# Source code for sqlalchemy.orm.attributes

    # orm/attributes.py
    # Copyright (C) 2005-2019 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

    """Defines instrumentation for class attributes and their interaction
    with instances.

    This module is usually not directly visible to user applications, but
    defines a large part of the ORM's interactivity.

    """

    import operator

    from . import collections
    from . import exc as orm_exc
    from . import interfaces
    from .base import ATTR_EMPTY
    from .base import ATTR_WAS_SET
    from .base import CALLABLES_OK
    from .base import INIT_OK
    from .base import instance_dict
    from .base import instance_state
    from .base import instance_str
    from .base import LOAD_AGAINST_COMMITTED
    from .base import manager_of_class
    from .base import NEVER_SET
    from .base import NO_AUTOFLUSH
    from .base import NO_CHANGE  # noqa
    from .base import NO_RAISE
    from .base import NO_VALUE
    from .base import NON_PERSISTENT_OK  # noqa
    from .base import PASSIVE_NO_FETCH
    from .base import PASSIVE_NO_FETCH_RELATED  # noqa
    from .base import PASSIVE_NO_INITIALIZE
    from .base import PASSIVE_NO_RESULT
    from .base import PASSIVE_OFF
    from .base import PASSIVE_ONLY_PERSISTENT
    from .base import PASSIVE_RETURN_NEVER_SET
    from .base import RELATED_OBJECT_OK  # noqa
    from .base import SQL_OK  # noqa
    from .base import state_str
    from .. import event
    from .. import inspection
    from .. import util

    @inspection._self_inspects
    class QueryableAttribute(
        interfaces._MappedAttribute,
        interfaces.InspectionAttr,
        interfaces.PropComparator,
    ):
        """Base class for :term:`descriptor` objects that intercept
        attribute events on behalf of a :class:`.MapperProperty`
        object.  The actual :class:`.MapperProperty` is accessible
        via the :attr:`.QueryableAttribute.property`
        attribute.

        .. seealso::

            :class:`.InstrumentedAttribute`

            :class:`.MapperProperty`

            :attr:`.Mapper.all_orm_descriptors`

            :attr:`.Mapper.attrs`
        """

        is_attribute = True

        def __init__(
            self,
            class_,
            key,
            impl=None,
            comparator=None,
            parententity=None,
            of_type=None,
        ):
            self.class_ = class_
            self.key = key
            self.impl = impl
            self.comparator = comparator
            self._parententity = parententity
            self._of_type = of_type

            manager = manager_of_class(class_)
            # manager is None in the case of AliasedClass
            if manager:
                # propagate existing event listeners from
                # immediate superclass
                for base in manager._bases:
                    if key in base:
                        self.dispatch._update(base[key].dispatch)
                        if base[key].dispatch._active_history:
                            self.dispatch._active_history = True

        @util.memoized_property
        def _supports_population(self):
            return self.impl.supports_population

        @property
        def _impl_uses_objects(self):
            return self.impl.uses_objects

        def get_history(self, instance, passive=PASSIVE_OFF):
            return self.impl.get_history(
                instance_state(instance), instance_dict(instance), passive
            )

        def __selectable__(self):
            # TODO: conditionally attach this method based on clause_element ?
            return self

        @util.memoized_property
        def info(self):
            """Return the 'info' dictionary for the underlying SQL element.

            The behavior here is as follows:

            * If the attribute is a column-mapped property, i.e.
              :class:`.ColumnProperty`, which is mapped directly
              to a schema-level :class:`.Column` object, this attribute
              will return the :attr:`.SchemaItem.info` dictionary associated
              with the core-level :class:`.Column` object.

            * If the attribute is a :class:`.ColumnProperty` but is mapped to
              any other kind of SQL expression other than a :class:`.Column`,
              the attribute will refer to the :attr:`.MapperProperty.info`
              dictionary associated directly with the :class:`.ColumnProperty`,
              assuming the SQL expression itself does not have its own ``.info``
              attribute (which should be the case, unless a user-defined SQL
              construct has defined one).

            * If the attribute refers to any other kind of
              :class:`.MapperProperty`, including :class:`.RelationshipProperty`,
              the attribute will refer to the :attr:`.MapperProperty.info`
              dictionary associated with that :class:`.MapperProperty`.

            * To access the :attr:`.MapperProperty.info` dictionary of the
              :class:`.MapperProperty` unconditionally, including for a
              :class:`.ColumnProperty` that's associated directly with a
              :class:`.schema.Column`, the attribute can be referred to using
              :attr:`.QueryableAttribute.property` attribute, as
              ``MyClass.someattribute.property.info``.

            .. seealso::

                :attr:`.SchemaItem.info`

                :attr:`.MapperProperty.info`

            """
            return self.comparator.info

        @util.memoized_property
        def parent(self):
            """Return an inspection instance representing the parent.

            This will be either an instance of :class:`.Mapper`
            or :class:`.AliasedInsp`, depending upon the nature
            of the parent entity which this attribute is associated
            with.

            """
            return inspection.inspect(self._parententity)

        @property
        def expression(self):
            return self.comparator.__clause_element__()

        def __clause_element__(self):
            return self.comparator.__clause_element__()

        def _query_clause_element(self):
            """like __clause_element__(), but called specifically
            by :class:`.Query` to allow special behavior."""

            return self.comparator._query_clause_element()

        def _bulk_update_tuples(self, value):
            """Return setter tuples for a bulk UPDATE."""

            return self.comparator._bulk_update_tuples(value)

        def adapt_to_entity(self, adapt_to_entity):
            assert not self._of_type
            return self.__class__(
                adapt_to_entity.entity,
                self.key,
                impl=self.impl,
                comparator=self.comparator.adapt_to_entity(adapt_to_entity),
                parententity=adapt_to_entity,
            )

        def of_type(self, cls):
            return QueryableAttribute(
                self.class_,
                self.key,
                self.impl,
                self.comparator.of_type(cls),
                self._parententity,
                of_type=cls,
            )

        def label(self, name):
            return self._query_clause_element().label(name)

        def operate(self, op, *other, **kwargs):
            return op(self.comparator, *other, **kwargs)

        def reverse_operate(self, op, other, **kwargs):
            return op(other, self.comparator, **kwargs)

        def hasparent(self, state, optimistic=False):
            return self.impl.hasparent(state, optimistic=optimistic) is not False

        def __getattr__(self, key):
            try:
                return getattr(self.comparator, key)
            except AttributeError:
                raise AttributeError(
                    "Neither %r object nor %r object associated with %s "
                    "has an attribute %r"
                    % (
                        type(self).__name__,
                        type(self.comparator).__name__,
                        self,
                        key,
                    )
                )

        def __str__(self):
            return "%s.%s" % (self.class_.__name__, self.key)

        @util.memoized_property
        def property(self):
            """Return the :class:`.MapperProperty` associated with this
            :class:`.QueryableAttribute`.

            Return values here will commonly be instances of
            :class:`.ColumnProperty` or :class:`.RelationshipProperty`.

            """
            return self.comparator.property

    class InstrumentedAttribute(QueryableAttribute):
        """Class bound instrumented attribute which adds basic
        :term:`descriptor` methods.

        See :class:`.QueryableAttribute` for a description of most features.

        """

        def __set__(self, instance, value):
            self.impl.set(
                instance_state(instance), instance_dict(instance), value, None
            )

        def __delete__(self, instance):
            self.impl.delete(instance_state(instance), instance_dict(instance))

        def __get__(self, instance, owner):
            if instance is None:
                return self

            dict_ = instance_dict(instance)
            if self._supports_population and self.key in dict_:
                return dict_[self.key]
            else:
                return self.impl.get(instance_state(instance), dict_)

    def create_proxied_attribute(descriptor):
        """Create an QueryableAttribute / user descriptor hybrid.

        Returns a new QueryableAttribute type that delegates descriptor
        behavior and getattr() to the given descriptor.
        """

        # TODO: can move this to descriptor_props if the need for this
        # function is removed from ext/hybrid.py

        class Proxy(QueryableAttribute):
            """Presents the :class:`.QueryableAttribute` interface as a
            proxy on top of a Python descriptor / :class:`.PropComparator`
            combination.

            """

            def __init__(
                self,
                class_,
                key,
                descriptor,
                comparator,
                adapt_to_entity=None,
                doc=None,
                original_property=None,
            ):
                self.class_ = class_
                self.key = key
                self.descriptor = descriptor
                self.original_property = original_property
                self._comparator = comparator
                self._adapt_to_entity = adapt_to_entity
                self.__doc__ = doc

            _is_internal_proxy = True

            @property
            def _impl_uses_objects(self):
                return (
                    self.original_property is not None
                    and getattr(self.class_, self.key).impl.uses_objects
                )

            @property
            def property(self):
                return self.comparator.property

            @util.memoized_property
            def comparator(self):
                if util.callable(self._comparator):
                    self._comparator = self._comparator()
                if self._adapt_to_entity:
                    self._comparator = self._comparator.adapt_to_entity(
                        self._adapt_to_entity
                    )
                return self._comparator

            def adapt_to_entity(self, adapt_to_entity):
                return self.__class__(
                    adapt_to_entity.entity,
                    self.key,
                    self.descriptor,
                    self._comparator,
                    adapt_to_entity,
                )

            def __get__(self, instance, owner):
                retval = self.descriptor.__get__(instance, owner)
                # detect if this is a plain Python @property, which just returns
                # itself for class level access.  If so, then return us.
                # Otherwise, return the object returned by the descriptor.
                if retval is self.descriptor and instance is None:
                    return self
                else:
                    return retval

            def __str__(self):
                return "%s.%s" % (self.class_.__name__, self.key)

            def __getattr__(self, attribute):
                """Delegate __getattr__ to the original descriptor and/or
                comparator."""
                try:
                    return getattr(descriptor, attribute)
                except AttributeError:
                    if attribute == "comparator":
                        raise AttributeError("comparator")
                    try:
                        # comparator itself might be unreachable
                        comparator = self.comparator
                    except AttributeError:
                        raise AttributeError(
                            "Neither %r object nor unconfigured comparator "
                            "object associated with %s has an attribute %r"
                            % (type(descriptor).__name__, self, attribute)
                        )
                    else:
                        try:
                            return getattr(comparator, attribute)
                        except AttributeError:
                            raise AttributeError(
                                "Neither %r object nor %r object "
                                "associated with %s has an attribute %r"
                                % (
                                    type(descriptor).__name__,
                                    type(comparator).__name__,
                                    self,
                                    attribute,
                                )
                            )

        Proxy.__name__ = type(descriptor).__name__ + "Proxy"

        util.monkeypatch_proxied_specials(
            Proxy, type(descriptor), name="descriptor", from_instance=descriptor
        )
        return Proxy

    OP_REMOVE = util.symbol("REMOVE")
    OP_APPEND = util.symbol("APPEND")
    OP_REPLACE = util.symbol("REPLACE")
    OP_BULK_REPLACE = util.symbol("BULK_REPLACE")
    OP_MODIFIED = util.symbol("MODIFIED")

    class Event(object):
        """A token propagated throughout the course of a chain of attribute
        events.

        Serves as an indicator of the source of the event and also provides
        a means of controlling propagation across a chain of attribute
        operations.

        The :class:`.Event` object is sent as the ``initiator`` argument
        when dealing with events such as :meth:`.AttributeEvents.append`,
        :meth:`.AttributeEvents.set`,
        and :meth:`.AttributeEvents.remove`.

        The :class:`.Event` object is currently interpreted by the backref
        event handlers, and is used to control the propagation of operations
        across two mutually-dependent attributes.

        .. versionadded:: 0.9.0

        :var impl: The :class:`.AttributeImpl` which is the current event
         initiator.

        :var op: The symbol :attr:`.OP_APPEND`, :attr:`.OP_REMOVE`,
         :attr:`.OP_REPLACE`, or :attr:`.OP_BULK_REPLACE`, indicating the
         source operation.

        """

        __slots__ = "impl", "op", "parent_token"

        def __init__(self, attribute_impl, op):
            self.impl = attribute_impl
            self.op = op
            self.parent_token = self.impl.parent_token

        def __eq__(self, other):
            return (
                isinstance(other, Event)
                and other.impl is self.impl
                and other.op == self.op
            )

        @property
        def key(self):
            return self.impl.key

        def hasparent(self, state):
            return self.impl.hasparent(state)

    class AttributeImpl(object):
        """internal implementation for instrumented attributes."""

        def __init__(
            self,
            class_,
            key,
            callable_,
            dispatch,
            trackparent=False,
            extension=None,
            compare_function=None,
            active_history=False,
            parent_token=None,
            expire_missing=True,
            send_modified_events=True,
            accepts_scalar_loader=None,
            **kwargs
        ):
            r"""Construct an AttributeImpl.

            :param \class_: associated class

            :param key: string name of the attribute

            :param \callable_:
              optional function which generates a callable based on a parent
              instance, which produces the "default" values for a scalar or
              collection attribute when it's first accessed, if not present
              already.

            :param trackparent:
              if True, attempt to track if an instance has a parent attached
              to it via this attribute.

            :param extension:
              a single or list of AttributeExtension object(s) which will
              receive set/delete/append/remove/etc. events.
              The event package is now used.

              .. deprecated::  1.3

                The :paramref:`.AttributeImpl.extension` parameter is deprecated
                and will be removed in a future release, corresponding to the
                "extension" parameter on the :class:`.MapperProprty` classes
                like :func:`.column_property` and :func:`.relationship`  The
                events system is now used.

            :param compare_function:
              a function that compares two values which are normally
              assignable to this attribute.

            :param active_history:
              indicates that get_history() should always return the "old" value,
              even if it means executing a lazy callable upon attribute change.

            :param parent_token:
              Usually references the MapperProperty, used as a key for
              the hasparent() function to identify an "owning" attribute.
              Allows multiple AttributeImpls to all match a single
              owner attribute.

            :param expire_missing:
              if False, don't add an "expiry" callable to this attribute
              during state.expire_attributes(None), if no value is present
              for this key.

            :param send_modified_events:
              if False, the InstanceState._modified_event method will have no
              effect; this means the attribute will never show up as changed in a
              history entry.

            """
            self.class_ = class_
            self.key = key
            self.callable_ = callable_
            self.dispatch = dispatch
            self.trackparent = trackparent
            self.parent_token = parent_token or self
            self.send_modified_events = send_modified_events
            if compare_function is None:
                self.is_equal = operator.eq
            else:
                self.is_equal = compare_function

            if accepts_scalar_loader is not None:
                self.accepts_scalar_loader = accepts_scalar_loader
            else:
                self.accepts_scalar_loader = self.default_accepts_scalar_loader

            # TODO: pass in the manager here
            # instead of doing a lookup
            attr = manager_of_class(class_)[key]

            for ext in util.to_list(extension or []):
                ext._adapt_listener(attr, ext)

            if active_history:
                self.dispatch._active_history = True

            self.expire_missing = expire_missing
            self._modified_token = Event(self, OP_MODIFIED)

        __slots__ = (
            "class_",
            "key",
            "callable_",
            "dispatch",
            "trackparent",
            "parent_token",
            "send_modified_events",
            "is_equal",
            "expire_missing",
            "_modified_token",
            "accepts_scalar_loader",
        )

        def __str__(self):
            return "%s.%s" % (self.class_.__name__, self.key)

        def _get_active_history(self):
            """Backwards compat for impl.active_history"""

            return self.dispatch._active_history

        def _set_active_history(self, value):
            self.dispatch._active_history = value

        active_history = property(_get_active_history, _set_active_history)

        def hasparent(self, state, optimistic=False):
            """Return the boolean value of a `hasparent` flag attached to
            the given state.

            The `optimistic` flag determines what the default return value
            should be if no `hasparent` flag can be located.

            As this function is used to determine if an instance is an
            *orphan*, instances that were loaded from storage should be
            assumed to not be orphans, until a True/False value for this
            flag is set.

            An instance attribute that is loaded by a callable function
            will also not have a `hasparent` flag.

            """
            msg = "This AttributeImpl is not configured to track parents."
            assert self.trackparent, msg

            return (
                state.parents.get(id(self.parent_token), optimistic) is not False
            )

        def sethasparent(self, state, parent_state, value):
            """Set a boolean flag on the given item corresponding to
            whether or not it is attached to a parent object via the
            attribute represented by this ``InstrumentedAttribute``.

            """
            msg = "This AttributeImpl is not configured to track parents."
            assert self.trackparent, msg

            id_ = id(self.parent_token)
            if value:
                state.parents[id_] = parent_state
            else:
                if id_ in state.parents:
                    last_parent = state.parents[id_]

                    if (
                        last_parent is not False
                        and last_parent.key != parent_state.key
                    ):

                        if last_parent.obj() is None:
                            raise orm_exc.StaleDataError(
                                "Removing state %s from parent "
                                "state %s along attribute '%s', "
                                "but the parent record "
                                "has gone stale, can't be sure this "
                                "is the most recent parent."
                                % (
                                    state_str(state),
                                    state_str(parent_state),
                                    self.key,
                                )
                            )

                        return

                state.parents[id_] = False

        def get_history(self, state, dict_, passive=PASSIVE_OFF):
            raise NotImplementedError()

        def get_all_pending(self, state, dict_, passive=PASSIVE_NO_INITIALIZE):
            """Return a list of tuples of (state, obj)
            for all objects in this attribute's current state
            + history.

            Only applies to object-based attributes.

            This is an inlining of existing functionality
            which roughly corresponds to:

                get_state_history(
                            state,
                            key,
                            passive=PASSIVE_NO_INITIALIZE).sum()

            """
            raise NotImplementedError()

        def initialize(self, state, dict_):
            """Initialize the given state's attribute with an empty value."""

            value = None
            for fn in self.dispatch.init_scalar:
                ret = fn(state, value, dict_)
                if ret is not ATTR_EMPTY:
                    value = ret

            return value

        def get(self, state, dict_, passive=PASSIVE_OFF):
            """Retrieve a value from the given object.
            If a callable is assembled on this object's attribute, and
            passive is False, the callable will be executed and the
            resulting value will be set as the new value for this attribute.
            """
            if self.key in dict_:
                return dict_[self.key]
            else:
                # if history present, don't load
                key = self.key
                if (
                    key not in state.committed_state
                    or state.committed_state[key] is NEVER_SET
                ):
                    if not passive & CALLABLES_OK:
                        return PASSIVE_NO_RESULT

                    if key in state.expired_attributes:
                        value = state._load_expired(state, passive)
                    elif key in state.callables:
                        callable_ = state.callables[key]
                        value = callable_(state, passive)
                    elif self.callable_:
                        value = self.callable_(state, passive)
                    else:
                        value = ATTR_EMPTY

                    if value is PASSIVE_NO_RESULT or value is NEVER_SET:
                        return value
                    elif value is ATTR_WAS_SET:
                        try:
                            return dict_[key]
                        except KeyError:
                            # TODO: no test coverage here.
                            raise KeyError(
                                "Deferred loader for attribute "
                                "%r failed to populate "
                                "correctly" % key
                            )
                    elif value is not ATTR_EMPTY:
                        return self.set_committed_value(state, dict_, value)

                if not passive & INIT_OK:
                    return NEVER_SET
                else:
                    # Return a new, empty value
                    return self.initialize(state, dict_)

        def append(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
            self.set(state, dict_, value, initiator, passive=passive)

        def remove(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
            self.set(
                state, dict_, None, initiator, passive=passive, check_old=value
            )

        def pop(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
            self.set(
                state,
                dict_,
                None,
                initiator,
                passive=passive,
                check_old=value,
                pop=True,
            )

        def set(
            self,
            state,
            dict_,
            value,
            initiator,
            passive=PASSIVE_OFF,
            check_old=None,
            pop=False,
        ):
            raise NotImplementedError()

        def get_committed_value(self, state, dict_, passive=PASSIVE_OFF):
            """return the unchanged value of this attribute"""

            if self.key in state.committed_state:
                value = state.committed_state[self.key]
                if value in (NO_VALUE, NEVER_SET):
                    return None
                else:
                    return value
            else:
                return self.get(state, dict_, passive=passive)

        def set_committed_value(self, state, dict_, value):
            """set an attribute value on the given instance and 'commit' it."""

            dict_[self.key] = value
            state._commit(dict_, [self.key])
            return value

    class ScalarAttributeImpl(AttributeImpl):
        """represents a scalar value-holding InstrumentedAttribute."""

        default_accepts_scalar_loader = True
        uses_objects = False
        supports_population = True
        collection = False
        dynamic = False

        __slots__ = "_replace_token", "_append_token", "_remove_token"

        def __init__(self, *arg, **kw):
            super(ScalarAttributeImpl, self).__init__(*arg, **kw)
            self._replace_token = self._append_token = Event(self, OP_REPLACE)
            self._remove_token = Event(self, OP_REMOVE)

        def delete(self, state, dict_):
            if self.dispatch._active_history:
                old = self.get(state, dict_, PASSIVE_RETURN_NEVER_SET)
            else:
                old = dict_.get(self.key, NO_VALUE)

            if self.dispatch.remove:
                self.fire_remove_event(state, dict_, old, self._remove_token)
            state._modified_event(dict_, self, old)

            existing = dict_.pop(self.key, NO_VALUE)
            if (
                existing is NO_VALUE
                and old is NO_VALUE
                and not state.expired
                and self.key not in state.expired_attributes
            ):
                raise AttributeError("%s object does not have a value" % self)

        def get_history(self, state, dict_, passive=PASSIVE_OFF):
            if self.key in dict_:
                return History.from_scalar_attribute(self, state, dict_[self.key])
            else:
                if passive & INIT_OK:
                    passive ^= INIT_OK
                current = self.get(state, dict_, passive=passive)
                if current is PASSIVE_NO_RESULT:
                    return HISTORY_BLANK
                else:
                    return History.from_scalar_attribute(self, state, current)

        def set(
            self,
            state,
            dict_,
            value,
            initiator,
            passive=PASSIVE_OFF,
            check_old=None,
            pop=False,
        ):
            if self.dispatch._active_history:
                old = self.get(state, dict_, PASSIVE_RETURN_NEVER_SET)
            else:
                old = dict_.get(self.key, NO_VALUE)

            if self.dispatch.set:
                value = self.fire_replace_event(
                    state, dict_, value, old, initiator
                )
            state._modified_event(dict_, self, old)
            dict_[self.key] = value

        def fire_replace_event(self, state, dict_, value, previous, initiator):
            for fn in self.dispatch.set:
                value = fn(
                    state, value, previous, initiator or self._replace_token
                )
            return value

        def fire_remove_event(self, state, dict_, value, initiator):
            for fn in self.dispatch.remove:
                fn(state, value, initiator or self._remove_token)

        @property
        def type(self):
            self.property.columns[0].type

    class ScalarObjectAttributeImpl(ScalarAttributeImpl):
        """represents a scalar-holding InstrumentedAttribute,
           where the target object is also instrumented.

           Adds events to delete/set operations.

        """

        default_accepts_scalar_loader = False
        uses_objects = True
        supports_population = True
        collection = False

        __slots__ = ()

        def delete(self, state, dict_):
            if self.dispatch._active_history:
                old = self.get(
                    state,
                    dict_,
                    passive=PASSIVE_ONLY_PERSISTENT
                    | NO_AUTOFLUSH
                    | LOAD_AGAINST_COMMITTED,
                )
            else:
                old = self.get(
                    state,
                    dict_,
                    passive=PASSIVE_NO_FETCH ^ INIT_OK
                    | LOAD_AGAINST_COMMITTED
                    | NO_RAISE,
                )

            self.fire_remove_event(state, dict_, old, self._remove_token)

            existing = dict_.pop(self.key, NO_VALUE)

            # if the attribute is expired, we currently have no way to tell
            # that an object-attribute was expired vs. not loaded.   So
            # for this test, we look to see if the object has a DB identity.
            if (
                existing is NO_VALUE
                and old is not PASSIVE_NO_RESULT
                and state.key is None
            ):
                raise AttributeError("%s object does not have a value" % self)

        def get_history(self, state, dict_, passive=PASSIVE_OFF):
            if self.key in dict_:
                return History.from_object_attribute(self, state, dict_[self.key])
            else:
                if passive & INIT_OK:
                    passive ^= INIT_OK
                current = self.get(state, dict_, passive=passive)
                if current is PASSIVE_NO_RESULT:
                    return HISTORY_BLANK
                else:
                    return History.from_object_attribute(self, state, current)

        def get_all_pending(self, state, dict_, passive=PASSIVE_NO_INITIALIZE):
            if self.key in dict_:
                current = dict_[self.key]
            elif passive & CALLABLES_OK:
                current = self.get(state, dict_, passive=passive)
            else:
                return []

            # can't use __hash__(), can't use __eq__() here
            if (
                current is not None
                and current is not PASSIVE_NO_RESULT
                and current is not NEVER_SET
            ):
                ret = [(instance_state(current), current)]
            else:
                ret = [(None, None)]

            if self.key in state.committed_state:
                original = state.committed_state[self.key]
                if (
                    original is not None
                    and original is not PASSIVE_NO_RESULT
                    and original is not NEVER_SET
                    and original is not current
                ):

                    ret.append((instance_state(original), original))
            return ret

        def set(
            self,
            state,
            dict_,
            value,
            initiator,
            passive=PASSIVE_OFF,
            check_old=None,
            pop=False,
        ):
            """Set a value on the given InstanceState.

            """
            if self.dispatch._active_history:
                old = self.get(
                    state,
                    dict_,
                    passive=PASSIVE_ONLY_PERSISTENT
                    | NO_AUTOFLUSH
                    | LOAD_AGAINST_COMMITTED,
                )
            else:
                old = self.get(
                    state,
                    dict_,
                    passive=PASSIVE_NO_FETCH ^ INIT_OK
                    | LOAD_AGAINST_COMMITTED
                    | NO_RAISE,
                )

            if (
                check_old is not None
                and old is not PASSIVE_NO_RESULT
                and check_old is not old
            ):
                if pop:
                    return
                else:
                    raise ValueError(
                        "Object %s not associated with %s on attribute '%s'"
                        % (instance_str(check_old), state_str(state), self.key)
                    )

            value = self.fire_replace_event(state, dict_, value, old, initiator)
            dict_[self.key] = value

        def fire_remove_event(self, state, dict_, value, initiator):
            if self.trackparent and value is not None:
                self.sethasparent(instance_state(value), state, False)

            for fn in self.dispatch.remove:
                fn(state, value, initiator or self._remove_token)

            state._modified_event(dict_, self, value)

        def fire_replace_event(self, state, dict_, value, previous, initiator):
            if self.trackparent:
                if previous is not value and previous not in (
                    None,
                    PASSIVE_NO_RESULT,
                    NEVER_SET,
                ):
                    self.sethasparent(instance_state(previous), state, False)

            for fn in self.dispatch.set:
                value = fn(
                    state, value, previous, initiator or self._replace_token
                )

            state._modified_event(dict_, self, previous)

            if self.trackparent:
                if value is not None:
                    self.sethasparent(instance_state(value), state, True)

            return value

    class CollectionAttributeImpl(AttributeImpl):
        """A collection-holding attribute that instruments changes in membership.

        Only handles collections of instrumented objects.

        InstrumentedCollectionAttribute holds an arbitrary, user-specified
        container object (defaulting to a list) and brokers access to the
        CollectionAdapter, a "view" onto that object that presents consistent bag
        semantics to the orm layer independent of the user data implementation.

        """

        default_accepts_scalar_loader = False
        uses_objects = True
        supports_population = True
        collection = True
        dynamic = False

        __slots__ = (
            "copy",
            "collection_factory",
            "_append_token",
            "_remove_token",
            "_bulk_replace_token",
            "_duck_typed_as",
        )

        def __init__(
            self,
            class_,
            key,
            callable_,
            dispatch,
            typecallable=None,
            trackparent=False,
            extension=None,
            copy_function=None,
            compare_function=None,
            **kwargs
        ):
            super(CollectionAttributeImpl, self).__init__(
                class_,
                key,
                callable_,
                dispatch,
                trackparent=trackparent,
                extension=extension,
                compare_function=compare_function,
                **kwargs
            )

            if copy_function is None:
                copy_function = self.__copy
            self.copy = copy_function
            self.collection_factory = typecallable
            self._append_token = Event(self, OP_APPEND)
            self._remove_token = Event(self, OP_REMOVE)
            self._bulk_replace_token = Event(self, OP_BULK_REPLACE)
            self._duck_typed_as = util.duck_type_collection(
                self.collection_factory()
            )

            if getattr(self.collection_factory, "_sa_linker", None):

                @event.listens_for(self, "init_collection")
                def link(target, collection, collection_adapter):
                    collection._sa_linker(collection_adapter)

                @event.listens_for(self, "dispose_collection")
                def unlink(target, collection, collection_adapter):
                    collection._sa_linker(None)

        def __copy(self, item):
            return [y for y in collections.collection_adapter(item)]

        def get_history(self, state, dict_, passive=PASSIVE_OFF):
            current = self.get(state, dict_, passive=passive)
            if current is PASSIVE_NO_RESULT:
                return HISTORY_BLANK
            else:
                return History.from_collection(self, state, current)

        def get_all_pending(self, state, dict_, passive=PASSIVE_NO_INITIALIZE):
            # NOTE: passive is ignored here at the moment

            if self.key not in dict_:
                return []

            current = dict_[self.key]
            current = getattr(current, "_sa_adapter")

            if self.key in state.committed_state:
                original = state.committed_state[self.key]
                if original not in (NO_VALUE, NEVER_SET):
                    current_states = [
                        ((c is not None) and instance_state(c) or None, c)
                        for c in current
                    ]
                    original_states = [
                        ((c is not None) and instance_state(c) or None, c)
                        for c in original
                    ]

                    current_set = dict(current_states)
                    original_set = dict(original_states)

                    return (
                        [
                            (s, o)
                            for s, o in current_states
                            if s not in original_set
                        ]
                        + [(s, o) for s, o in current_states if s in original_set]
                        + [
                            (s, o)
                            for s, o in original_states
                            if s not in current_set
                        ]
                    )

            return [(instance_state(o), o) for o in current]

        def fire_append_event(self, state, dict_, value, initiator):
            for fn in self.dispatch.append:
                value = fn(state, value, initiator or self._append_token)

            state._modified_event(dict_, self, NEVER_SET, True)

            if self.trackparent and value is not None:
                self.sethasparent(instance_state(value), state, True)

            return value

        def fire_pre_remove_event(self, state, dict_, initiator):
            """A special event used for pop() operations.

            The "remove" event needs to have the item to be removed passed to
            it, which in the case of pop from a set, we don't have a way to access
            the item before the operation.   the event is used for all pop()
            operations (even though set.pop is the one where it is really needed).

            """
            state._modified_event(dict_, self, NEVER_SET, True)

        def fire_remove_event(self, state, dict_, value, initiator):
            if self.trackparent and value is not None:
                self.sethasparent(instance_state(value), state, False)

            for fn in self.dispatch.remove:
                fn(state, value, initiator or self._remove_token)

            state._modified_event(dict_, self, NEVER_SET, True)

        def delete(self, state, dict_):
            if self.key not in dict_:
                return

            state._modified_event(dict_, self, NEVER_SET, True)

            collection = self.get_collection(state, state.dict)
            collection.clear_with_event()

            # key is always present because we checked above.  e.g.
            # del is a no-op if collection not present.
            del dict_[self.key]

        def initialize(self, state, dict_):
            """Initialize this attribute with an empty collection."""

            _, user_data = self._initialize_collection(state)
            dict_[self.key] = user_data
            return user_data

        def _initialize_collection(self, state):

            adapter, collection = state.manager.initialize_collection(
                self.key, state, self.collection_factory
            )

            self.dispatch.init_collection(state, collection, adapter)

            return adapter, collection

        def append(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
            collection = self.get_collection(state, dict_, passive=passive)
            if collection is PASSIVE_NO_RESULT:
                value = self.fire_append_event(state, dict_, value, initiator)
                assert (
                    self.key not in dict_
                ), "Collection was loaded during event handling."
                state._get_pending_mutation(self.key).append(value)
            else:
                collection.append_with_event(value, initiator)

        def remove(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
            collection = self.get_collection(state, state.dict, passive=passive)
            if collection is PASSIVE_NO_RESULT:
                self.fire_remove_event(state, dict_, value, initiator)
                assert (
                    self.key not in dict_
                ), "Collection was loaded during event handling."
                state._get_pending_mutation(self.key).remove(value)
            else:
                collection.remove_with_event(value, initiator)

        def pop(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
            try:
                # TODO: better solution here would be to add
                # a "popper" role to collections.py to complement
                # "remover".
                self.remove(state, dict_, value, initiator, passive=passive)
            except (ValueError, KeyError, IndexError):
                pass

        def set(
            self,
            state,
            dict_,
            value,
            initiator=None,
            passive=PASSIVE_OFF,
            pop=False,
            _adapt=True,
        ):
            iterable = orig_iterable = value

            # pulling a new collection first so that an adaptation exception does
            # not trigger a lazy load of the old collection.
            new_collection, user_data = self._initialize_collection(state)
            if _adapt:
                if new_collection._converter is not None:
                    iterable = new_collection._converter(iterable)
                else:
                    setting_type = util.duck_type_collection(iterable)
                    receiving_type = self._duck_typed_as

                    if setting_type is not receiving_type:
                        given = (
                            iterable is None
                            and "None"
                            or iterable.__class__.__name__
                        )
                        wanted = self._duck_typed_as.__name__
                        raise TypeError(
                            "Incompatible collection type: %s is not %s-like"
                            % (given, wanted)
                        )

                    # If the object is an adapted collection, return the (iterable)
                    # adapter.
                    if hasattr(iterable, "_sa_iterator"):
                        iterable = iterable._sa_iterator()
                    elif setting_type is dict:
                        if util.py3k:
                            iterable = iterable.values()
                        else:
                            iterable = getattr(
                                iterable, "itervalues", iterable.values
                            )()
                    else:
                        iterable = iter(iterable)
            new_values = list(iterable)

            evt = self._bulk_replace_token

            self.dispatch.bulk_replace(state, new_values, evt)

            old = self.get(state, dict_, passive=PASSIVE_ONLY_PERSISTENT)
            if old is PASSIVE_NO_RESULT:
                old = self.initialize(state, dict_)
            elif old is orig_iterable:
                # ignore re-assignment of the current collection, as happens
                # implicitly with in-place operators (foo.collection |= other)
                return

            # place a copy of "old" in state.committed_state
            state._modified_event(dict_, self, old, True)

            old_collection = old._sa_adapter

            dict_[self.key] = user_data

            collections.bulk_replace(
                new_values, old_collection, new_collection, initiator=evt
            )

            del old._sa_adapter
            self.dispatch.dispose_collection(state, old, old_collection)

        def _invalidate_collection(self, collection):
            adapter = getattr(collection, "_sa_adapter")
            adapter.invalidated = True

        def set_committed_value(self, state, dict_, value):
            """Set an attribute value on the given instance and 'commit' it."""

            collection, user_data = self._initialize_collection(state)

            if value:
                collection.append_multiple_without_event(value)

            state.dict[self.key] = user_data

            state._commit(dict_, [self.key])

            if self.key in state._pending_mutations:
                # pending items exist.  issue a modified event,
                # add/remove new items.
                state._modified_event(dict_, self, user_data, True)

                pending = state._pending_mutations.pop(self.key)
                added = pending.added_items
                removed = pending.deleted_items
                for item in added:
                    collection.append_without_event(item)
                for item in removed:
                    collection.remove_without_event(item)

            return user_data

        def get_collection(
            self, state, dict_, user_data=None, passive=PASSIVE_OFF
        ):
            """Retrieve the CollectionAdapter associated with the given state.

            Creates a new CollectionAdapter if one does not exist.

            """
            if user_data is None:
                user_data = self.get(state, dict_, passive=passive)
                if user_data is PASSIVE_NO_RESULT:
                    return user_data

            return getattr(user_data, "_sa_adapter")

    def backref_listeners(attribute, key, uselist):
        """Apply listeners to synchronize a two-way relationship."""

        # use easily recognizable names for stack traces.

        # in the sections marked "tokens to test for a recursive loop",
        # this is somewhat brittle and very performance-sensitive logic
        # that is specific to how we might arrive at each event.  a marker
        # that can target us directly to arguments being invoked against
        # the impl might be simpler, but could interfere with other systems.

        parent_token = attribute.impl.parent_token
        parent_impl = attribute.impl

        def _acceptable_key_err(child_state, initiator, child_impl):
            raise ValueError(
                "Bidirectional attribute conflict detected: "
                'Passing object %s to attribute "%s" '
                'triggers a modify event on attribute "%s" '
                'via the backref "%s".'
                % (
                    state_str(child_state),
                    initiator.parent_token,
                    child_impl.parent_token,
                    attribute.impl.parent_token,
                )
            )

        def emit_backref_from_scalar_set_event(state, child, oldchild, initiator):
            if oldchild is child:
                return child
            if (
                oldchild is not None
                and oldchild is not PASSIVE_NO_RESULT
                and oldchild is not NEVER_SET
            ):
                # With lazy=None, there's no guarantee that the full collection is
                # present when updating via a backref.
                old_state, old_dict = (
                    instance_state(oldchild),
                    instance_dict(oldchild),
                )
                impl = old_state.manager[key].impl

                # tokens to test for a recursive loop.
                if not impl.collection and not impl.dynamic:
                    check_recursive_token = impl._replace_token
                else:
                    check_recursive_token = impl._remove_token

                if initiator is not check_recursive_token:
                    impl.pop(
                        old_state,
                        old_dict,
                        state.obj(),
                        parent_impl._append_token,
                        passive=PASSIVE_NO_FETCH,
                    )

            if child is not None:
                child_state, child_dict = (
                    instance_state(child),
                    instance_dict(child),
                )
                child_impl = child_state.manager[key].impl

                if (
                    initiator.parent_token is not parent_token
                    and initiator.parent_token is not child_impl.parent_token
                ):
                    _acceptable_key_err(state, initiator, child_impl)

                # tokens to test for a recursive loop.
                check_append_token = child_impl._append_token
                check_bulk_replace_token = (
                    child_impl._bulk_replace_token
                    if child_impl.collection
                    else None
                )

                if (
                    initiator is not check_append_token
                    and initiator is not check_bulk_replace_token
                ):
                    child_impl.append(
                        child_state,
                        child_dict,
                        state.obj(),
                        initiator,
                        passive=PASSIVE_NO_FETCH,
                    )
            return child

        def emit_backref_from_collection_append_event(state, child, initiator):
            if child is None:
                return

            child_state, child_dict = instance_state(child), instance_dict(child)
            child_impl = child_state.manager[key].impl

            if (
                initiator.parent_token is not parent_token
                and initiator.parent_token is not child_impl.parent_token
            ):
                _acceptable_key_err(state, initiator, child_impl)

            # tokens to test for a recursive loop.
            check_append_token = child_impl._append_token
            check_bulk_replace_token = (
                child_impl._bulk_replace_token if child_impl.collection else None
            )

            if (
                initiator is not check_append_token
                and initiator is not check_bulk_replace_token
            ):
                child_impl.append(
                    child_state,
                    child_dict,
                    state.obj(),
                    initiator,
                    passive=PASSIVE_NO_FETCH,
                )
            return child

        def emit_backref_from_collection_remove_event(state, child, initiator):
            if (
                child is not None
                and child is not PASSIVE_NO_RESULT
                and child is not NEVER_SET
            ):
                child_state, child_dict = (
                    instance_state(child),
                    instance_dict(child),
                )
                child_impl = child_state.manager[key].impl

                # tokens to test for a recursive loop.
                if not child_impl.collection and not child_impl.dynamic:
                    check_remove_token = child_impl._remove_token
                    check_replace_token = child_impl._replace_token
                    check_for_dupes_on_remove = uselist and not parent_impl.dynamic
                else:
                    check_remove_token = child_impl._remove_token
                    check_replace_token = (
                        child_impl._bulk_replace_token
                        if child_impl.collection
                        else None
                    )
                    check_for_dupes_on_remove = False

                if (
                    initiator is not check_remove_token
                    and initiator is not check_replace_token
                ):

                    if not check_for_dupes_on_remove or not util.has_dupes(
                        # when this event is called, the item is usually
                        # present in the list, except for a pop() operation.
                        state.dict[parent_impl.key],
                        child,
                    ):
                        child_impl.pop(
                            child_state,
                            child_dict,
                            state.obj(),
                            initiator,
                            passive=PASSIVE_NO_FETCH,
                        )

        if uselist:
            event.listen(
                attribute,
                "append",
                emit_backref_from_collection_append_event,
                retval=True,
                raw=True,
            )
        else:
            event.listen(
                attribute,
                "set",
                emit_backref_from_scalar_set_event,
                retval=True,
                raw=True,
            )
        # TODO: need coverage in test/orm/ of remove event
        event.listen(
            attribute,
            "remove",
            emit_backref_from_collection_remove_event,
            retval=True,
            raw=True,
        )

    _NO_HISTORY = util.symbol("NO_HISTORY")
    _NO_STATE_SYMBOLS = frozenset(
        [id(PASSIVE_NO_RESULT), id(NO_VALUE), id(NEVER_SET)]
    )

    History = util.namedtuple("History", ["added", "unchanged", "deleted"])

    class History(History):
        """A 3-tuple of added, unchanged and deleted values,
        representing the changes which have occurred on an instrumented
        attribute.

        The easiest way to get a :class:`.History` object for a particular
        attribute on an object is to use the :func:`.inspect` function::

            from sqlalchemy import inspect

            hist = inspect(myobject).attrs.myattribute.history

        Each tuple member is an iterable sequence:

        * ``added`` - the collection of items added to the attribute (the first
          tuple element).

        * ``unchanged`` - the collection of items that have not changed on the
          attribute (the second tuple element).

        * ``deleted`` - the collection of items that have been removed from the
          attribute (the third tuple element).

        """

        def __bool__(self):
            return self != HISTORY_BLANK

        __nonzero__ = __bool__

        def empty(self):
            """Return True if this :class:`.History` has no changes
            and no existing, unchanged state.

            """

            return not bool((self.added or self.deleted) or self.unchanged)

        def sum(self):
            """Return a collection of added + unchanged + deleted."""

            return (
                (self.added or []) + (self.unchanged or []) + (self.deleted or [])
            )

        def non_deleted(self):
            """Return a collection of added + unchanged."""

            return (self.added or []) + (self.unchanged or [])

        def non_added(self):
            """Return a collection of unchanged + deleted."""

            return (self.unchanged or []) + (self.deleted or [])

        def has_changes(self):
            """Return True if this :class:`.History` has changes."""

            return bool(self.added or self.deleted)

        def as_state(self):
            return History(
                [
                    (c is not None) and instance_state(c) or None
                    for c in self.added
                ],
                [
                    (c is not None) and instance_state(c) or None
                    for c in self.unchanged
                ],
                [
                    (c is not None) and instance_state(c) or None
                    for c in self.deleted
                ],
            )

        @classmethod
        def from_scalar_attribute(cls, attribute, state, current):
            original = state.committed_state.get(attribute.key, _NO_HISTORY)

            if original is _NO_HISTORY:
                if current is NEVER_SET:
                    return cls((), (), ())
                else:
                    return cls((), [current], ())
            # don't let ClauseElement expressions here trip things up
            elif attribute.is_equal(current, original) is True:
                return cls((), [current], ())
            else:
                # current convention on native scalars is to not
                # include information
                # about missing previous value in "deleted", but
                # we do include None, which helps in some primary
                # key situations
                if id(original) in _NO_STATE_SYMBOLS:
                    deleted = ()
                    # indicate a "del" operation occurred when we don't have
                    # the previous value as: ([None], (), ())
                    if id(current) in _NO_STATE_SYMBOLS:
                        current = None
                else:
                    deleted = [original]
                if current is NEVER_SET:
                    return cls((), (), deleted)
                else:
                    return cls([current], (), deleted)

        @classmethod
        def from_object_attribute(cls, attribute, state, current):
            original = state.committed_state.get(attribute.key, _NO_HISTORY)

            if original is _NO_HISTORY:
                if current is NO_VALUE or current is NEVER_SET:
                    return cls((), (), ())
                else:
                    return cls((), [current], ())
            elif current is original and current is not NEVER_SET:
                return cls((), [current], ())
            else:
                # current convention on related objects is to not
                # include information
                # about missing previous value in "deleted", and
                # to also not include None - the dependency.py rules
                # ignore the None in any case.
                if id(original) in _NO_STATE_SYMBOLS or original is None:
                    deleted = ()
                    # indicate a "del" operation occurred when we don't have
                    # the previous value as: ([None], (), ())
                    if id(current) in _NO_STATE_SYMBOLS:
                        current = None
                else:
                    deleted = [original]
                if current is NO_VALUE or current is NEVER_SET:
                    return cls((), (), deleted)
                else:
                    return cls([current], (), deleted)

        @classmethod
        def from_collection(cls, attribute, state, current):
            original = state.committed_state.get(attribute.key, _NO_HISTORY)

            if current is NO_VALUE or current is NEVER_SET:
                return cls((), (), ())

            current = getattr(current, "_sa_adapter")
            if original in (NO_VALUE, NEVER_SET):
                return cls(list(current), (), ())
            elif original is _NO_HISTORY:
                return cls((), list(current), ())
            else:

                current_states = [
                    ((c is not None) and instance_state(c) or None, c)
                    for c in current
                ]
                original_states = [
                    ((c is not None) and instance_state(c) or None, c)
                    for c in original
                ]

                current_set = dict(current_states)
                original_set = dict(original_states)

                return cls(
                    [o for s, o in current_states if s not in original_set],
                    [o for s, o in current_states if s in original_set],
                    [o for s, o in original_states if s not in current_set],
                )

    HISTORY_BLANK = History(None, None, None)

    def get_history(obj, key, passive=PASSIVE_OFF):
        """Return a :class:`.History` record for the given object
        and attribute key.

        This is the **pre-flush** history for a given attribute, which is
        reset each time the :class:`.Session` flushes changes to the
        current database transaction.

        .. note::

            Prefer to use the :attr:`.AttributeState.history` and
            :meth:`.AttributeState.load_history` accessors to retrieve the
            :class:`.History` for instance attributes.

        :param obj: an object whose class is instrumented by the
          attributes package.

        :param key: string attribute name.

        :param passive: indicates loading behavior for the attribute
           if the value is not already present.   This is a
           bitflag attribute, which defaults to the symbol
           :attr:`.PASSIVE_OFF` indicating all necessary SQL
           should be emitted.

        .. seealso::

            :attr:`.AttributeState.history`

            :meth:`.AttributeState.load_history` - retrieve history
            using loader callables if the value is not locally present.

        """
        if passive is True:
            util.warn_deprecated(
                "Passing True for 'passive' is deprecated. "
                "Use attributes.PASSIVE_NO_INITIALIZE"
            )
            passive = PASSIVE_NO_INITIALIZE
        elif passive is False:
            util.warn_deprecated(
                "Passing False for 'passive' is "
                "deprecated.  Use attributes.PASSIVE_OFF"
            )
            passive = PASSIVE_OFF

        return get_state_history(instance_state(obj), key, passive)

    def get_state_history(state, key, passive=PASSIVE_OFF):
        return state.get_history(key, passive)

    def has_parent(cls, obj, key, optimistic=False):
        """TODO"""
        manager = manager_of_class(cls)
        state = instance_state(obj)
        return manager.has_parent(state, key, optimistic)

    def register_attribute(class_, key, **kw):
        comparator = kw.pop("comparator", None)
        parententity = kw.pop("parententity", None)
        doc = kw.pop("doc", None)
        desc = register_descriptor(class_, key, comparator, parententity, doc=doc)
        register_attribute_impl(class_, key, **kw)
        return desc

    def register_attribute_impl(
        class_,
        key,
        uselist=False,
        callable_=None,
        useobject=False,
        impl_class=None,
        backref=None,
        **kw
    ):

        manager = manager_of_class(class_)
        if uselist:
            factory = kw.pop("typecallable", None)
            typecallable = manager.instrument_collection_class(
                key, factory or list
            )
        else:
            typecallable = kw.pop("typecallable", None)

        dispatch = manager[key].dispatch

        if impl_class:
            impl = impl_class(class_, key, typecallable, dispatch, **kw)
        elif uselist:
            impl = CollectionAttributeImpl(
                class_, key, callable_, dispatch, typecallable=typecallable, **kw
            )
        elif useobject:
            impl = ScalarObjectAttributeImpl(
                class_, key, callable_, dispatch, **kw
            )
        else:
            impl = ScalarAttributeImpl(class_, key, callable_, dispatch, **kw)

        manager[key].impl = impl

        if backref:
            backref_listeners(manager[key], backref, uselist)

        manager.post_configure_attribute(key)
        return manager[key]

    def register_descriptor(
        class_, key, comparator=None, parententity=None, doc=None
    ):
        manager = manager_of_class(class_)

        descriptor = InstrumentedAttribute(
            class_, key, comparator=comparator, parententity=parententity
        )

        descriptor.__doc__ = doc

        manager.instrument_attribute(key, descriptor)
        return descriptor

    def unregister_attribute(class_, key):
        manager_of_class(class_).uninstrument_attribute(key)

    def init_collection(obj, key):
        """Initialize a collection attribute and return the collection adapter.

        This function is used to provide direct access to collection internals
        for a previously unloaded attribute.  e.g.::

            collection_adapter = init_collection(someobject, 'elements')
            for elem in values:
                collection_adapter.append_without_event(elem)

        For an easier way to do the above, see
        :func:`~sqlalchemy.orm.attributes.set_committed_value`.

        :param obj: a mapped object

        :param key: string attribute name where the collection is located.

        """
        state = instance_state(obj)
        dict_ = state.dict
        return init_state_collection(state, dict_, key)

    def init_state_collection(state, dict_, key):
        """Initialize a collection attribute and return the collection adapter."""

        attr = state.manager[key].impl
        user_data = attr.initialize(state, dict_)
        return attr.get_collection(state, dict_, user_data)

    def set_committed_value(instance, key, value):
        """Set the value of an attribute with no history events.

        Cancels any previous history present.  The value should be
        a scalar value for scalar-holding attributes, or
        an iterable for any collection-holding attribute.

        This is the same underlying method used when a lazy loader
        fires off and loads additional data from the database.
        In particular, this method can be used by application code
        which has loaded additional attributes or collections through
        separate queries, which can then be attached to an instance
        as though it were part of its original loaded state.

        """
        state, dict_ = instance_state(instance), instance_dict(instance)
        state.manager[key].impl.set_committed_value(state, dict_, value)

    def set_attribute(instance, key, value, initiator=None):
        """Set the value of an attribute, firing history events.

        This function may be used regardless of instrumentation
        applied directly to the class, i.e. no descriptors are required.
        Custom attribute management schemes will need to make usage
        of this method to establish attribute state as understood
        by SQLAlchemy.

        :param instance: the object that will be modified

        :param key: string name of the attribute

        :param value: value to assign

        :param initiator: an instance of :class:`.Event` that would have
         been propagated from a previous event listener.  This argument
         is used when the :func:`.set_attribute` function is being used within
         an existing event listening function where an :class:`.Event` object
         is being supplied; the object may be used to track the origin of the
         chain of events.

         .. versionadded:: 1.2.3

        """
        state, dict_ = instance_state(instance), instance_dict(instance)
        state.manager[key].impl.set(state, dict_, value, initiator)

    def get_attribute(instance, key):
        """Get the value of an attribute, firing any callables required.

        This function may be used regardless of instrumentation
        applied directly to the class, i.e. no descriptors are required.
        Custom attribute management schemes will need to make usage
        of this method to make usage of attribute state as understood
        by SQLAlchemy.

        """
        state, dict_ = instance_state(instance), instance_dict(instance)
        return state.manager[key].impl.get(state, dict_)

    def del_attribute(instance, key):
        """Delete the value of an attribute, firing history events.

        This function may be used regardless of instrumentation
        applied directly to the class, i.e. no descriptors are required.
        Custom attribute management schemes will need to make usage
        of this method to establish attribute state as understood
        by SQLAlchemy.

        """
        state, dict_ = instance_state(instance), instance_dict(instance)
        state.manager[key].impl.delete(state, dict_)

    def flag_modified(instance, key):
        """Mark an attribute on an instance as 'modified'.

        This sets the 'modified' flag on the instance and
        establishes an unconditional change event for the given attribute.
        The attribute must have a value present, else an
        :class:`.InvalidRequestError` is raised.

        To mark an object "dirty" without referring to any specific attribute
        so that it is considered within a flush, use the
        :func:`.attributes.flag_dirty` call.

        .. seealso::

            :func:`.attributes.flag_dirty`

        """
        state, dict_ = instance_state(instance), instance_dict(instance)
        impl = state.manager[key].impl
        impl.dispatch.modified(state, impl._modified_token)
        state._modified_event(dict_, impl, NO_VALUE, is_userland=True)

    def flag_dirty(instance):
        """Mark an instance as 'dirty' without any specific attribute mentioned.

        This is a special operation that will allow the object to travel through
        the flush process for interception by events such as
        :meth:`.SessionEvents.before_flush`.   Note that no SQL will be emitted in
        the flush process for an object that has no changes, even if marked dirty
        via this method.  However, a :meth:`.SessionEvents.before_flush` handler
        will be able to see the object in the :attr:`.Session.dirty` collection and
        may establish changes on it, which will then be included in the SQL
        emitted.

        .. versionadded:: 1.2

        .. seealso::

            :func:`.attributes.flag_modified`

        """

        state, dict_ = instance_state(instance), instance_dict(instance)
        state._modified_event(dict_, None, NO_VALUE, is_userland=True)