sqlalchemy.orm.events.AttributeEvents

Receive a collection append event. The append event is invoked for each element as it is appended to the collection. This occurs for single-item appends as well as for a "bulk replace" operation. :param target: the object instance receiving the event. If the listener is registered with ``raw=True``, this will be the :class:`.InstanceState` object. :param 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. :param initiator: An instance of :class:`.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, as well as be inspected for information about the source of the event. :param key: When the event is established using the :paramref:`.AttributeEvents.include_key` parameter set to True, this will be the key used in the operation, such as ``collection[some_key_or_index] = value``. The parameter is not passed to the event at all if the the :paramref:`.AttributeEvents.include_key` was not used to set up the event; this is to allow backwards compatibility with existing event handlers that don't include the ``key`` parameter. .. versionadded:: 2.0 :return: if the event was registered with ``retval=True``, the given value, or a new effective value, should be returned. .. seealso:: :class:`.AttributeEvents` - background on listener options such as propagation to subclasses. :meth:`.AttributeEvents.bulk_replace`

Receive a collection append event where the collection was not actually mutated. This event differs from :meth:`_orm.AttributeEvents.append` in that it is fired off for de-duplicating collections such as sets and dictionaries, when the object already exists in the target collection. The event does not have a return value and the identity of the given object cannot be changed. The event is used for cascading objects into a :class:`_orm.Session` when the collection has already been mutated via a backref event. :param target: the object instance receiving the event. If the listener is registered with ``raw=True``, this will be the :class:`.InstanceState` object. :param value: the value that would be appended if the object did not already exist in the collection. :param initiator: An instance of :class:`.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, as well as be inspected for information about the source of the event. :param key: When the event is established using the :paramref:`.AttributeEvents.include_key` parameter set to True, this will be the key used in the operation, such as ``collection[some_key_or_index] = value``. The parameter is not passed to the event at all if the the :paramref:`.AttributeEvents.include_key` was not used to set up the event; this is to allow backwards compatibility with existing event handlers that don't include the ``key`` parameter. .. versionadded:: 2.0 :return: No return value is defined for this event. .. versionadded:: 1.4.15

Receive a collection 'bulk replace' event. This event is invoked for a sequence of values as they are incoming to a bulk collection set operation, which can be modified in place before the values are treated as ORM objects. This is an "early hook" that runs before the bulk replace routine attempts to reconcile which objects are already present in the collection and which are being removed by the net replace operation. It is typical that this method be combined with use of the :meth:`.AttributeEvents.append` event. When using both of these events, note that a bulk replace operation will invoke the :meth:`.AttributeEvents.append` event for all new items, even after :meth:`.AttributeEvents.bulk_replace` has been invoked for the collection as a whole. In order to determine if an :meth:`.AttributeEvents.append` event is part of a bulk replace, use the symbol :attr:`~.attributes.OP_BULK_REPLACE` to test the incoming initiator:: from sqlalchemy.orm.attributes import OP_BULK_REPLACE @event.listens_for(SomeObject.collection, "bulk_replace") def process_collection(target, values, initiator): values[:] = [_make_value(value) for value in values] @event.listens_for(SomeObject.collection, "append", retval=True) def process_collection(target, value, initiator): # make sure bulk_replace didn't already do it if initiator is None or initiator.op is not OP_BULK_REPLACE: return _make_value(value) else: return value .. versionadded:: 1.2 :param target: the object instance receiving the event. If the listener is registered with ``raw=True``, this will be the :class:`.InstanceState` object. :param value: a sequence (e.g. a list) of the values being set. The handler can modify this list in place. :param initiator: An instance of :class:`.attributes.Event` representing the initiation of the event. :param keys: When the event is established using the :paramref:`.AttributeEvents.include_key` parameter set to True, this will be the sequence of keys used in the operation, typically only for a dictionary update. The parameter is not passed to the event at all if the the :paramref:`.AttributeEvents.include_key` was not used to set up the event; this is to allow backwards compatibility with existing event handlers that don't include the ``key`` parameter. .. versionadded:: 2.0 .. seealso:: :class:`.AttributeEvents` - background on listener options such as propagation to subclasses.

Receive a 'collection dispose' event. 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 old collection received will contain its previous contents. .. versionchanged:: 1.2 The collection passed to :meth:`.AttributeEvents.dispose_collection` will now have its contents before the dispose intact; previously, the collection would be empty. .. versionadded:: 1.0.0 the :meth:`.AttributeEvents.init_collection` and :meth:`.AttributeEvents.dispose_collection` events. .. seealso:: :class:`.AttributeEvents` - background on listener options such as propagation to subclasses.

Receive a 'collection init' event. 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 :param target: the object instance receiving the event. If the listener is registered with ``raw=True``, this will be the :class:`.InstanceState` object. :param collection: the new collection. This will always be generated from what was specified as :paramref:`_orm.relationship.collection_class`, and will always be empty. :param collection_adapter: the :class:`.CollectionAdapter` that will mediate internal access to the collection. .. versionadded:: 1.0.0 :meth:`.AttributeEvents.init_collection` and :meth:`.AttributeEvents.dispose_collection` events. .. seealso:: :class:`.AttributeEvents` - background on listener options such as propagation to subclasses. :meth:`.AttributeEvents.init_scalar` - "scalar" version of this event.

Receive a scalar "init" event. This event is invoked when an uninitialized, unpersisted scalar attribute is accessed, e.g. read:: x = my_object.some_attribute The ORM's default behavior when this occurs for an un-initialized attribute is to return the value ``None``; note this differs from Python's usual behavior of raising ``AttributeError``. The event here can be used to customize what value is actually returned, with the assumption that the event listener would be mirroring a default generator that is configured on the Core :class:`_schema.Column` object as well. Since a default generator on a :class:`_schema.Column` might also produce a changing value such as a timestamp, the :meth:`.AttributeEvents.init_scalar` event handler can also be used to **set** the newly returned value, so that a Core-level default generation function effectively fires off only once, but at the moment the attribute is accessed on the non-persisted object. Normally, no change to the object's state is made when an uninitialized attribute is accessed (much older SQLAlchemy versions did in fact change the object's state). If a default generator on a column returned a particular constant, a handler might be used as follows:: SOME_CONSTANT = 3.1415926 class MyClass(Base): # ... some_attribute = Column(Numeric, default=SOME_CONSTANT) @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 of ``SOME_CONSTANT``. The above code includes the following features: * By setting the value ``SOME_CONSTANT`` in the given ``dict_``, we indicate that this value is to be persisted to the database. This supersedes the use of ``SOME_CONSTANT`` in the default generator for the :class:`_schema.Column`. The ``active_column_defaults.py`` example given at :ref:`examples_instrumentation` illustrates using the same approach for a changing default, e.g. a timestamp generator. In this particular example, it is not strictly necessary to do this since ``SOME_CONSTANT`` would be part of the INSERT statement in either case. * 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. In the above example, the attribute set event :meth:`.AttributeEvents.set` as well as the related validation feature provided by :obj:`_orm.validates` is **not** invoked when we apply our value to the given ``dict_``. 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 the ``value`` argument of the next listener. .. versionadded:: 1.1 :param target: the object instance receiving the event. If the listener is registered with ``raw=True``, this will be the :class:`.InstanceState` object. :param 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. :param 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. .. seealso:: :meth:`.AttributeEvents.init_collection` - collection version of this event :class:`.AttributeEvents` - background on listener options such as propagation to subclasses. :ref:`examples_instrumentation` - see the ``active_column_defaults.py`` example.

Receive a 'modified' event. This event is triggered when the :func:`.attributes.flag_modified` function is used to trigger a modify event on an attribute without any specific value being set. .. versionadded:: 1.2 :param target: the object instance receiving the event. If the listener is registered with ``raw=True``, this will be the :class:`.InstanceState` object. :param initiator: An instance of :class:`.attributes.Event` representing the initiation of the event. .. seealso:: :class:`.AttributeEvents` - background on listener options such as propagation to subclasses.

Receive a collection remove event. :param target: the object instance receiving the event. If the listener is registered with ``raw=True``, this will be the :class:`.InstanceState` object. :param value: the value being removed. :param initiator: An instance of :class:`.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. .. versionchanged:: 0.9.0 the ``initiator`` argument is now passed as a :class:`.attributes.Event` object, and may be modified by backref handlers within a chain of backref-linked events. :param key: When the event is established using the :paramref:`.AttributeEvents.include_key` parameter set to True, this will be the key used in the operation, such as ``del collection[some_key_or_index]``. The parameter is not passed to the event at all if the the :paramref:`.AttributeEvents.include_key` was not used to set up the event; this is to allow backwards compatibility with existing event handlers that don't include the ``key`` parameter. .. versionadded:: 2.0 :return: No return value is defined for this event. .. seealso:: :class:`.AttributeEvents` - background on listener options such as propagation to subclasses.

Receive a scalar set event. :param target: the object instance receiving the event. If the listener is registered with ``raw=True``, this will be the :class:`.InstanceState` object. :param 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. :param 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. :param initiator: An instance of :class:`.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. .. versionchanged:: 0.9.0 the ``initiator`` argument is now passed as a :class:`.attributes.Event` object, and may be modified by backref handlers within a chain of backref-linked events. :return: if the event was registered with ``retval=True``, the given value, or a new effective value, should be returned. .. seealso:: :class:`.AttributeEvents` - background on listener options such as propagation to subclasses.