
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 themapper()
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 theevent.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 theInstanceState
state-management object associated with the instance. - attrs¶ – sequence of attribute names which were expired, or None if all attributes were expired.
- target¶ – the mapped instance. If the event is configured with
-
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) ...
This event is called when the
__init__
method of a class is called the first time for that particular class. The event invokes before__init__
actually proceeds as well as before theInstanceEvents.init()
event is invoked.
-
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, in conjunction with the object’s constructor, e.g. its
__init__
method. It is not called when an object is loaded from the database; see theInstanceEvents.load()
event in order to intercept a database load.The event is called before the actual
__init__
constructor of the object is called. Thekwargs
dictionary may be modified in-place in order to affect what is passed to__init__
.Parameters: - target¶ – the mapped instance. If the event is configured with
raw=True
, this will instead be theInstanceState
state-management object associated with the instance. - args¶ – positional arguments passed to the
__init__
method. This is passed as a tuple and is currently immutable. - kwargs¶ – keyword arguments passed to the
__init__
method. This structure can be altered in place.
- target¶ – the mapped instance. If the event is configured with
-
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, in conjunction with the object’s constructor, e.g. its
__init__
method. It is not called when an object is loaded from the database.The event is invoked after an exception raised by the
__init__
method is caught. After the event is invoked, the original exception is re-raised outwards, so that the construction of the object still raises an exception. The actual exception and stack trace raised should be present insys.exc_info()
.Parameters: - target¶ – the mapped instance. If the event is configured with
raw=True
, this will instead be theInstanceState
state-management object associated with the instance. - args¶ – positional arguments that were passed to the
__init__
method. - kwargs¶ – keyword arguments that were passed to the
__init__
method.
- target¶ – the mapped instance. If the event is configured with
-
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 theInstanceState
state-management object associated with the instance. - context¶ – the
QueryContext
corresponding to the currentQuery
in progress. This argument may beNone
if the load does not correspond to aQuery
, such as duringSession.merge()
.
- target¶ – the mapped instance. If the event is configured with
-
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 theInstanceState
state-management object associated with the instance. - state_dict¶ – the dictionary returned by
InstanceState.__getstate__
, containing the state to be pickled.
- target¶ – the mapped instance. If the event is configured with
-
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) ...
Contrast this to the
InstanceEvents.load()
method, which is invoked when the object is first loaded from a query.Parameters: - target¶ – the mapped instance. If the event is configured with
raw=True
, this will instead be theInstanceState
state-management object associated with the instance. - context¶ – the
QueryContext
corresponding to the currentQuery
in progress. - attrs¶ – sequence of attribute names which were populated, or None if all column-mapped, non-deferred attributes were populated.
See also
- target¶ – the mapped instance. If the event is configured with
-
refresh_flush
(target, flush_context, attrs)¶ Receive an object instance after one or more attributes have been refreshed within the persistence of the object.
Example argument forms:
from sqlalchemy import event # standard decorator style @event.listens_for(SomeClass, 'refresh_flush') def receive_refresh_flush(target, flush_context, attrs): "listen for the 'refresh_flush' event" # ... (event handling logic) ...
This event is the same as
InstanceEvents.refresh()
except it is invoked within the unit of work flush process, and the values here typically come from the process of handling an INSERT or UPDATE, such as via the RETURNING clause or from Python-side default values.New in version 1.0.5.
Parameters: - target¶ – the mapped instance. If the event is configured with
raw=True
, this will instead be theInstanceState
state-management object associated with the instance. - flush_context¶ – Internal
UOWTransaction
object which handles the details of the flush. - attrs¶ – sequence of attribute names which were populated.
- target¶ – the mapped instance. If the event is configured with
-
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 theInstanceState
state-management object associated with the instance. - state_dict¶ – the dictionary sent to
InstanceState.__setstate__
, containing the state dictionary which was pickled.
- target¶ – the mapped instance. If the event is configured with
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 acceptSession
objects as well as the return result ofsessionmaker()
andscoped_session()
.Additionally, it accepts the
Session
class which will apply listeners to allSession
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.
-
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: - session¶ – The target
Session
. - transaction¶ – The
SessionTransaction
. - connection¶ – The
Connection
object which will be used for SQL statements.
- session¶ – The target
-
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 argumentsdelete_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
- theSession
involvedquery
-theQuery
object that this update operation was called upon.context
TheQueryContext
object, corresponding to the invocation of an ORM query.result
theResultProxy
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 argumentsupdate_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
- theSession
involvedquery
-theQuery
object that this update operation was called upon.context
TheQueryContext
object, corresponding to the invocation of an ORM query.result
theResultProxy
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, theSession
can emit SQL to the database many times within the scope of a transaction. For interception of these events, use thebefore_flush()
,after_flush()
, orafter_flush_postexec()
events.Note
The
Session
is not in an active transaction when theafter_commit()
event is invoked, and therefore can not emit SQL. To emit SQL corresponding to every transaction, use thebefore_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.
- session¶ – The target
-
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.
- session¶ – The target
-
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, theSession
will not be in an “active” state during this event, as the current transaction is not valid. To acquire aSession
which is active after the outermost rollback has proceeded, use theSessionEvents.after_soft_rollback()
event, checking theSession.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 andSession.query()
operations after an outermost rollback by first checking theSession.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 currentSessionTransaction
for the givenSession
is available via theSession.transaction
attribute.
New in version 0.7.3.
- session¶ – The target
-
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 eachSessionTransaction
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 correspondingafter_transaction_end()
event (assuming normal operation of theSession
).Parameters: - session¶ – the target
Session
. - transaction¶ – the target
SessionTransaction
.
New in version 0.8.
See also
- session¶ – the target
-
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 allSessionTransaction
objects in use, including those for nested transactions and subtransactions, and is always matched by a correspondingafter_transaction_create()
event.Parameters: - session¶ – the target
Session
. - transaction¶ – the target
SessionTransaction
.
New in version 0.8.
See also
- session¶ – the target
-
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.
-
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, theSession
can emit SQL to the database many times within the scope of a transaction. For interception of these events, use thebefore_flush()
,after_flush()
, orafter_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 theSession.flush()
method (note this usage is deprecated).
- session¶ – The target
-
deleted_to_detached
(session, instance)¶ Intercept the “deleted to detached” transition for a specific object.
Example argument forms:
from sqlalchemy import event # standard decorator style @event.listens_for(SomeSessionOrFactory, 'deleted_to_detached') def receive_deleted_to_detached(session, instance): "listen for the 'deleted_to_detached' event" # ... (event handling logic) ...
This event is invoked when a deleted object is evicted from the session. The typical case when this occurs is when the transaction for a
Session
in which the object was deleted is committed; the object moves from the deleted state to the detached state.It is also invoked for objects that were deleted in a flush when the
Session.expunge_all()
orSession.close()
events are called, as well as if the object is individually expunged from its deleted state viaSession.expunge()
.New in version 1.1.
See also
-
deleted_to_persistent
(session, instance)¶ Intercept the “deleted to persistent” transition for a specific object.
Example argument forms:
from sqlalchemy import event # standard decorator style @event.listens_for(SomeSessionOrFactory, 'deleted_to_persistent') def receive_deleted_to_persistent(session, instance): "listen for the 'deleted_to_persistent' event" # ... (event handling logic) ...
This transition occurs only when an object that’s been deleted successfully in a flush is restored due to a call to
Session.rollback()
. The event is not called under any other circumstances.New in version 1.1.
See also
-
detached_to_persistent
(session, instance)¶ Intercept the “detached to persistent” transition for a specific object.
Example argument forms:
from sqlalchemy import event # standard decorator style @event.listens_for(SomeSessionOrFactory, 'detached_to_persistent') def receive_detached_to_persistent(session, instance): "listen for the 'detached_to_persistent' event" # ... (event handling logic) ...
This event is a specialization of the
SessionEvents.after_attach()
event which is only invoked for this specific transition. It is invoked typically during theSession.add()
call, as well as during theSession.delete()
call if the object was not previously associated with theSession
(note that an object marked as “deleted” remains in the “persistent” state until the flush proceeds).Note
If the object becomes persistent as part of a call to
Session.delete()
, the object is not yet marked as deleted when this event is called. To detect deleted objects, check thedeleted
flag sent to theSessionEvents.persistent_to_detached()
to event after the flush proceeds, or check theSession.deleted
collection within theSessionEvents.before_flush()
event if deleted objects need to be intercepted before the flush.Parameters: New in version 1.1.
See also
-
loaded_as_persistent
(session, instance)¶ Intercept the “loaded as peristent” transition for a specific object.
Example argument forms:
from sqlalchemy import event # standard decorator style @event.listens_for(SomeSessionOrFactory, 'loaded_as_persistent') def receive_loaded_as_persistent(session, instance): "listen for the 'loaded_as_persistent' event" # ... (event handling logic) ...
This event is invoked within the ORM loading process, and is invoked very similarly to the
InstanceEvents.load()
event. However, the event here is linkable to aSession
class or instance, rather than to a mapper or class hierarchy, and integrates with the other session lifecycle events smoothly. The object is guaranteed to be present in the session’s identity map when this event is called.Parameters: New in version 1.1.
See also
-
pending_to_persistent
(session, instance)¶ Intercept the “pending to persistent”” transition for a specific object.
Example argument forms:
from sqlalchemy import event # standard decorator style @event.listens_for(SomeSessionOrFactory, 'pending_to_persistent') def receive_pending_to_persistent(session, instance): "listen for the 'pending_to_persistent' event" # ... (event handling logic) ...
This event is invoked within the flush process, and is similar to scanning the
Session.new
collection within theSessionEvents.after_flush()
event. However, in this case the object has already been moved to the persistent state when the event is called.Parameters: New in version 1.1.
See also
-
pending_to_transient
(session, instance)¶ Intercept the “pending to transient” transition for a specific object.
Example argument forms:
from sqlalchemy import event # standard decorator style @event.listens_for(SomeSessionOrFactory, 'pending_to_transient') def receive_pending_to_transient(session, instance): "listen for the 'pending_to_transient' event" # ... (event handling logic) ...
This less common transition occurs when an pending object that has not been flushed is evicted from the session; this can occur when the
Session.rollback()
method rolls back the transaction, or when theSession.expunge()
method is used.Parameters: New in version 1.1.
See also
-
persistent_to_deleted
(session, instance)¶ Intercept the “persistent to deleted” transition for a specific object.
Example argument forms:
from sqlalchemy import event # standard decorator style @event.listens_for(SomeSessionOrFactory, 'persistent_to_deleted') def receive_persistent_to_deleted(session, instance): "listen for the 'persistent_to_deleted' event" # ... (event handling logic) ...
This event is invoked when a persistent object’s identity is deleted from the database within a flush, however the object still remains associated with the
Session
until the transaction completes.If the transaction is rolled back, the object moves again to the persistent state, and the
SessionEvents.deleted_to_persistent()
event is called. If the transaction is committed, the object becomes detached, which will emit theSessionEvents.deleted_to_detached()
event.Note that while the
Session.delete()
method is the primary public interface to mark an object as deleted, many objects get deleted due to cascade rules, which are not always determined until flush time. Therefore, there’s no way to catch every object that will be deleted until the flush has proceeded. theSessionEvents.persistent_to_deleted()
event is therefore invoked at the end of a flush.New in version 1.1.
See also
-
persistent_to_detached
(session, instance)¶ Intercept the “persistent to detached” transition for a specific object.
Example argument forms:
from sqlalchemy import event # standard decorator style @event.listens_for(SomeSessionOrFactory, 'persistent_to_detached') def receive_persistent_to_detached(session, instance): "listen for the 'persistent_to_detached' event" # ... (event handling logic) ...
This event is invoked when a persistent object is evicted from the session. There are many conditions that cause this to happen, including:
- using a method such as
Session.expunge()
orSession.close()
- Calling the
Session.rollback()
method, when the object was part of an INSERT statement for that session’s transaction
Parameters: New in version 1.1.
See also
- using a method such as
-
persistent_to_transient
(session, instance)¶ Intercept the “persistent to transient” transition for a specific object.
Example argument forms:
from sqlalchemy import event # standard decorator style @event.listens_for(SomeSessionOrFactory, 'persistent_to_transient') def receive_persistent_to_transient(session, instance): "listen for the 'persistent_to_transient' event" # ... (event handling logic) ...
This less common transition occurs when an pending object that has has been flushed is evicted from the session; this can occur when the
Session.rollback()
method rolls back the transaction.Parameters: New in version 1.1.
See also
-
transient_to_pending
(session, instance)¶ Intercept the “transient to pending” transition for a specific object.
Example argument forms:
from sqlalchemy import event # standard decorator style @event.listens_for(SomeSessionOrFactory, 'transient_to_pending') def receive_transient_to_pending(session, instance): "listen for the 'transient_to_pending' event" # ... (event handling logic) ...
This event is a specialization of the
SessionEvents.after_attach()
event which is only invoked for this specific transition. It is invoked typically during theSession.add()
call.Parameters: New in version 1.1.
See also
-
Query Events¶
- class
sqlalchemy.orm.events.
QueryEvents
¶ Bases:
sqlalchemy.event.base.Events
Represent events within the construction of a
Query
object.The events here are intended to be used with an as-yet-unreleased inspection system for
Query
. Some very basic operations are possible now, however the inspection system is intended to allow complex query manipulations to be automated.New in version 1.0.0.
-
before_compile
(query)¶ Receive the
Query
object before it is composed into a coreSelect
object.Example argument forms:
from sqlalchemy import event # standard decorator style @event.listens_for(SomeQuery, 'before_compile') def receive_before_compile(query): "listen for the 'before_compile' event" # ... (event handling logic) ...
This event is intended to allow changes to the query given:
@event.listens_for(Query, "before_compile", retval=True) def no_deleted(query): for desc in query.column_descriptions: if desc['type'] is User: entity = desc['entity'] query = query.filter(entity.deleted == False) return query
The event should normally be listened with the
retval=True
parameter set, so that the modified query may be returned.
-
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 toFalse
. 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
, usemanager_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
, usemanager_of_class()
.
-