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dibbler/sqlalchemy/orm/events.py

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# orm/events.py
# Copyright (C) 2005-2017 the SQLAlchemy authors and contributors
# <see AUTHORS file>
#
# 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)
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