# orm/events.py # Copyright (C) 2005-2017 the SQLAlchemy authors and contributors # # # This module is part of SQLAlchemy and is released under # the MIT License: http://www.opensource.org/licenses/mit-license.php """ORM event interfaces. """ from .. import event, exc, util from .base import _mapper_or_none import inspect import weakref from . import interfaces from . import mapperlib, instrumentation from .session import Session, sessionmaker from .scoping import scoped_session from .attributes import QueryableAttribute from .query import Query from sqlalchemy.util.compat import inspect_getargspec class InstrumentationEvents(event.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. .. versionchanged:: 0.8 - events here will emit based on comparing the incoming class to the type of class passed to :func:`.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. """ _target_class_doc = "SomeBaseClass" _dispatch_target = instrumentation.InstrumentationFactory @classmethod def _accept_with(cls, target): if isinstance(target, type): return _InstrumentationEventsHold(target) else: return None @classmethod def _listen(cls, event_key, propagate=True, **kw): target, identifier, fn = \ event_key.dispatch_target, event_key.identifier, \ event_key._listen_fn def listen(target_cls, *arg): listen_cls = target() if propagate and issubclass(target_cls, listen_cls): return fn(target_cls, *arg) elif not propagate and target_cls is listen_cls: return fn(target_cls, *arg) def remove(ref): key = event.registry._EventKey( None, identifier, listen, instrumentation._instrumentation_factory) getattr(instrumentation._instrumentation_factory.dispatch, identifier).remove(key) target = weakref.ref(target.class_, remove) event_key.\ with_dispatch_target(instrumentation._instrumentation_factory).\ with_wrapper(listen).base_listen(**kw) @classmethod def _clear(cls): super(InstrumentationEvents, cls)._clear() instrumentation._instrumentation_factory.dispatch._clear() def class_instrument(self, cls): """Called after the given class is instrumented. To get at the :class:`.ClassManager`, use :func:`.manager_of_class`. """ def class_uninstrument(self, cls): """Called before the given class is uninstrumented. To get at the :class:`.ClassManager`, use :func:`.manager_of_class`. """ def attribute_instrument(self, cls, key, inst): """Called when an attribute is instrumented.""" class _InstrumentationEventsHold(object): """temporary marker object used to transfer from _accept_with() to _listen() on the InstrumentationEvents class. """ def __init__(self, class_): self.class_ = class_ dispatch = event.dispatcher(InstrumentationEvents) class InstanceEvents(event.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) * :class:`.Mapper` objects * the :class:`.Mapper` class itself and the :func:`.mapper` function indicate listening for all mappers. .. versionchanged:: 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 :class:`.InstanceEvents`, several modifiers are available to the :func:`.event.listen` function. :param 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. :param raw=False: When True, the "target" argument passed to applicable event listener functions will be the instance's :class:`.InstanceState` management object, rather than the mapped instance itself. """ _target_class_doc = "SomeClass" _dispatch_target = instrumentation.ClassManager @classmethod def _new_classmanager_instance(cls, class_, classmanager): _InstanceEventsHold.populate(class_, classmanager) @classmethod @util.dependencies("sqlalchemy.orm") def _accept_with(cls, orm, target): if isinstance(target, instrumentation.ClassManager): return target elif isinstance(target, mapperlib.Mapper): return target.class_manager elif target is orm.mapper: return instrumentation.ClassManager elif isinstance(target, type): if issubclass(target, mapperlib.Mapper): return instrumentation.ClassManager else: manager = instrumentation.manager_of_class(target) if manager: return manager else: return _InstanceEventsHold(target) return None @classmethod def _listen(cls, event_key, raw=False, propagate=False, **kw): target, identifier, fn = \ event_key.dispatch_target, event_key.identifier, \ event_key._listen_fn if not raw: def wrap(state, *arg, **kw): return fn(state.obj(), *arg, **kw) event_key = event_key.with_wrapper(wrap) event_key.base_listen(propagate=propagate, **kw) if propagate: for mgr in target.subclass_managers(True): event_key.with_dispatch_target(mgr).base_listen( propagate=True) @classmethod def _clear(cls): super(InstanceEvents, cls)._clear() _InstanceEventsHold._clear() def first_init(self, manager, cls): """Called when the first instance of a particular mapping is called. 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 the :meth:`.InstanceEvents.init` event is invoked. """ def init(self, target, args, kwargs): """Receive an instance when its constructor is called. 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 the :meth:`.InstanceEvents.load` event in order to intercept a database load. The event is called before the actual ``__init__`` constructor of the object is called. The ``kwargs`` dictionary may be modified in-place in order to affect what is passed to ``__init__``. :param target: the mapped instance. If the event is configured with ``raw=True``, this will instead be the :class:`.InstanceState` state-management object associated with the instance. :param args: positional arguments passed to the ``__init__`` method. This is passed as a tuple and is currently immutable. :param kwargs: keyword arguments passed to the ``__init__`` method. This structure *can* be altered in place. .. seealso:: :meth:`.InstanceEvents.init_failure` :meth:`.InstanceEvents.load` """ def init_failure(self, target, args, kwargs): """Receive an instance when its constructor has been called, and raised an exception. 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 in ``sys.exc_info()``. :param target: the mapped instance. If the event is configured with ``raw=True``, this will instead be the :class:`.InstanceState` state-management object associated with the instance. :param args: positional arguments that were passed to the ``__init__`` method. :param kwargs: keyword arguments that were passed to the ``__init__`` method. .. seealso:: :meth:`.InstanceEvents.init` :meth:`.InstanceEvents.load` """ def load(self, target, context): """Receive an object instance after it has been created via ``__new__``, and after initial attribute population has occurred. This typically occurs when the instance is created based on incoming result rows, and is only called once for that instance's lifetime. Note that during a result-row load, this method is called upon the first row received for this instance. Note that some attributes and collections may or may not be loaded or even initialized, depending on what's present in the result rows. :param target: the mapped instance. If the event is configured with ``raw=True``, this will instead be the :class:`.InstanceState` state-management object associated with the instance. :param context: the :class:`.QueryContext` corresponding to the current :class:`.Query` in progress. This argument may be ``None`` if the load does not correspond to a :class:`.Query`, such as during :meth:`.Session.merge`. .. seealso:: :meth:`.InstanceEvents.init` :meth:`.InstanceEvents.refresh` :meth:`.SessionEvents.loaded_as_persistent` """ def refresh(self, target, context, attrs): """Receive an object instance after one or more attributes have been refreshed from a query. Contrast this to the :meth:`.InstanceEvents.load` method, which is invoked when the object is first loaded from a query. :param target: the mapped instance. If the event is configured with ``raw=True``, this will instead be the :class:`.InstanceState` state-management object associated with the instance. :param context: the :class:`.QueryContext` corresponding to the current :class:`.Query` in progress. :param attrs: sequence of attribute names which were populated, or None if all column-mapped, non-deferred attributes were populated. .. seealso:: :meth:`.InstanceEvents.load` """ def refresh_flush(self, target, flush_context, attrs): """Receive an object instance after one or more attributes have been refreshed within the persistence of the object. This event is the same as :meth:`.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. .. versionadded:: 1.0.5 :param target: the mapped instance. If the event is configured with ``raw=True``, this will instead be the :class:`.InstanceState` state-management object associated with the instance. :param flush_context: Internal :class:`.UOWTransaction` object which handles the details of the flush. :param attrs: sequence of attribute names which were populated. """ def expire(self, target, attrs): """Receive an object instance after its attributes or some subset have been expired. 'keys' is a list of attribute names. If None, the entire state was expired. :param target: the mapped instance. If the event is configured with ``raw=True``, this will instead be the :class:`.InstanceState` state-management object associated with the instance. :param attrs: sequence of attribute names which were expired, or None if all attributes were expired. """ def pickle(self, target, state_dict): """Receive an object instance when its associated state is being pickled. :param target: the mapped instance. If the event is configured with ``raw=True``, this will instead be the :class:`.InstanceState` state-management object associated with the instance. :param state_dict: the dictionary returned by :class:`.InstanceState.__getstate__`, containing the state to be pickled. """ def unpickle(self, target, state_dict): """Receive an object instance after its associated state has been unpickled. :param target: the mapped instance. If the event is configured with ``raw=True``, this will instead be the :class:`.InstanceState` state-management object associated with the instance. :param state_dict: the dictionary sent to :class:`.InstanceState.__setstate__`, containing the state dictionary which was pickled. """ class _EventsHold(event.RefCollection): """Hold onto listeners against unmapped, uninstrumented classes. Establish _listen() for that class' mapper/instrumentation when those objects are created for that class. """ def __init__(self, class_): self.class_ = class_ @classmethod def _clear(cls): cls.all_holds.clear() class HoldEvents(object): _dispatch_target = None @classmethod def _listen(cls, event_key, raw=False, propagate=False, **kw): target, identifier, fn = \ event_key.dispatch_target, event_key.identifier, event_key.fn if target.class_ in target.all_holds: collection = target.all_holds[target.class_] else: collection = target.all_holds[target.class_] = {} event.registry._stored_in_collection(event_key, target) collection[event_key._key] = (event_key, raw, propagate) if propagate: stack = list(target.class_.__subclasses__()) while stack: subclass = stack.pop(0) stack.extend(subclass.__subclasses__()) subject = target.resolve(subclass) if subject is not None: # we are already going through __subclasses__() # so leave generic propagate flag False event_key.with_dispatch_target(subject).\ listen(raw=raw, propagate=False, **kw) def remove(self, event_key): target, identifier, fn = \ event_key.dispatch_target, event_key.identifier, event_key.fn if isinstance(target, _EventsHold): collection = target.all_holds[target.class_] del collection[event_key._key] @classmethod def populate(cls, class_, subject): for subclass in class_.__mro__: if subclass in cls.all_holds: collection = cls.all_holds[subclass] for event_key, raw, propagate in collection.values(): if propagate or subclass is class_: # since we can't be sure in what order different # classes in a hierarchy are triggered with # populate(), we rely upon _EventsHold for all event # assignment, instead of using the generic propagate # flag. event_key.with_dispatch_target(subject).\ listen(raw=raw, propagate=False) class _InstanceEventsHold(_EventsHold): all_holds = weakref.WeakKeyDictionary() def resolve(self, class_): return instrumentation.manager_of_class(class_) class HoldInstanceEvents(_EventsHold.HoldEvents, InstanceEvents): pass dispatch = event.dispatcher(HoldInstanceEvents) class MapperEvents(event.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) * :class:`.Mapper` objects * the :class:`.Mapper` class itself and the :func:`.mapper` function indicate listening for all mappers. .. versionchanged:: 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 :meth:`~.MapperEvents.before_insert`, and :meth:`~.MapperEvents.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 :meth:`.SessionEvents.before_flush` and :meth:`.SessionEvents.after_flush` methods as more flexible and user-friendly hooks in which to apply additional database state during a flush. When using :class:`.MapperEvents`, several modifiers are available to the :func:`.event.listen` function. :param 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. :param raw=False: When True, the "target" argument passed to applicable event listener functions will be the instance's :class:`.InstanceState` management object, rather than the mapped instance itself. :param 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. """ _target_class_doc = "SomeClass" _dispatch_target = mapperlib.Mapper @classmethod def _new_mapper_instance(cls, class_, mapper): _MapperEventsHold.populate(class_, mapper) @classmethod @util.dependencies("sqlalchemy.orm") def _accept_with(cls, orm, target): if target is orm.mapper: return mapperlib.Mapper elif isinstance(target, type): if issubclass(target, mapperlib.Mapper): return target else: mapper = _mapper_or_none(target) if mapper is not None: return mapper else: return _MapperEventsHold(target) else: return target @classmethod def _listen( cls, event_key, raw=False, retval=False, propagate=False, **kw): target, identifier, fn = \ event_key.dispatch_target, event_key.identifier, \ event_key._listen_fn if identifier in ("before_configured", "after_configured") and \ target is not mapperlib.Mapper: util.warn( "'before_configured' and 'after_configured' ORM events " "only invoke with the mapper() function or Mapper class " "as the target.") if not raw or not retval: if not raw: meth = getattr(cls, identifier) try: target_index = \ inspect_getargspec(meth)[0].index('target') - 1 except ValueError: target_index = None def wrap(*arg, **kw): if not raw and target_index is not None: arg = list(arg) arg[target_index] = arg[target_index].obj() if not retval: fn(*arg, **kw) return interfaces.EXT_CONTINUE else: return fn(*arg, **kw) event_key = event_key.with_wrapper(wrap) if propagate: for mapper in target.self_and_descendants: event_key.with_dispatch_target(mapper).base_listen( propagate=True, **kw) else: event_key.base_listen(**kw) @classmethod def _clear(cls): super(MapperEvents, cls)._clear() _MapperEventsHold._clear() def instrument_class(self, mapper, class_): r"""Receive a class when the mapper is first constructed, before instrumentation is applied to the mapped class. This event is the earliest phase of mapper construction. Most attributes of the mapper are not yet initialized. This listener can either be applied to the :class:`.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_): " ... " :param mapper: the :class:`.Mapper` which is the target of this event. :param class\_: the mapped class. """ def mapper_configured(self, mapper, class_): r"""Called when a specific mapper has completed its own configuration within the scope of the :func:`.configure_mappers` call. The :meth:`.MapperEvents.mapper_configured` event is invoked for each mapper that is encountered when the :func:`.orm.configure_mappers` function proceeds through the current list of not-yet-configured mappers. :func:`.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 :paramref:`.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 :meth:`.MapperEvents.after_configured` event; this event invokes only after all known mappings have been fully configured. The :meth:`.MapperEvents.mapper_configured` event, unlike :meth:`.MapperEvents.before_configured` or :meth:`.MapperEvents.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. :param mapper: the :class:`.Mapper` which is the target of this event. :param class\_: the mapped class. .. seealso:: :meth:`.MapperEvents.before_configured` :meth:`.MapperEvents.after_configured` """ # TODO: need coverage for this event def before_configured(self): """Called before a series of mappers have been configured. The :meth:`.MapperEvents.before_configured` event is invoked each time the :func:`.orm.configure_mappers` function is invoked, before the function has done any of its work. :func:`.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 :class:`.Mapper` class or :func:`.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(): # ... Constrast this event to :meth:`.MapperEvents.after_configured`, which is invoked after the series of mappers has been configured, as well as :meth:`.MapperEvents.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 :func:`.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, 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(): # ... .. versionadded:: 0.9.3 .. seealso:: :meth:`.MapperEvents.mapper_configured` :meth:`.MapperEvents.after_configured` """ def after_configured(self): """Called after a series of mappers have been configured. The :meth:`.MapperEvents.after_configured` event is invoked each time the :func:`.orm.configure_mappers` function is invoked, after the function has completed its work. :func:`.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 :meth:`.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 to :meth:`.MapperEvents.before_configured`, which is invoked before the series of mappers has been configured. This event can **only** be applied to the :class:`.Mapper` class or :func:`.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 :func:`.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, 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(): # ... .. seealso:: :meth:`.MapperEvents.mapper_configured` :meth:`.MapperEvents.before_configured` """ def before_insert(self, mapper, connection, target): """Receive an object instance before an INSERT statement is emitted corresponding to that instance. This event is used to modify local, non-object related attributes on the instance before an INSERT occurs, as well as to emit additional SQL statements on the given connection. The event is often called for a batch of objects of the same class before their INSERT statements are emitted at once in a later step. In the extremely rare case that this is not desirable, the :func:`.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 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 :class:`.Connection`. **Please read fully** the notes at :ref:`session_persistence_mapper` for guidelines on using these methods; generally, the :meth:`.SessionEvents.before_flush` method should be preferred for general on-flush changes. :param mapper: the :class:`.Mapper` which is the target of this event. :param connection: the :class:`.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. :param target: the mapped instance being persisted. If the event is configured with ``raw=True``, this will instead be the :class:`.InstanceState` state-management object associated with the instance. :return: No return value is supported by this event. .. seealso:: :ref:`session_persistence_events` """ def after_insert(self, mapper, connection, target): """Receive an object instance after an INSERT statement is emitted corresponding to that instance. This event is used to modify in-Python-only state on the instance after an INSERT occurs, as well as to emit additional SQL statements on the given connection. The event is often called for a batch of objects of the same class after their INSERT statements have been emitted at once in a previous step. In the extremely rare case that this is not desirable, the :func:`.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 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 :class:`.Connection`. **Please read fully** the notes at :ref:`session_persistence_mapper` for guidelines on using these methods; generally, the :meth:`.SessionEvents.before_flush` method should be preferred for general on-flush changes. :param mapper: the :class:`.Mapper` which is the target of this event. :param connection: the :class:`.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. :param target: the mapped instance being persisted. If the event is configured with ``raw=True``, this will instead be the :class:`.InstanceState` state-management object associated with the instance. :return: No return value is supported by this event. .. seealso:: :ref:`session_persistence_events` """ def before_update(self, mapper, connection, target): """Receive an object instance before an UPDATE statement is emitted corresponding to that instance. This event is used to modify local, non-object related attributes on the instance before an UPDATE occurs, as well as to emit additional SQL statements on the given connection. This method is called for all instances that are marked as "dirty", *even those which have no net changes to their column-based attributes*. An object is marked as dirty when any of its column-based attributes have a "set attribute" operation called or when any of its collections are modified. If, at update time, no column-based attributes have any net changes, no UPDATE statement will be issued. This means that an instance being sent to :meth:`~.MapperEvents.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 :func:`.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 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 :class:`.Connection`. **Please read fully** the notes at :ref:`session_persistence_mapper` for guidelines on using these methods; generally, the :meth:`.SessionEvents.before_flush` method should be preferred for general on-flush changes. :param mapper: the :class:`.Mapper` which is the target of this event. :param connection: the :class:`.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. :param target: the mapped instance being persisted. If the event is configured with ``raw=True``, this will instead be the :class:`.InstanceState` state-management object associated with the instance. :return: No return value is supported by this event. .. seealso:: :ref:`session_persistence_events` """ def after_update(self, mapper, connection, target): """Receive an object instance after an UPDATE statement is emitted corresponding to that instance. This event is used to modify in-Python-only state on the instance after an UPDATE occurs, as well as to emit additional SQL statements on the given connection. This method is called for all instances that are marked as "dirty", *even those which have no net changes to their column-based attributes*, and for which no UPDATE statement has proceeded. An object is marked as dirty when any of its column-based attributes have a "set attribute" operation called or when any of its collections are modified. If, at update time, no column-based attributes have any net changes, no UPDATE statement will be issued. This means that an instance being sent to :meth:`~.MapperEvents.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 :func:`.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 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 :class:`.Connection`. **Please read fully** the notes at :ref:`session_persistence_mapper` for guidelines on using these methods; generally, the :meth:`.SessionEvents.before_flush` method should be preferred for general on-flush changes. :param mapper: the :class:`.Mapper` which is the target of this event. :param connection: the :class:`.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. :param target: the mapped instance being persisted. If the event is configured with ``raw=True``, this will instead be the :class:`.InstanceState` state-management object associated with the instance. :return: No return value is supported by this event. .. seealso:: :ref:`session_persistence_events` """ def before_delete(self, mapper, connection, target): """Receive an object instance before a DELETE statement is emitted corresponding to that instance. This event is used to emit additional SQL statements on the given connection as well as to perform application specific bookkeeping related to a deletion event. The event is often called for a batch of objects of the same class before their DELETE statements are emitted at once in a later step. .. 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 :class:`.Connection`. **Please read fully** the notes at :ref:`session_persistence_mapper` for guidelines on using these methods; generally, the :meth:`.SessionEvents.before_flush` method should be preferred for general on-flush changes. :param mapper: the :class:`.Mapper` which is the target of this event. :param connection: the :class:`.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. :param target: the mapped instance being deleted. If the event is configured with ``raw=True``, this will instead be the :class:`.InstanceState` state-management object associated with the instance. :return: No return value is supported by this event. .. seealso:: :ref:`session_persistence_events` """ def after_delete(self, mapper, connection, target): """Receive an object instance after a DELETE statement has been emitted corresponding to that instance. This event is used to emit additional SQL statements on the given connection as well as to perform application specific bookkeeping related to a deletion event. The event is often called for a batch of objects of the same class after their DELETE statements have been emitted at once in a previous step. .. 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 :class:`.Connection`. **Please read fully** the notes at :ref:`session_persistence_mapper` for guidelines on using these methods; generally, the :meth:`.SessionEvents.before_flush` method should be preferred for general on-flush changes. :param mapper: the :class:`.Mapper` which is the target of this event. :param connection: the :class:`.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. :param target: the mapped instance being deleted. If the event is configured with ``raw=True``, this will instead be the :class:`.InstanceState` state-management object associated with the instance. :return: No return value is supported by this event. .. seealso:: :ref:`session_persistence_events` """ class _MapperEventsHold(_EventsHold): all_holds = weakref.WeakKeyDictionary() def resolve(self, class_): return _mapper_or_none(class_) class HoldMapperEvents(_EventsHold.HoldEvents, MapperEvents): pass dispatch = event.dispatcher(HoldMapperEvents) class SessionEvents(event.Events): """Define events specific to :class:`.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 :func:`~.event.listen` function will accept :class:`.Session` objects as well as the return result of :class:`~.sessionmaker()` and :class:`~.scoped_session()`. Additionally, it accepts the :class:`.Session` class which will apply listeners to all :class:`.Session` instances globally. """ _target_class_doc = "SomeSessionOrFactory" _dispatch_target = Session @classmethod def _accept_with(cls, target): if isinstance(target, scoped_session): target = target.session_factory if not isinstance(target, sessionmaker) and \ ( not isinstance(target, type) or not issubclass(target, Session) ): raise exc.ArgumentError( "Session event listen on a scoped_session " "requires that its creation callable " "is associated with the Session class.") if isinstance(target, sessionmaker): return target.class_ elif isinstance(target, type): if issubclass(target, scoped_session): return Session elif issubclass(target, Session): return target elif isinstance(target, Session): return target else: return None def after_transaction_create(self, session, transaction): """Execute when a new :class:`.SessionTransaction` is created. This event differs from :meth:`~.SessionEvents.after_begin` in that it occurs for each :class:`.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 :meth:`~.SessionEvents.after_transaction_end` event (assuming normal operation of the :class:`.Session`). :param session: the target :class:`.Session`. :param transaction: the target :class:`.SessionTransaction`. To detect if this is the outermost :class:`.SessionTransaction`, as opposed to a "subtransaction" or a SAVEPOINT, test that the :attr:`.SessionTransaction.parent` attribute is ``None``:: @event.listens_for(session, "after_transaction_create") def after_transaction_create(session, transaction): if transaction.parent is None: # work with top-level transaction To detect if the :class:`.SessionTransaction` is a SAVEPOINT, use the :attr:`.SessionTransaction.nested` attribute:: @event.listens_for(session, "after_transaction_create") def after_transaction_create(session, transaction): if transaction.nested: # work with SAVEPOINT transaction .. seealso:: :class:`.SessionTransaction` :meth:`~.SessionEvents.after_transaction_end` """ def after_transaction_end(self, session, transaction): """Execute when the span of a :class:`.SessionTransaction` ends. This event differs from :meth:`~.SessionEvents.after_commit` in that it corresponds to all :class:`.SessionTransaction` objects in use, including those for nested transactions and subtransactions, and is always matched by a corresponding :meth:`~.SessionEvents.after_transaction_create` event. :param session: the target :class:`.Session`. :param transaction: the target :class:`.SessionTransaction`. To detect if this is the outermost :class:`.SessionTransaction`, as opposed to a "subtransaction" or a SAVEPOINT, test that the :attr:`.SessionTransaction.parent` attribute is ``None``:: @event.listens_for(session, "after_transaction_create") def after_transaction_end(session, transaction): if transaction.parent is None: # work with top-level transaction To detect if the :class:`.SessionTransaction` is a SAVEPOINT, use the :attr:`.SessionTransaction.nested` attribute:: @event.listens_for(session, "after_transaction_create") def after_transaction_end(session, transaction): if transaction.nested: # work with SAVEPOINT transaction .. seealso:: :class:`.SessionTransaction` :meth:`~.SessionEvents.after_transaction_create` """ def before_commit(self, session): """Execute before commit is called. .. note:: The :meth:`~.SessionEvents.before_commit` hook is *not* per-flush, that is, the :class:`.Session` can emit SQL to the database many times within the scope of a transaction. For interception of these events, use the :meth:`~.SessionEvents.before_flush`, :meth:`~.SessionEvents.after_flush`, or :meth:`~.SessionEvents.after_flush_postexec` events. :param session: The target :class:`.Session`. .. seealso:: :meth:`~.SessionEvents.after_commit` :meth:`~.SessionEvents.after_begin` :meth:`~.SessionEvents.after_transaction_create` :meth:`~.SessionEvents.after_transaction_end` """ def after_commit(self, session): """Execute after a commit has occurred. .. note:: The :meth:`~.SessionEvents.after_commit` hook is *not* per-flush, that is, the :class:`.Session` can emit SQL to the database many times within the scope of a transaction. For interception of these events, use the :meth:`~.SessionEvents.before_flush`, :meth:`~.SessionEvents.after_flush`, or :meth:`~.SessionEvents.after_flush_postexec` events. .. note:: The :class:`.Session` is not in an active transaction when the :meth:`~.SessionEvents.after_commit` event is invoked, and therefore can not emit SQL. To emit SQL corresponding to every transaction, use the :meth:`~.SessionEvents.before_commit` event. :param session: The target :class:`.Session`. .. seealso:: :meth:`~.SessionEvents.before_commit` :meth:`~.SessionEvents.after_begin` :meth:`~.SessionEvents.after_transaction_create` :meth:`~.SessionEvents.after_transaction_end` """ def after_rollback(self, session): """Execute after a real DBAPI rollback has occurred. Note that this event only fires when the *actual* rollback against the database occurs - it does *not* fire each time the :meth:`.Session.rollback` method is called, if the underlying DBAPI transaction has already been rolled back. In many cases, the :class:`.Session` will not be in an "active" state during this event, as the current transaction is not valid. To acquire a :class:`.Session` which is active after the outermost rollback has proceeded, use the :meth:`.SessionEvents.after_soft_rollback` event, checking the :attr:`.Session.is_active` flag. :param session: The target :class:`.Session`. """ def after_soft_rollback(self, session, previous_transaction): """Execute after any rollback has occurred, including "soft" rollbacks that don't actually emit at the DBAPI level. This corresponds to both nested and outer rollbacks, i.e. the innermost rollback that calls the DBAPI's rollback() method, as well as the enclosing rollback calls that only pop themselves from the transaction stack. The given :class:`.Session` can be used to invoke SQL and :meth:`.Session.query` operations after an outermost rollback by first checking the :attr:`.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") :param session: The target :class:`.Session`. :param previous_transaction: The :class:`.SessionTransaction` transactional marker object which was just closed. The current :class:`.SessionTransaction` for the given :class:`.Session` is available via the :attr:`.Session.transaction` attribute. .. versionadded:: 0.7.3 """ def before_flush(self, session, flush_context, instances): """Execute before flush process has started. :param session: The target :class:`.Session`. :param flush_context: Internal :class:`.UOWTransaction` object which handles the details of the flush. :param instances: Usually ``None``, this is the collection of objects which can be passed to the :meth:`.Session.flush` method (note this usage is deprecated). .. seealso:: :meth:`~.SessionEvents.after_flush` :meth:`~.SessionEvents.after_flush_postexec` :ref:`session_persistence_events` """ def after_flush(self, session, flush_context): """Execute after flush has completed, but before commit has been called. Note that the session's state is still in pre-flush, i.e. 'new', 'dirty', and 'deleted' lists still show pre-flush state as well as the history settings on instance attributes. :param session: The target :class:`.Session`. :param flush_context: Internal :class:`.UOWTransaction` object which handles the details of the flush. .. seealso:: :meth:`~.SessionEvents.before_flush` :meth:`~.SessionEvents.after_flush_postexec` :ref:`session_persistence_events` """ def after_flush_postexec(self, session, flush_context): """Execute after flush has completed, and after the post-exec state occurs. This will be when the 'new', 'dirty', and 'deleted' lists are in their final state. An actual commit() may or may not have occurred, depending on whether or not the flush started its own transaction or participated in a larger transaction. :param session: The target :class:`.Session`. :param flush_context: Internal :class:`.UOWTransaction` object which handles the details of the flush. .. seealso:: :meth:`~.SessionEvents.before_flush` :meth:`~.SessionEvents.after_flush` :ref:`session_persistence_events` """ def after_begin(self, session, transaction, connection): """Execute after a transaction is begun on a connection :param session: The target :class:`.Session`. :param transaction: The :class:`.SessionTransaction`. :param connection: The :class:`~.engine.Connection` object which will be used for SQL statements. .. seealso:: :meth:`~.SessionEvents.before_commit` :meth:`~.SessionEvents.after_commit` :meth:`~.SessionEvents.after_transaction_create` :meth:`~.SessionEvents.after_transaction_end` """ def before_attach(self, session, instance): """Execute before an instance is attached to a session. This is called before an add, delete or merge causes the object to be part of the session. .. versionadded:: 0.8. Note that :meth:`~.SessionEvents.after_attach` now fires off after the item is part of the session. :meth:`.before_attach` is provided for those cases where the item should not yet be part of the session state. .. seealso:: :meth:`~.SessionEvents.after_attach` :ref:`session_lifecycle_events` """ def after_attach(self, session, instance): """Execute after an instance is attached to a session. 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 :meth:`.before_attach` event. .. seealso:: :meth:`~.SessionEvents.before_attach` :ref:`session_lifecycle_events` """ @event._legacy_signature("0.9", ["session", "query", "query_context", "result"], lambda update_context: ( update_context.session, update_context.query, update_context.context, update_context.result)) def after_bulk_update(self, update_context): """Execute after a bulk update operation to the session. This is called as a result of the :meth:`.Query.update` method. :param update_context: an "update context" object which contains details about the update, including these attributes: * ``session`` - the :class:`.Session` involved * ``query`` -the :class:`.Query` object that this update operation was called upon. * ``context`` The :class:`.QueryContext` object, corresponding to the invocation of an ORM query. * ``result`` the :class:`.ResultProxy` returned as a result of the bulk UPDATE operation. """ @event._legacy_signature("0.9", ["session", "query", "query_context", "result"], lambda delete_context: ( delete_context.session, delete_context.query, delete_context.context, delete_context.result)) def after_bulk_delete(self, delete_context): """Execute after a bulk delete operation to the session. This is called as a result of the :meth:`.Query.delete` method. :param delete_context: a "delete context" object which contains details about the update, including these attributes: * ``session`` - the :class:`.Session` involved * ``query`` -the :class:`.Query` object that this update operation was called upon. * ``context`` The :class:`.QueryContext` object, corresponding to the invocation of an ORM query. * ``result`` the :class:`.ResultProxy` returned as a result of the bulk DELETE operation. """ def transient_to_pending(self, session, instance): """Intercept the "transient to pending" transition for a specific object. This event is a specialization of the :meth:`.SessionEvents.after_attach` event which is only invoked for this specific transition. It is invoked typically during the :meth:`.Session.add` call. :param session: target :class:`.Session` :param instance: the ORM-mapped instance being operated upon. .. versionadded:: 1.1 .. seealso:: :ref:`session_lifecycle_events` """ def pending_to_transient(self, session, instance): """Intercept the "pending to transient" transition for a specific object. This less common transition occurs when an pending object that has not been flushed is evicted from the session; this can occur when the :meth:`.Session.rollback` method rolls back the transaction, or when the :meth:`.Session.expunge` method is used. :param session: target :class:`.Session` :param instance: the ORM-mapped instance being operated upon. .. versionadded:: 1.1 .. seealso:: :ref:`session_lifecycle_events` """ def persistent_to_transient(self, session, instance): """Intercept the "persistent to transient" transition for a specific object. This less common transition occurs when an pending object that has has been flushed is evicted from the session; this can occur when the :meth:`.Session.rollback` method rolls back the transaction. :param session: target :class:`.Session` :param instance: the ORM-mapped instance being operated upon. .. versionadded:: 1.1 .. seealso:: :ref:`session_lifecycle_events` """ def pending_to_persistent(self, session, instance): """Intercept the "pending to persistent"" transition for a specific object. This event is invoked within the flush process, and is similar to scanning the :attr:`.Session.new` collection within the :meth:`.SessionEvents.after_flush` event. However, in this case the object has already been moved to the persistent state when the event is called. :param session: target :class:`.Session` :param instance: the ORM-mapped instance being operated upon. .. versionadded:: 1.1 .. seealso:: :ref:`session_lifecycle_events` """ def detached_to_persistent(self, session, instance): """Intercept the "detached to persistent" transition for a specific object. This event is a specialization of the :meth:`.SessionEvents.after_attach` event which is only invoked for this specific transition. It is invoked typically during the :meth:`.Session.add` call, as well as during the :meth:`.Session.delete` call if the object was not previously associated with the :class:`.Session` (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 :meth:`.Session.delete`, the object is **not** yet marked as deleted when this event is called. To detect deleted objects, check the ``deleted`` flag sent to the :meth:`.SessionEvents.persistent_to_detached` to event after the flush proceeds, or check the :attr:`.Session.deleted` collection within the :meth:`.SessionEvents.before_flush` event if deleted objects need to be intercepted before the flush. :param session: target :class:`.Session` :param instance: the ORM-mapped instance being operated upon. .. versionadded:: 1.1 .. seealso:: :ref:`session_lifecycle_events` """ def loaded_as_persistent(self, session, instance): """Intercept the "loaded as persistent" transition for a specific object. This event is invoked within the ORM loading process, and is invoked very similarly to the :meth:`.InstanceEvents.load` event. However, the event here is linkable to a :class:`.Session` 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. :param session: target :class:`.Session` :param instance: the ORM-mapped instance being operated upon. .. versionadded:: 1.1 .. seealso:: :ref:`session_lifecycle_events` """ def persistent_to_deleted(self, session, instance): """Intercept the "persistent to deleted" transition for a specific object. 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 :class:`.Session` until the transaction completes. If the transaction is rolled back, the object moves again to the persistent state, and the :meth:`.SessionEvents.deleted_to_persistent` event is called. If the transaction is committed, the object becomes detached, which will emit the :meth:`.SessionEvents.deleted_to_detached` event. Note that while the :meth:`.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. the :meth:`.SessionEvents.persistent_to_deleted` event is therefore invoked at the end of a flush. .. versionadded:: 1.1 .. seealso:: :ref:`session_lifecycle_events` """ def deleted_to_persistent(self, session, instance): """Intercept the "deleted to persistent" transition for a specific object. This transition occurs only when an object that's been deleted successfully in a flush is restored due to a call to :meth:`.Session.rollback`. The event is not called under any other circumstances. .. versionadded:: 1.1 .. seealso:: :ref:`session_lifecycle_events` """ def deleted_to_detached(self, session, instance): """Intercept the "deleted to detached" transition for a specific object. 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 :class:`.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 :meth:`.Session.expunge_all` or :meth:`.Session.close` events are called, as well as if the object is individually expunged from its deleted state via :meth:`.Session.expunge`. .. versionadded:: 1.1 .. seealso:: :ref:`session_lifecycle_events` """ def persistent_to_detached(self, session, instance): """Intercept the "persistent to detached" transition for a specific object. 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 :meth:`.Session.expunge` or :meth:`.Session.close` * Calling the :meth:`.Session.rollback` method, when the object was part of an INSERT statement for that session's transaction :param session: target :class:`.Session` :param instance: the ORM-mapped instance being operated upon. :param deleted: boolean. If True, indicates this object moved to the detached state because it was marked as deleted and flushed. .. versionadded:: 1.1 .. seealso:: :ref:`session_lifecycle_events` """ class AttributeEvents(event.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 :func:`~.event.listen`:: def validate_phone(target, value, oldvalue, initiator): "Strip non-numeric characters from a phone number" return re.sub(r'\D', '', 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 :exc:`ValueError` to halt the operation. Several modifiers are available to the :func:`~.event.listen` function. :param 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 :func:`.column_property` and :func:`.relationship`. :param 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. :param raw=False: When True, the "target" argument to the event will be the :class:`.InstanceState` management object, rather than the mapped instance itself. :param 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. """ _target_class_doc = "SomeClass.some_attribute" _dispatch_target = QueryableAttribute @staticmethod def _set_dispatch(cls, dispatch_cls): dispatch = event.Events._set_dispatch(cls, dispatch_cls) dispatch_cls._active_history = False return dispatch @classmethod def _accept_with(cls, target): # TODO: coverage if isinstance(target, interfaces.MapperProperty): return getattr(target.parent.class_, target.key) else: return target @classmethod def _listen(cls, event_key, active_history=False, raw=False, retval=False, propagate=False): target, identifier, fn = \ event_key.dispatch_target, event_key.identifier, \ event_key._listen_fn if active_history: target.dispatch._active_history = True if not raw or not retval: def wrap(target, value, *arg): if not raw: target = target.obj() if not retval: fn(target, value, *arg) return value else: return fn(target, value, *arg) event_key = event_key.with_wrapper(wrap) event_key.base_listen(propagate=propagate) if propagate: manager = instrumentation.manager_of_class(target.class_) for mgr in manager.subclass_managers(True): event_key.with_dispatch_target( mgr[target.key]).base_listen(propagate=True) def append(self, target, value, initiator): """Receive a collection append 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 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. .. 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. """ def remove(self, target, value, initiator): """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. :return: No return value is defined for this event. """ def set(self, target, value, oldvalue, initiator): """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. """ def init_scalar(self, target, value, dict_): """Receive a scalar "init" event. 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 of ``SOME_CONSTANT``. The above code includes the following features: * By setting the value ``SOME_CONSTANT`` in the given ``dict_``, 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**. The ``dict_`` 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 the :meth:`.AttributeEvents.set` event should be awkwardly fired off during an attribute access operation. This does not impact the INSERT operation since the ``None`` value matches the value of ``NULL`` that goes into the database in any case; note that ``None`` is skipped during the INSERT to ensure that column and SQL-level default functions can fire off. 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. The :meth:`.AttributeEvents.init_scalar` event may be used to extract values from the default values and/or callables established on mapped :class:`.Column` objects. See the "active column defaults" example in :ref:`examples_instrumentation` for an example of this. .. 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:: :ref:`examples_instrumentation` - see the ``active_column_defaults.py`` example. """ def init_collection(self, target, collection, collection_adapter): """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:`.RelationshipProperty.collection_class`, and will always be empty. :param collection_adpater: the :class:`.CollectionAdapter` that will mediate internal access to the collection. .. versionadded:: 1.0.0 the :meth:`.AttributeEvents.init_collection` and :meth:`.AttributeEvents.dispose_collection` events supersede the :class:`.collection.linker` hook. """ def dispose_collection(self, target, collection, collection_adpater): """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 mechanics of the event will typically include that the given collection is empty, even if it stored objects while being replaced. .. versionadded:: 1.0.0 the :meth:`.AttributeEvents.init_collection` and :meth:`.AttributeEvents.dispose_collection` events supersede the :class:`.collection.linker` hook. """ class QueryEvents(event.Events): """Represent events within the construction of a :class:`.Query` object. The events here are intended to be used with an as-yet-unreleased inspection system for :class:`.Query`. Some very basic operations are possible now, however the inspection system is intended to allow complex query manipulations to be automated. .. versionadded:: 1.0.0 """ _target_class_doc = "SomeQuery" _dispatch_target = Query def before_compile(self, query): """Receive the :class:`.Query` object before it is composed into a core :class:`.Select` object. 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. """ @classmethod def _listen( cls, event_key, retval=False, **kw): fn = event_key._listen_fn if not retval: def wrap(*arg, **kw): if not retval: query = arg[0] fn(*arg, **kw) return query else: return fn(*arg, **kw) event_key = event_key.with_wrapper(wrap) event_key.base_listen(**kw)