# 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, *arg):
                if not raw:
                    target = target.obj()
                if not retval:
                    if arg:
                        value = arg[0]
                    else:
                        value = None
                    fn(target, *arg)
                    return value
                else:
                    return fn(target, *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.

        The append event is invoked for each element as it is appended
        to the collection.  This occurs for single-item appends as well
        as for a "bulk replace" operation.

        :param target: the object instance receiving the event.
          If the listener is registered with ``raw=True``, this will
          be the :class:`.InstanceState` object.
        :param value: the value being appended.  If this listener
          is registered with ``retval=True``, the listener
          function must return this value, or a new value which
          replaces it.
        :param initiator: An instance of :class:`.attributes.Event`
          representing the initiation of the event.  May be modified
          from its original value by backref handlers in order to control
          chained event propagation, as well as be inspected for information
          about the source of the event.
        :return: if the event was registered with ``retval=True``,
         the given value, or a new effective value, should be returned.

        .. seealso::

            :meth:`.AttributeEvents.bulk_replace`

        """

    def bulk_replace(self, target, values, initiator):
        """Receive a collection 'bulk replace' event.

        This event is invoked for a sequence of values as they are incoming
        to a bulk collection set operation, which can be
        modified in place before the values are treated as ORM objects.
        This is an "early hook" that runs before the bulk replace routine
        attempts to reconcile which objects are already present in the
        collection and which are being removed by the net replace operation.

        It is typical that this method be combined with use of the
        :meth:`.AttributeEvents.append` event.    When using both of these
        events, note that a bulk replace operation will invoke
        the :meth:`.AttributeEvents.append` event for all new items,
        even after :meth:`.AttributeEvents.bulk_replace` has been invoked
        for the collection as a whole.  In order to determine if an
        :meth:`.AttributeEvents.append` event is part of a bulk replace,
        use the symbol :attr:`~.attributes.OP_BULK_REPLACE` to test the
        incoming initiator::

            from sqlalchemy.orm.attributes import OP_BULK_REPLACE

            @event.listens_for(SomeObject.collection, "bulk_replace")
            def process_collection(target, values, initiator):
                values[:] = [_make_value(value) for value in values]

            @event.listens_for(SomeObject.collection, "append", retval=True)
            def process_collection(target, value, initiator):
                # make sure bulk_replace didn't already do it
                if initiator is None or initiator.op is not OP_BULK_REPLACE:
                    return _make_value(value)
                else:
                    return value



        .. versionadded:: 1.2

        :param target: the object instance receiving the event.
          If the listener is registered with ``raw=True``, this will
          be the :class:`.InstanceState` object.
        :param value: a sequence (e.g. a list) of the values being set.  The
          handler can modify this list in place.
        :param initiator: An instance of :class:`.attributes.Event`
          representing the initiation of the event.

        """

    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 old collection received will contain its previous contents.

        .. versionchanged:: 1.2 The collection passed to
           :meth:`.AttributeEvents.dispose_collection` will now have its
           contents before the dispose intact; previously, the collection
           would be empty.

        .. versionadded:: 1.0.0 the :meth:`.AttributeEvents.init_collection`
           and :meth:`.AttributeEvents.dispose_collection` events supersede
           the :class:`.collection.linker` hook.

        """

    def modified(self, target, initiator):
        """Receive a 'modified' event.

        This event is triggered when the :func:`.attributes.flag_modified`
        function is used to trigger a modify event on an attribute without
        any specific value being set.

        .. versionadded:: 1.2

        :param target: the object instance receiving the event.
          If the listener is registered with ``raw=True``, this will
          be the :class:`.InstanceState` object.

        :param initiator: An instance of :class:`.attributes.Event`
          representing the initiation of the event.

        """


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)
