# ext/associationproxy.py
# Copyright (C) 2005-2025 the SQLAlchemy authors and contributors
# <see AUTHORS file>
#
# This module is part of SQLAlchemy and is released under
# the MIT License: https://www.opensource.org/licenses/mit-license.php

"""Contain the ``AssociationProxy`` class.

The ``AssociationProxy`` is a Python property object which provides
transparent proxied access to the endpoint of an association object.

See the example ``examples/association/proxied_association.py``.

"""
from __future__ import annotations

import operator
import typing
from typing import AbstractSet
from typing import Any
from typing import Callable
from typing import cast
from typing import Collection
from typing import Dict
from typing import Generic
from typing import ItemsView
from typing import Iterable
from typing import Iterator
from typing import KeysView
from typing import List
from typing import Mapping
from typing import MutableMapping
from typing import MutableSequence
from typing import MutableSet
from typing import NoReturn
from typing import Optional
from typing import overload
from typing import Set
from typing import Tuple
from typing import Type
from typing import TypeVar
from typing import Union
from typing import ValuesView

from .. import ColumnElement
from .. import exc
from .. import inspect
from .. import orm
from .. import util
from ..orm import collections
from ..orm import InspectionAttrExtensionType
from ..orm import interfaces
from ..orm import ORMDescriptor
from ..orm.base import SQLORMOperations
from ..orm.interfaces import _AttributeOptions
from ..orm.interfaces import _DCAttributeOptions
from ..orm.interfaces import _DEFAULT_ATTRIBUTE_OPTIONS
from ..sql import operators
from ..sql import or_
from ..sql.base import _NoArg
from ..util.typing import Literal
from ..util.typing import Protocol
from ..util.typing import Self
from ..util.typing import SupportsIndex
from ..util.typing import SupportsKeysAndGetItem

if typing.TYPE_CHECKING:
    from ..orm.interfaces import MapperProperty
    from ..orm.interfaces import PropComparator
    from ..orm.mapper import Mapper
    from ..sql._typing import _ColumnExpressionArgument
    from ..sql._typing import _InfoType


_T = TypeVar("_T", bound=Any)
_T_co = TypeVar("_T_co", bound=Any, covariant=True)
_T_con = TypeVar("_T_con", bound=Any, contravariant=True)
_S = TypeVar("_S", bound=Any)
_KT = TypeVar("_KT", bound=Any)
_VT = TypeVar("_VT", bound=Any)


def association_proxy(
    target_collection: str,
    attr: str,
    *,
    creator: Optional[_CreatorProtocol] = None,
    getset_factory: Optional[_GetSetFactoryProtocol] = None,
    proxy_factory: Optional[_ProxyFactoryProtocol] = None,
    proxy_bulk_set: Optional[_ProxyBulkSetProtocol] = None,
    info: Optional[_InfoType] = None,
    cascade_scalar_deletes: bool = False,
    create_on_none_assignment: bool = False,
    init: Union[_NoArg, bool] = _NoArg.NO_ARG,
    repr: Union[_NoArg, bool] = _NoArg.NO_ARG,  # noqa: A002
    default: Optional[Any] = _NoArg.NO_ARG,
    default_factory: Union[_NoArg, Callable[[], _T]] = _NoArg.NO_ARG,
    compare: Union[_NoArg, bool] = _NoArg.NO_ARG,
    kw_only: Union[_NoArg, bool] = _NoArg.NO_ARG,
    hash: Union[_NoArg, bool, None] = _NoArg.NO_ARG,  # noqa: A002
    dataclass_metadata: Union[_NoArg, Mapping[Any, Any], None] = _NoArg.NO_ARG,
) -> AssociationProxy[Any]:
    r"""Return a Python property implementing a view of a target
    attribute which references an attribute on members of the
    target.

    The returned value is an instance of :class:`.AssociationProxy`.

    Implements a Python property representing a relationship as a collection
    of simpler values, or a scalar value.  The proxied property will mimic
    the collection type of the target (list, dict or set), or, in the case of
    a one to one relationship, a simple scalar value.

    :param target_collection: Name of the attribute that is the immediate
      target.  This attribute is typically mapped by
      :func:`~sqlalchemy.orm.relationship` to link to a target collection, but
      can also be a many-to-one or non-scalar relationship.

    :param attr: Attribute on the associated instance or instances that
      are available on instances of the target object.

    :param creator: optional.

      Defines custom behavior when new items are added to the proxied
      collection.

      By default, adding new items to the collection will trigger a
      construction of an instance of the target object, passing the given
      item as a positional argument to the target constructor.  For cases
      where this isn't sufficient, :paramref:`.association_proxy.creator`
      can supply a callable that will construct the object in the
      appropriate way, given the item that was passed.

      For list- and set- oriented collections, a single argument is
      passed to the callable. For dictionary oriented collections, two
      arguments are passed, corresponding to the key and value.

      The :paramref:`.association_proxy.creator` callable is also invoked
      for scalar (i.e. many-to-one, one-to-one) relationships. If the
      current value of the target relationship attribute is ``None``, the
      callable is used to construct a new object.  If an object value already
      exists, the given attribute value is populated onto that object.

      .. seealso::

        :ref:`associationproxy_creator`

    :param cascade_scalar_deletes: when True, indicates that setting
        the proxied value to ``None``, or deleting it via ``del``, should
        also remove the source object.  Only applies to scalar attributes.
        Normally, removing the proxied target will not remove the proxy
        source, as this object may have other state that is still to be
        kept.

        .. versionadded:: 1.3

        .. seealso::

            :ref:`cascade_scalar_deletes` - complete usage example

    :param create_on_none_assignment: when True, indicates that setting
      the proxied value to ``None`` should **create** the source object
      if it does not exist, using the creator.  Only applies to scalar
      attributes.  This is mutually exclusive
      vs. the :paramref:`.assocation_proxy.cascade_scalar_deletes`.

      .. versionadded:: 2.0.18

    :param init: Specific to :ref:`orm_declarative_native_dataclasses`,
     specifies if the mapped attribute should be part of the ``__init__()``
     method as generated by the dataclass process.

     .. versionadded:: 2.0.0b4

    :param repr: Specific to :ref:`orm_declarative_native_dataclasses`,
     specifies if the attribute established by this :class:`.AssociationProxy`
     should be part of the ``__repr__()`` method as generated by the dataclass
     process.

     .. versionadded:: 2.0.0b4

    :param default_factory: Specific to
     :ref:`orm_declarative_native_dataclasses`, specifies a default-value
     generation function that will take place as part of the ``__init__()``
     method as generated by the dataclass process.

     .. versionadded:: 2.0.0b4

    :param compare: Specific to
     :ref:`orm_declarative_native_dataclasses`, indicates if this field
     should be included in comparison operations when generating the
     ``__eq__()`` and ``__ne__()`` methods for the mapped class.

     .. versionadded:: 2.0.0b4

    :param kw_only: Specific to :ref:`orm_declarative_native_dataclasses`,
     indicates if this field should be marked as keyword-only when generating
     the ``__init__()`` method as generated by the dataclass process.

     .. versionadded:: 2.0.0b4

    :param hash: Specific to
     :ref:`orm_declarative_native_dataclasses`, controls if this field
     is included when generating the ``__hash__()`` method for the mapped
     class.

     .. versionadded:: 2.0.36

    :param dataclass_metadata: Specific to
     :ref:`orm_declarative_native_dataclasses`, supplies metadata
     to be attached to the generated dataclass field.

     .. versionadded:: 2.0.42

    :param info: optional, will be assigned to
     :attr:`.AssociationProxy.info` if present.


    The following additional parameters involve injection of custom behaviors
    within the :class:`.AssociationProxy` object and are for advanced use
    only:

    :param getset_factory: Optional.  Proxied attribute access is
        automatically handled by routines that get and set values based on
        the `attr` argument for this proxy.

        If you would like to customize this behavior, you may supply a
        `getset_factory` callable that produces a tuple of `getter` and
        `setter` functions.  The factory is called with two arguments, the
        abstract type of the underlying collection and this proxy instance.

    :param proxy_factory: Optional.  The type of collection to emulate is
        determined by sniffing the target collection.  If your collection
        type can't be determined by duck typing or you'd like to use a
        different collection implementation, you may supply a factory
        function to produce those collections.  Only applicable to
        non-scalar relationships.

    :param proxy_bulk_set: Optional, use with proxy_factory.


    """
    return AssociationProxy(
        target_collection,
        attr,
        creator=creator,
        getset_factory=getset_factory,
        proxy_factory=proxy_factory,
        proxy_bulk_set=proxy_bulk_set,
        info=info,
        cascade_scalar_deletes=cascade_scalar_deletes,
        create_on_none_assignment=create_on_none_assignment,
        attribute_options=_AttributeOptions(
            init,
            repr,
            default,
            default_factory,
            compare,
            kw_only,
            hash,
            dataclass_metadata,
        ),
    )


class AssociationProxyExtensionType(InspectionAttrExtensionType):
    ASSOCIATION_PROXY = "ASSOCIATION_PROXY"
    """Symbol indicating an :class:`.InspectionAttr` that's
    of type :class:`.AssociationProxy`.

    Is assigned to the :attr:`.InspectionAttr.extension_type`
    attribute.

    """


class _GetterProtocol(Protocol[_T_co]):
    def __call__(self, instance: Any) -> _T_co: ...


# mypy 0.990 we are no longer allowed to make this Protocol[_T_con]
class _SetterProtocol(Protocol): ...


class _PlainSetterProtocol(_SetterProtocol, Protocol[_T_con]):
    def __call__(self, instance: Any, value: _T_con) -> None: ...


class _DictSetterProtocol(_SetterProtocol, Protocol[_T_con]):
    def __call__(self, instance: Any, key: Any, value: _T_con) -> None: ...


# mypy 0.990 we are no longer allowed to make this Protocol[_T_con]
class _CreatorProtocol(Protocol): ...


class _PlainCreatorProtocol(_CreatorProtocol, Protocol[_T_con]):
    def __call__(self, value: _T_con) -> Any: ...


class _KeyCreatorProtocol(_CreatorProtocol, Protocol[_T_con]):
    def __call__(self, key: Any, value: Optional[_T_con]) -> Any: ...


class _LazyCollectionProtocol(Protocol[_T]):
    def __call__(
        self,
    ) -> Union[
        MutableSet[_T], MutableMapping[Any, _T], MutableSequence[_T]
    ]: ...


class _GetSetFactoryProtocol(Protocol):
    def __call__(
        self,
        collection_class: Optional[Type[Any]],
        assoc_instance: AssociationProxyInstance[Any],
    ) -> Tuple[_GetterProtocol[Any], _SetterProtocol]: ...


class _ProxyFactoryProtocol(Protocol):
    def __call__(
        self,
        lazy_collection: _LazyCollectionProtocol[Any],
        creator: _CreatorProtocol,
        value_attr: str,
        parent: AssociationProxyInstance[Any],
    ) -> Any: ...


class _ProxyBulkSetProtocol(Protocol):
    def __call__(
        self, proxy: _AssociationCollection[Any], collection: Iterable[Any]
    ) -> None: ...


class _AssociationProxyProtocol(Protocol[_T]):
    """describes the interface of :class:`.AssociationProxy`
    without including descriptor methods in the interface."""

    creator: Optional[_CreatorProtocol]
    key: str
    target_collection: str
    value_attr: str
    cascade_scalar_deletes: bool
    create_on_none_assignment: bool
    getset_factory: Optional[_GetSetFactoryProtocol]
    proxy_factory: Optional[_ProxyFactoryProtocol]
    proxy_bulk_set: Optional[_ProxyBulkSetProtocol]

    @util.ro_memoized_property
    def info(self) -> _InfoType: ...

    def for_class(
        self, class_: Type[Any], obj: Optional[object] = None
    ) -> AssociationProxyInstance[_T]: ...

    def _default_getset(
        self, collection_class: Any
    ) -> Tuple[_GetterProtocol[Any], _SetterProtocol]: ...


class AssociationProxy(
    interfaces.InspectionAttrInfo,
    ORMDescriptor[_T],
    _DCAttributeOptions,
    _AssociationProxyProtocol[_T],
):
    """A descriptor that presents a read/write view of an object attribute."""

    is_attribute = True
    extension_type = AssociationProxyExtensionType.ASSOCIATION_PROXY

    def __init__(
        self,
        target_collection: str,
        attr: str,
        *,
        creator: Optional[_CreatorProtocol] = None,
        getset_factory: Optional[_GetSetFactoryProtocol] = None,
        proxy_factory: Optional[_ProxyFactoryProtocol] = None,
        proxy_bulk_set: Optional[_ProxyBulkSetProtocol] = None,
        info: Optional[_InfoType] = None,
        cascade_scalar_deletes: bool = False,
        create_on_none_assignment: bool = False,
        attribute_options: Optional[_AttributeOptions] = None,
    ):
        """Construct a new :class:`.AssociationProxy`.

        The :class:`.AssociationProxy` object is typically constructed using
        the :func:`.association_proxy` constructor function. See the
        description of :func:`.association_proxy` for a description of all
        parameters.


        """
        self.target_collection = target_collection
        self.value_attr = attr
        self.creator = creator
        self.getset_factory = getset_factory
        self.proxy_factory = proxy_factory
        self.proxy_bulk_set = proxy_bulk_set

        if cascade_scalar_deletes and create_on_none_assignment:
            raise exc.ArgumentError(
                "The cascade_scalar_deletes and create_on_none_assignment "
                "parameters are mutually exclusive."
            )
        self.cascade_scalar_deletes = cascade_scalar_deletes
        self.create_on_none_assignment = create_on_none_assignment

        self.key = "_%s_%s_%s" % (
            type(self).__name__,
            target_collection,
            id(self),
        )
        if info:
            self.info = info  # type: ignore

        if (
            attribute_options
            and attribute_options != _DEFAULT_ATTRIBUTE_OPTIONS
        ):
            self._has_dataclass_arguments = True
            self._attribute_options = attribute_options
        else:
            self._has_dataclass_arguments = False
            self._attribute_options = _DEFAULT_ATTRIBUTE_OPTIONS

    @overload
    def __get__(
        self, instance: Literal[None], owner: Literal[None]
    ) -> Self: ...

    @overload
    def __get__(
        self, instance: Literal[None], owner: Any
    ) -> AssociationProxyInstance[_T]: ...

    @overload
    def __get__(self, instance: object, owner: Any) -> _T: ...

    def __get__(
        self, instance: object, owner: Any
    ) -> Union[AssociationProxyInstance[_T], _T, AssociationProxy[_T]]:
        if owner is None:
            return self
        inst = self._as_instance(owner, instance)
        if inst:
            return inst.get(instance)

        assert instance is None

        return self

    def __set__(self, instance: object, values: _T) -> None:
        class_ = type(instance)
        self._as_instance(class_, instance).set(instance, values)

    def __delete__(self, instance: object) -> None:
        class_ = type(instance)
        self._as_instance(class_, instance).delete(instance)

    def for_class(
        self, class_: Type[Any], obj: Optional[object] = None
    ) -> AssociationProxyInstance[_T]:
        r"""Return the internal state local to a specific mapped class.

        E.g., given a class ``User``::

            class User(Base):
                # ...

                keywords = association_proxy("kws", "keyword")

        If we access this :class:`.AssociationProxy` from
        :attr:`_orm.Mapper.all_orm_descriptors`, and we want to view the
        target class for this proxy as mapped by ``User``::

            inspect(User).all_orm_descriptors["keywords"].for_class(User).target_class

        This returns an instance of :class:`.AssociationProxyInstance` that
        is specific to the ``User`` class.   The :class:`.AssociationProxy`
        object remains agnostic of its parent class.

        :param class\_: the class that we are returning state for.

        :param obj: optional, an instance of the class that is required
         if the attribute refers to a polymorphic target, e.g. where we have
         to look at the type of the actual destination object to get the
         complete path.

        .. versionadded:: 1.3 - :class:`.AssociationProxy` no longer stores
           any state specific to a particular parent class; the state is now
           stored in per-class :class:`.AssociationProxyInstance` objects.


        """
        return self._as_instance(class_, obj)

    def _as_instance(
        self, class_: Any, obj: Any
    ) -> AssociationProxyInstance[_T]:
        try:
            inst = class_.__dict__[self.key + "_inst"]
        except KeyError:
            inst = None

        # avoid exception context
        if inst is None:
            owner = self._calc_owner(class_)
            if owner is not None:
                inst = AssociationProxyInstance.for_proxy(self, owner, obj)
                setattr(class_, self.key + "_inst", inst)
            else:
                inst = None

        if inst is not None and not inst._is_canonical:
            # the AssociationProxyInstance can't be generalized
            # since the proxied attribute is not on the targeted
            # class, only on subclasses of it, which might be
            # different.  only return for the specific
            # object's current value
            return inst._non_canonical_get_for_object(obj)  # type: ignore
        else:
            return inst  # type: ignore  # TODO

    def _calc_owner(self, target_cls: Any) -> Any:
        # we might be getting invoked for a subclass
        # that is not mapped yet, in some declarative situations.
        # save until we are mapped
        try:
            insp = inspect(target_cls)
        except exc.NoInspectionAvailable:
            # can't find a mapper, don't set owner. if we are a not-yet-mapped
            # subclass, we can also scan through __mro__ to find a mapped
            # class, but instead just wait for us to be called again against a
            # mapped class normally.
            return None
        else:
            return insp.mapper.class_manager.class_

    def _default_getset(
        self, collection_class: Any
    ) -> Tuple[_GetterProtocol[Any], _SetterProtocol]:
        attr = self.value_attr
        _getter = operator.attrgetter(attr)

        def getter(instance: Any) -> Optional[Any]:
            return _getter(instance) if instance is not None else None

        if collection_class is dict:

            def dict_setter(instance: Any, k: Any, value: Any) -> None:
                setattr(instance, attr, value)

            return getter, dict_setter

        else:

            def plain_setter(o: Any, v: Any) -> None:
                setattr(o, attr, v)

            return getter, plain_setter

    def __repr__(self) -> str:
        return "AssociationProxy(%r, %r)" % (
            self.target_collection,
            self.value_attr,
        )


# the pep-673 Self type does not work in Mypy for a "hybrid"
# style method that returns type or Self, so for one specific case
# we still need to use the pre-pep-673 workaround.
_Self = TypeVar("_Self", bound="AssociationProxyInstance[Any]")


class AssociationProxyInstance(SQLORMOperations[_T]):
    """A per-class object that serves class- and object-specific results.

    This is used by :class:`.AssociationProxy` when it is invoked
    in terms of a specific class or instance of a class, i.e. when it is
    used as a regular Python descriptor.

    When referring to the :class:`.AssociationProxy` as a normal Python
    descriptor, the :class:`.AssociationProxyInstance` is the object that
    actually serves the information.   Under normal circumstances, its presence
    is transparent::

        >>> User.keywords.scalar
        False

    In the special case that the :class:`.AssociationProxy` object is being
    accessed directly, in order to get an explicit handle to the
    :class:`.AssociationProxyInstance`, use the
    :meth:`.AssociationProxy.for_class` method::

        proxy_state = inspect(User).all_orm_descriptors["keywords"].for_class(User)

        # view if proxy object is scalar or not
        >>> proxy_state.scalar
        False

    .. versionadded:: 1.3

    """  # noqa

    collection_class: Optional[Type[Any]]
    parent: _AssociationProxyProtocol[_T]

    def __init__(
        self,
        parent: _AssociationProxyProtocol[_T],
        owning_class: Type[Any],
        target_class: Type[Any],
        value_attr: str,
    ):
        self.parent = parent
        self.key = parent.key
        self.owning_class = owning_class
        self.target_collection = parent.target_collection
        self.collection_class = None
        self.target_class = target_class
        self.value_attr = value_attr

    target_class: Type[Any]
    """The intermediary class handled by this
    :class:`.AssociationProxyInstance`.

    Intercepted append/set/assignment events will result
    in the generation of new instances of this class.

    """

    @classmethod
    def for_proxy(
        cls,
        parent: AssociationProxy[_T],
        owning_class: Type[Any],
        parent_instance: Any,
    ) -> AssociationProxyInstance[_T]:
        target_collection = parent.target_collection
        value_attr = parent.value_attr
        prop = cast(
            "orm.RelationshipProperty[_T]",
            orm.class_mapper(owning_class).get_property(target_collection),
        )

        # this was never asserted before but this should be made clear.
        if not isinstance(prop, orm.RelationshipProperty):
            raise NotImplementedError(
                "association proxy to a non-relationship "
                "intermediary is not supported"
            ) from None

        target_class = prop.mapper.class_

        try:
            target_assoc = cast(
                "AssociationProxyInstance[_T]",
                cls._cls_unwrap_target_assoc_proxy(target_class, value_attr),
            )
        except AttributeError:
            # the proxied attribute doesn't exist on the target class;
            # return an "ambiguous" instance that will work on a per-object
            # basis
            return AmbiguousAssociationProxyInstance(
                parent, owning_class, target_class, value_attr
            )
        except Exception as err:
            raise exc.InvalidRequestError(
                f"Association proxy received an unexpected error when "
                f"trying to retreive attribute "
                f'"{target_class.__name__}.{parent.value_attr}" from '
                f'class "{target_class.__name__}": {err}'
            ) from err
        else:
            return cls._construct_for_assoc(
                target_assoc, parent, owning_class, target_class, value_attr
            )

    @classmethod
    def _construct_for_assoc(
        cls,
        target_assoc: Optional[AssociationProxyInstance[_T]],
        parent: _AssociationProxyProtocol[_T],
        owning_class: Type[Any],
        target_class: Type[Any],
        value_attr: str,
    ) -> AssociationProxyInstance[_T]:
        if target_assoc is not None:
            return ObjectAssociationProxyInstance(
                parent, owning_class, target_class, value_attr
            )

        attr = getattr(target_class, value_attr)
        if not hasattr(attr, "_is_internal_proxy"):
            return AmbiguousAssociationProxyInstance(
                parent, owning_class, target_class, value_attr
            )
        is_object = attr._impl_uses_objects
        if is_object:
            return ObjectAssociationProxyInstance(
                parent, owning_class, target_class, value_attr
            )
        else:
            return ColumnAssociationProxyInstance(
                parent, owning_class, target_class, value_attr
            )

    def _get_property(self) -> MapperProperty[Any]:
        return orm.class_mapper(self.owning_class).get_property(
            self.target_collection
        )

    @property
    def _comparator(self) -> PropComparator[Any]:
        return getattr(  # type: ignore
            self.owning_class, self.target_collection
        ).comparator

    def __clause_element__(self) -> NoReturn:
        raise NotImplementedError(
            "The association proxy can't be used as a plain column "
            "expression; it only works inside of a comparison expression"
        )

    @classmethod
    def _cls_unwrap_target_assoc_proxy(
        cls, target_class: Any, value_attr: str
    ) -> Optional[AssociationProxyInstance[_T]]:
        attr = getattr(target_class, value_attr)
        assert not isinstance(attr, AssociationProxy)
        if isinstance(attr, AssociationProxyInstance):
            return attr
        return None

    @util.memoized_property
    def _unwrap_target_assoc_proxy(
        self,
    ) -> Optional[AssociationProxyInstance[_T]]:
        return self._cls_unwrap_target_assoc_proxy(
            self.target_class, self.value_attr
        )

    @property
    def remote_attr(self) -> SQLORMOperations[_T]:
        """The 'remote' class attribute referenced by this
        :class:`.AssociationProxyInstance`.

        .. seealso::

            :attr:`.AssociationProxyInstance.attr`

            :attr:`.AssociationProxyInstance.local_attr`

        """
        return cast(
            "SQLORMOperations[_T]", getattr(self.target_class, self.value_attr)
        )

    @property
    def local_attr(self) -> SQLORMOperations[Any]:
        """The 'local' class attribute referenced by this
        :class:`.AssociationProxyInstance`.

        .. seealso::

            :attr:`.AssociationProxyInstance.attr`

            :attr:`.AssociationProxyInstance.remote_attr`

        """
        return cast(
            "SQLORMOperations[Any]",
            getattr(self.owning_class, self.target_collection),
        )

    @property
    def attr(self) -> Tuple[SQLORMOperations[Any], SQLORMOperations[_T]]:
        """Return a tuple of ``(local_attr, remote_attr)``.

        This attribute was originally intended to facilitate using the
        :meth:`_query.Query.join` method to join across the two relationships
        at once, however this makes use of a deprecated calling style.

        To use :meth:`_sql.select.join` or :meth:`_orm.Query.join` with
        an association proxy, the current method is to make use of the
        :attr:`.AssociationProxyInstance.local_attr` and
        :attr:`.AssociationProxyInstance.remote_attr` attributes separately::

            stmt = (
                select(Parent)
                .join(Parent.proxied.local_attr)
                .join(Parent.proxied.remote_attr)
            )

        A future release may seek to provide a more succinct join pattern
        for association proxy attributes.

        .. seealso::

            :attr:`.AssociationProxyInstance.local_attr`

            :attr:`.AssociationProxyInstance.remote_attr`

        """
        return (self.local_attr, self.remote_attr)

    @util.memoized_property
    def scalar(self) -> bool:
        """Return ``True`` if this :class:`.AssociationProxyInstance`
        proxies a scalar relationship on the local side."""

        scalar = not self._get_property().uselist
        if scalar:
            self._initialize_scalar_accessors()
        return scalar

    @util.memoized_property
    def _value_is_scalar(self) -> bool:
        return (
            not self._get_property()
            .mapper.get_property(self.value_attr)
            .uselist
        )

    @property
    def _target_is_object(self) -> bool:
        raise NotImplementedError()

    _scalar_get: _GetterProtocol[_T]
    _scalar_set: _PlainSetterProtocol[_T]

    def _initialize_scalar_accessors(self) -> None:
        if self.parent.getset_factory:
            get, set_ = self.parent.getset_factory(None, self)
        else:
            get, set_ = self.parent._default_getset(None)
        self._scalar_get, self._scalar_set = get, cast(
            "_PlainSetterProtocol[_T]", set_
        )

    def _default_getset(
        self, collection_class: Any
    ) -> Tuple[_GetterProtocol[Any], _SetterProtocol]:
        attr = self.value_attr
        _getter = operator.attrgetter(attr)

        def getter(instance: Any) -> Optional[_T]:
            return _getter(instance) if instance is not None else None

        if collection_class is dict:

            def dict_setter(instance: Any, k: Any, value: _T) -> None:
                setattr(instance, attr, value)

            return getter, dict_setter
        else:

            def plain_setter(o: Any, v: _T) -> None:
                setattr(o, attr, v)

            return getter, plain_setter

    @util.ro_non_memoized_property
    def info(self) -> _InfoType:
        return self.parent.info

    @overload
    def get(self: _Self, obj: Literal[None]) -> _Self: ...

    @overload
    def get(self, obj: Any) -> _T: ...

    def get(
        self, obj: Any
    ) -> Union[Optional[_T], AssociationProxyInstance[_T]]:
        if obj is None:
            return self

        proxy: _T

        if self.scalar:
            target = getattr(obj, self.target_collection)
            return self._scalar_get(target)
        else:
            try:
                # If the owning instance is reborn (orm session resurrect,
                # etc.), refresh the proxy cache.
                creator_id, self_id, proxy = cast(
                    "Tuple[int, int, _T]", getattr(obj, self.key)
                )
            except AttributeError:
                pass
            else:
                if id(obj) == creator_id and id(self) == self_id:
                    assert self.collection_class is not None
                    return proxy

            self.collection_class, proxy = self._new(
                _lazy_collection(obj, self.target_collection)
            )
            setattr(obj, self.key, (id(obj), id(self), proxy))
            return proxy

    def set(self, obj: Any, values: _T) -> None:
        if self.scalar:
            creator = cast(
                "_PlainCreatorProtocol[_T]",
                (
                    self.parent.creator
                    if self.parent.creator
                    else self.target_class
                ),
            )
            target = getattr(obj, self.target_collection)
            if target is None:
                if (
                    values is None
                    and not self.parent.create_on_none_assignment
                ):
                    return
                setattr(obj, self.target_collection, creator(values))
            else:
                self._scalar_set(target, values)
                if values is None and self.parent.cascade_scalar_deletes:
                    setattr(obj, self.target_collection, None)
        else:
            proxy = self.get(obj)
            assert self.collection_class is not None
            if proxy is not values:
                proxy._bulk_replace(self, values)

    def delete(self, obj: Any) -> None:
        if self.owning_class is None:
            self._calc_owner(obj, None)

        if self.scalar:
            target = getattr(obj, self.target_collection)
            if target is not None:
                delattr(target, self.value_attr)
        delattr(obj, self.target_collection)

    def _new(
        self, lazy_collection: _LazyCollectionProtocol[_T]
    ) -> Tuple[Type[Any], _T]:
        creator = (
            self.parent.creator
            if self.parent.creator is not None
            else cast("_CreatorProtocol", self.target_class)
        )
        collection_class = util.duck_type_collection(lazy_collection())

        if collection_class is None:
            raise exc.InvalidRequestError(
                f"lazy collection factory did not return a "
                f"valid collection type, got {collection_class}"
            )
        if self.parent.proxy_factory:
            return (
                collection_class,
                self.parent.proxy_factory(
                    lazy_collection, creator, self.value_attr, self
                ),
            )

        if self.parent.getset_factory:
            getter, setter = self.parent.getset_factory(collection_class, self)
        else:
            getter, setter = self.parent._default_getset(collection_class)

        if collection_class is list:
            return (
                collection_class,
                cast(
                    _T,
                    _AssociationList(
                        lazy_collection, creator, getter, setter, self
                    ),
                ),
            )
        elif collection_class is dict:
            return (
                collection_class,
                cast(
                    _T,
                    _AssociationDict(
                        lazy_collection, creator, getter, setter, self
                    ),
                ),
            )
        elif collection_class is set:
            return (
                collection_class,
                cast(
                    _T,
                    _AssociationSet(
                        lazy_collection, creator, getter, setter, self
                    ),
                ),
            )
        else:
            raise exc.ArgumentError(
                "could not guess which interface to use for "
                'collection_class "%s" backing "%s"; specify a '
                "proxy_factory and proxy_bulk_set manually"
                % (self.collection_class, self.target_collection)
            )

    def _set(
        self, proxy: _AssociationCollection[Any], values: Iterable[Any]
    ) -> None:
        if self.parent.proxy_bulk_set:
            self.parent.proxy_bulk_set(proxy, values)
        elif self.collection_class is list:
            cast("_AssociationList[Any]", proxy).extend(values)
        elif self.collection_class is dict:
            cast("_AssociationDict[Any, Any]", proxy).update(values)
        elif self.collection_class is set:
            cast("_AssociationSet[Any]", proxy).update(values)
        else:
            raise exc.ArgumentError(
                "no proxy_bulk_set supplied for custom "
                "collection_class implementation"
            )

    def _inflate(self, proxy: _AssociationCollection[Any]) -> None:
        creator = (
            self.parent.creator
            and self.parent.creator
            or cast(_CreatorProtocol, self.target_class)
        )

        if self.parent.getset_factory:
            getter, setter = self.parent.getset_factory(
                self.collection_class, self
            )
        else:
            getter, setter = self.parent._default_getset(self.collection_class)

        proxy.creator = creator
        proxy.getter = getter
        proxy.setter = setter

    def _criterion_exists(
        self,
        criterion: Optional[_ColumnExpressionArgument[bool]] = None,
        **kwargs: Any,
    ) -> ColumnElement[bool]:
        is_has = kwargs.pop("is_has", None)

        target_assoc = self._unwrap_target_assoc_proxy
        if target_assoc is not None:
            inner = target_assoc._criterion_exists(
                criterion=criterion, **kwargs
            )
            return self._comparator._criterion_exists(inner)

        if self._target_is_object:
            attr = getattr(self.target_class, self.value_attr)
            value_expr = attr.comparator._criterion_exists(criterion, **kwargs)
        else:
            if kwargs:
                raise exc.ArgumentError(
                    "Can't apply keyword arguments to column-targeted "
                    "association proxy; use =="
                )
            elif is_has and criterion is not None:
                raise exc.ArgumentError(
                    "Non-empty has() not allowed for "
                    "column-targeted association proxy; use =="
                )

            value_expr = criterion

        return self._comparator._criterion_exists(value_expr)

    def any(
        self,
        criterion: Optional[_ColumnExpressionArgument[bool]] = None,
        **kwargs: Any,
    ) -> ColumnElement[bool]:
        """Produce a proxied 'any' expression using EXISTS.

        This expression will be a composed product
        using the :meth:`.Relationship.Comparator.any`
        and/or :meth:`.Relationship.Comparator.has`
        operators of the underlying proxied attributes.

        """
        if self._unwrap_target_assoc_proxy is None and (
            self.scalar
            and (not self._target_is_object or self._value_is_scalar)
        ):
            raise exc.InvalidRequestError(
                "'any()' not implemented for scalar attributes. Use has()."
            )
        return self._criterion_exists(
            criterion=criterion, is_has=False, **kwargs
        )

    def has(
        self,
        criterion: Optional[_ColumnExpressionArgument[bool]] = None,
        **kwargs: Any,
    ) -> ColumnElement[bool]:
        """Produce a proxied 'has' expression using EXISTS.

        This expression will be a composed product
        using the :meth:`.Relationship.Comparator.any`
        and/or :meth:`.Relationship.Comparator.has`
        operators of the underlying proxied attributes.

        """
        if self._unwrap_target_assoc_proxy is None and (
            not self.scalar
            or (self._target_is_object and not self._value_is_scalar)
        ):
            raise exc.InvalidRequestError(
                "'has()' not implemented for collections. Use any()."
            )
        return self._criterion_exists(
            criterion=criterion, is_has=True, **kwargs
        )

    def __repr__(self) -> str:
        return "%s(%r)" % (self.__class__.__name__, self.parent)


class AmbiguousAssociationProxyInstance(AssociationProxyInstance[_T]):
    """an :class:`.AssociationProxyInstance` where we cannot determine
    the type of target object.
    """

    _is_canonical = False

    def _ambiguous(self) -> NoReturn:
        raise AttributeError(
            "Association proxy %s.%s refers to an attribute '%s' that is not "
            "directly mapped on class %s; therefore this operation cannot "
            "proceed since we don't know what type of object is referred "
            "towards"
            % (
                self.owning_class.__name__,
                self.target_collection,
                self.value_attr,
                self.target_class,
            )
        )

    def get(self, obj: Any) -> Any:
        if obj is None:
            return self
        else:
            return super().get(obj)

    def __eq__(self, obj: object) -> NoReturn:
        self._ambiguous()

    def __ne__(self, obj: object) -> NoReturn:
        self._ambiguous()

    def any(
        self,
        criterion: Optional[_ColumnExpressionArgument[bool]] = None,
        **kwargs: Any,
    ) -> NoReturn:
        self._ambiguous()

    def has(
        self,
        criterion: Optional[_ColumnExpressionArgument[bool]] = None,
        **kwargs: Any,
    ) -> NoReturn:
        self._ambiguous()

    @util.memoized_property
    def _lookup_cache(self) -> Dict[Type[Any], AssociationProxyInstance[_T]]:
        # mapping of <subclass>->AssociationProxyInstance.
        # e.g. proxy is A-> A.b -> B -> B.b_attr, but B.b_attr doesn't exist;
        # only B1(B) and B2(B) have "b_attr", keys in here would be B1, B2
        return {}

    def _non_canonical_get_for_object(
        self, parent_instance: Any
    ) -> AssociationProxyInstance[_T]:
        if parent_instance is not None:
            actual_obj = getattr(parent_instance, self.target_collection)
            if actual_obj is not None:
                try:
                    insp = inspect(actual_obj)
                except exc.NoInspectionAvailable:
                    pass
                else:
                    mapper = insp.mapper
                    instance_class = mapper.class_
                    if instance_class not in self._lookup_cache:
                        self._populate_cache(instance_class, mapper)

                    try:
                        return self._lookup_cache[instance_class]
                    except KeyError:
                        pass

        # no object or ambiguous object given, so return "self", which
        # is a proxy with generally only instance-level functionality
        return self

    def _populate_cache(
        self, instance_class: Any, mapper: Mapper[Any]
    ) -> None:
        prop = orm.class_mapper(self.owning_class).get_property(
            self.target_collection
        )

        if mapper.isa(prop.mapper):
            target_class = instance_class
            try:
                target_assoc = self._cls_unwrap_target_assoc_proxy(
                    target_class, self.value_attr
                )
            except AttributeError:
                pass
            else:
                self._lookup_cache[instance_class] = self._construct_for_assoc(
                    cast("AssociationProxyInstance[_T]", target_assoc),
                    self.parent,
                    self.owning_class,
                    target_class,
                    self.value_attr,
                )


class ObjectAssociationProxyInstance(AssociationProxyInstance[_T]):
    """an :class:`.AssociationProxyInstance` that has an object as a target."""

    _target_is_object: bool = True
    _is_canonical = True

    def contains(self, other: Any, **kw: Any) -> ColumnElement[bool]:
        """Produce a proxied 'contains' expression using EXISTS.

        This expression will be a composed product
        using the :meth:`.Relationship.Comparator.any`,
        :meth:`.Relationship.Comparator.has`,
        and/or :meth:`.Relationship.Comparator.contains`
        operators of the underlying proxied attributes.
        """

        target_assoc = self._unwrap_target_assoc_proxy
        if target_assoc is not None:
            return self._comparator._criterion_exists(
                target_assoc.contains(other)
                if not target_assoc.scalar
                else target_assoc == other
            )
        elif (
            self._target_is_object
            and self.scalar
            and not self._value_is_scalar
        ):
            return self._comparator.has(
                getattr(self.target_class, self.value_attr).contains(other)
            )
        elif self._target_is_object and self.scalar and self._value_is_scalar:
            raise exc.InvalidRequestError(
                "contains() doesn't apply to a scalar object endpoint; use =="
            )
        else:
            return self._comparator._criterion_exists(
                **{self.value_attr: other}
            )

    def __eq__(self, obj: Any) -> ColumnElement[bool]:  # type: ignore[override]  # noqa: E501
        # note the has() here will fail for collections; eq_()
        # is only allowed with a scalar.
        if obj is None:
            return or_(
                self._comparator.has(**{self.value_attr: obj}),
                self._comparator == None,
            )
        else:
            return self._comparator.has(**{self.value_attr: obj})

    def __ne__(self, obj: Any) -> ColumnElement[bool]:  # type: ignore[override]  # noqa: E501
        # note the has() here will fail for collections; eq_()
        # is only allowed with a scalar.
        return self._comparator.has(
            getattr(self.target_class, self.value_attr) != obj
        )


class ColumnAssociationProxyInstance(AssociationProxyInstance[_T]):
    """an :class:`.AssociationProxyInstance` that has a database column as a
    target.
    """

    _target_is_object: bool = False
    _is_canonical = True

    def __eq__(self, other: Any) -> ColumnElement[bool]:  # type: ignore[override]  # noqa: E501
        # special case "is None" to check for no related row as well
        expr = self._criterion_exists(
            self.remote_attr.operate(operators.eq, other)
        )
        if other is None:
            return or_(expr, self._comparator == None)
        else:
            return expr

    def operate(
        self, op: operators.OperatorType, *other: Any, **kwargs: Any
    ) -> ColumnElement[Any]:
        return self._criterion_exists(
            self.remote_attr.operate(op, *other, **kwargs)
        )


class _lazy_collection(_LazyCollectionProtocol[_T]):
    def __init__(self, obj: Any, target: str):
        self.parent = obj
        self.target = target

    def __call__(
        self,
    ) -> Union[MutableSet[_T], MutableMapping[Any, _T], MutableSequence[_T]]:
        return getattr(self.parent, self.target)  # type: ignore[no-any-return]

    def __getstate__(self) -> Any:
        return {"obj": self.parent, "target": self.target}

    def __setstate__(self, state: Any) -> None:
        self.parent = state["obj"]
        self.target = state["target"]


_IT = TypeVar("_IT", bound="Any")
"""instance type - this is the type of object inside a collection.

this is not the same as the _T of AssociationProxy and
AssociationProxyInstance itself, which will often refer to the
collection[_IT] type.

"""


class _AssociationCollection(Generic[_IT]):
    getter: _GetterProtocol[_IT]
    """A function.  Given an associated object, return the 'value'."""

    creator: _CreatorProtocol
    """
    A function that creates new target entities.  Given one parameter:
    value.  This assertion is assumed::

    obj = creator(somevalue)
    assert getter(obj) == somevalue
    """

    parent: AssociationProxyInstance[_IT]
    setter: _SetterProtocol
    """A function.  Given an associated object and a value, store that
        value on the object.
    """

    lazy_collection: _LazyCollectionProtocol[_IT]
    """A callable returning a list-based collection of entities (usually an
          object attribute managed by a SQLAlchemy relationship())"""

    def __init__(
        self,
        lazy_collection: _LazyCollectionProtocol[_IT],
        creator: _CreatorProtocol,
        getter: _GetterProtocol[_IT],
        setter: _SetterProtocol,
        parent: AssociationProxyInstance[_IT],
    ):
        """Constructs an _AssociationCollection.

        This will always be a subclass of either _AssociationList,
        _AssociationSet, or _AssociationDict.

        """
        self.lazy_collection = lazy_collection
        self.creator = creator
        self.getter = getter
        self.setter = setter
        self.parent = parent

    if typing.TYPE_CHECKING:
        col: Collection[_IT]
    else:
        col = property(lambda self: self.lazy_collection())

    def __len__(self) -> int:
        return len(self.col)

    def __bool__(self) -> bool:
        return bool(self.col)

    def __getstate__(self) -> Any:
        return {"parent": self.parent, "lazy_collection": self.lazy_collection}

    def __setstate__(self, state: Any) -> None:
        self.parent = state["parent"]
        self.lazy_collection = state["lazy_collection"]
        self.parent._inflate(self)

    def clear(self) -> None:
        raise NotImplementedError()


class _AssociationSingleItem(_AssociationCollection[_T]):
    setter: _PlainSetterProtocol[_T]
    creator: _PlainCreatorProtocol[_T]

    def _create(self, value: _T) -> Any:
        return self.creator(value)

    def _get(self, object_: Any) -> _T:
        return self.getter(object_)

    def _bulk_replace(
        self, assoc_proxy: AssociationProxyInstance[Any], values: Iterable[_IT]
    ) -> None:
        self.clear()
        assoc_proxy._set(self, values)


class _AssociationList(_AssociationSingleItem[_T], MutableSequence[_T]):
    """Generic, converting, list-to-list proxy."""

    col: MutableSequence[_T]

    def _set(self, object_: Any, value: _T) -> None:
        self.setter(object_, value)

    @overload
    def __getitem__(self, index: int) -> _T: ...

    @overload
    def __getitem__(self, index: slice) -> MutableSequence[_T]: ...

    def __getitem__(
        self, index: Union[int, slice]
    ) -> Union[_T, MutableSequence[_T]]:
        if not isinstance(index, slice):
            return self._get(self.col[index])
        else:
            return [self._get(member) for member in self.col[index]]

    @overload
    def __setitem__(self, index: int, value: _T) -> None: ...

    @overload
    def __setitem__(self, index: slice, value: Iterable[_T]) -> None: ...

    def __setitem__(
        self, index: Union[int, slice], value: Union[_T, Iterable[_T]]
    ) -> None:
        if not isinstance(index, slice):
            self._set(self.col[index], cast("_T", value))
        else:
            if index.stop is None:
                stop = len(self)
            elif index.stop < 0:
                stop = len(self) + index.stop
            else:
                stop = index.stop
            step = index.step or 1

            start = index.start or 0
            rng = list(range(index.start or 0, stop, step))

            sized_value = list(value)

            if step == 1:
                for i in rng:
                    del self[start]
                i = start
                for item in sized_value:
                    self.insert(i, item)
                    i += 1
            else:
                if len(sized_value) != len(rng):
                    raise ValueError(
                        "attempt to assign sequence of size %s to "
                        "extended slice of size %s"
                        % (len(sized_value), len(rng))
                    )
                for i, item in zip(rng, value):
                    self._set(self.col[i], item)

    @overload
    def __delitem__(self, index: int) -> None: ...

    @overload
    def __delitem__(self, index: slice) -> None: ...

    def __delitem__(self, index: Union[slice, int]) -> None:
        del self.col[index]

    def __contains__(self, value: object) -> bool:
        for member in self.col:
            # testlib.pragma exempt:__eq__
            if self._get(member) == value:
                return True
        return False

    def __iter__(self) -> Iterator[_T]:
        """Iterate over proxied values.

        For the actual domain objects, iterate over .col instead or
        just use the underlying collection directly from its property
        on the parent.
        """

        for member in self.col:
            yield self._get(member)
        return

    def append(self, value: _T) -> None:
        col = self.col
        item = self._create(value)
        col.append(item)

    def count(self, value: Any) -> int:
        count = 0
        for v in self:
            if v == value:
                count += 1
        return count

    def extend(self, values: Iterable[_T]) -> None:
        for v in values:
            self.append(v)

    def insert(self, index: int, value: _T) -> None:
        self.col[index:index] = [self._create(value)]

    def pop(self, index: int = -1) -> _T:
        return self.getter(self.col.pop(index))

    def remove(self, value: _T) -> None:
        for i, val in enumerate(self):
            if val == value:
                del self.col[i]
                return
        raise ValueError("value not in list")

    def reverse(self) -> NoReturn:
        """Not supported, use reversed(mylist)"""

        raise NotImplementedError()

    def sort(self) -> NoReturn:
        """Not supported, use sorted(mylist)"""

        raise NotImplementedError()

    def clear(self) -> None:
        del self.col[0 : len(self.col)]

    def __eq__(self, other: object) -> bool:
        return list(self) == other

    def __ne__(self, other: object) -> bool:
        return list(self) != other

    def __lt__(self, other: List[_T]) -> bool:
        return list(self) < other

    def __le__(self, other: List[_T]) -> bool:
        return list(self) <= other

    def __gt__(self, other: List[_T]) -> bool:
        return list(self) > other

    def __ge__(self, other: List[_T]) -> bool:
        return list(self) >= other

    def __add__(self, other: List[_T]) -> List[_T]:
        try:
            other = list(other)
        except TypeError:
            return NotImplemented
        return list(self) + other

    def __radd__(self, other: List[_T]) -> List[_T]:
        try:
            other = list(other)
        except TypeError:
            return NotImplemented
        return other + list(self)

    def __mul__(self, n: SupportsIndex) -> List[_T]:
        if not isinstance(n, int):
            return NotImplemented
        return list(self) * n

    def __rmul__(self, n: SupportsIndex) -> List[_T]:
        if not isinstance(n, int):
            return NotImplemented
        return n * list(self)

    def __iadd__(self, iterable: Iterable[_T]) -> Self:
        self.extend(iterable)
        return self

    def __imul__(self, n: SupportsIndex) -> Self:
        # unlike a regular list *=, proxied __imul__ will generate unique
        # backing objects for each copy.  *= on proxied lists is a bit of
        # a stretch anyhow, and this interpretation of the __imul__ contract
        # is more plausibly useful than copying the backing objects.
        if not isinstance(n, int):
            raise NotImplementedError()
        if n == 0:
            self.clear()
        elif n > 1:
            self.extend(list(self) * (n - 1))
        return self

    if typing.TYPE_CHECKING:
        # TODO: no idea how to do this without separate "stub"
        def index(
            self, value: Any, start: int = ..., stop: int = ...
        ) -> int: ...

    else:

        def index(self, value: Any, *arg) -> int:
            ls = list(self)
            return ls.index(value, *arg)

    def copy(self) -> List[_T]:
        return list(self)

    def __repr__(self) -> str:
        return repr(list(self))

    def __hash__(self) -> NoReturn:
        raise TypeError("%s objects are unhashable" % type(self).__name__)

    if not typing.TYPE_CHECKING:
        for func_name, func in list(locals().items()):
            if (
                callable(func)
                and func.__name__ == func_name
                and not func.__doc__
                and hasattr(list, func_name)
            ):
                func.__doc__ = getattr(list, func_name).__doc__
        del func_name, func


class _AssociationDict(_AssociationCollection[_VT], MutableMapping[_KT, _VT]):
    """Generic, converting, dict-to-dict proxy."""

    setter: _DictSetterProtocol[_VT]
    creator: _KeyCreatorProtocol[_VT]
    col: MutableMapping[_KT, Optional[_VT]]

    def _create(self, key: _KT, value: Optional[_VT]) -> Any:
        return self.creator(key, value)

    def _get(self, object_: Any) -> _VT:
        return self.getter(object_)

    def _set(self, object_: Any, key: _KT, value: _VT) -> None:
        return self.setter(object_, key, value)

    def __getitem__(self, key: _KT) -> _VT:
        return self._get(self.col[key])

    def __setitem__(self, key: _KT, value: _VT) -> None:
        if key in self.col:
            self._set(self.col[key], key, value)
        else:
            self.col[key] = self._create(key, value)

    def __delitem__(self, key: _KT) -> None:
        del self.col[key]

    def __contains__(self, key: object) -> bool:
        return key in self.col

    def __iter__(self) -> Iterator[_KT]:
        return iter(self.col.keys())

    def clear(self) -> None:
        self.col.clear()

    def __eq__(self, other: object) -> bool:
        return dict(self) == other

    def __ne__(self, other: object) -> bool:
        return dict(self) != other

    def __repr__(self) -> str:
        return repr(dict(self))

    @overload
    def get(self, __key: _KT) -> Optional[_VT]: ...

    @overload
    def get(self, __key: _KT, default: Union[_VT, _T]) -> Union[_VT, _T]: ...

    def get(
        self, key: _KT, default: Optional[Union[_VT, _T]] = None
    ) -> Union[_VT, _T, None]:
        try:
            return self[key]
        except KeyError:
            return default

    def setdefault(self, key: _KT, default: Optional[_VT] = None) -> _VT:
        # TODO: again, no idea how to create an actual MutableMapping.
        # default must allow None, return type can't include None,
        # the stub explicitly allows for default of None with a cryptic message
        # "This overload should be allowed only if the value type is
        # compatible with None.".
        if key not in self.col:
            self.col[key] = self._create(key, default)
            return default  # type: ignore
        else:
            return self[key]

    def keys(self) -> KeysView[_KT]:
        return self.col.keys()

    def items(self) -> ItemsView[_KT, _VT]:
        return ItemsView(self)

    def values(self) -> ValuesView[_VT]:
        return ValuesView(self)

    @overload
    def pop(self, __key: _KT) -> _VT: ...

    @overload
    def pop(
        self, __key: _KT, default: Union[_VT, _T] = ...
    ) -> Union[_VT, _T]: ...

    def pop(self, __key: _KT, *arg: Any, **kw: Any) -> Union[_VT, _T]:
        member = self.col.pop(__key, *arg, **kw)
        return self._get(member)

    def popitem(self) -> Tuple[_KT, _VT]:
        item = self.col.popitem()
        return (item[0], self._get(item[1]))

    @overload
    def update(
        self, __m: SupportsKeysAndGetItem[_KT, _VT], **kwargs: _VT
    ) -> None: ...

    @overload
    def update(
        self, __m: Iterable[tuple[_KT, _VT]], **kwargs: _VT
    ) -> None: ...

    @overload
    def update(self, **kwargs: _VT) -> None: ...

    def update(self, *a: Any, **kw: Any) -> None:
        up: Dict[_KT, _VT] = {}
        up.update(*a, **kw)

        for key, value in up.items():
            self[key] = value

    def _bulk_replace(
        self,
        assoc_proxy: AssociationProxyInstance[Any],
        values: Mapping[_KT, _VT],
    ) -> None:
        existing = set(self)
        constants = existing.intersection(values or ())
        additions = set(values or ()).difference(constants)
        removals = existing.difference(constants)

        for key, member in values.items() or ():
            if key in additions:
                self[key] = member
            elif key in constants:
                self[key] = member

        for key in removals:
            del self[key]

    def copy(self) -> Dict[_KT, _VT]:
        return dict(self.items())

    def __hash__(self) -> NoReturn:
        raise TypeError("%s objects are unhashable" % type(self).__name__)

    if not typing.TYPE_CHECKING:
        for func_name, func in list(locals().items()):
            if (
                callable(func)
                and func.__name__ == func_name
                and not func.__doc__
                and hasattr(dict, func_name)
            ):
                func.__doc__ = getattr(dict, func_name).__doc__
        del func_name, func


class _AssociationSet(_AssociationSingleItem[_T], MutableSet[_T]):
    """Generic, converting, set-to-set proxy."""

    col: MutableSet[_T]

    def __len__(self) -> int:
        return len(self.col)

    def __bool__(self) -> bool:
        if self.col:
            return True
        else:
            return False

    def __contains__(self, __o: object) -> bool:
        for member in self.col:
            if self._get(member) == __o:
                return True
        return False

    def __iter__(self) -> Iterator[_T]:
        """Iterate over proxied values.

        For the actual domain objects, iterate over .col instead or just use
        the underlying collection directly from its property on the parent.

        """
        for member in self.col:
            yield self._get(member)
        return

    def add(self, __element: _T) -> None:
        if __element not in self:
            self.col.add(self._create(__element))

    # for discard and remove, choosing a more expensive check strategy rather
    # than call self.creator()
    def discard(self, __element: _T) -> None:
        for member in self.col:
            if self._get(member) == __element:
                self.col.discard(member)
                break

    def remove(self, __element: _T) -> None:
        for member in self.col:
            if self._get(member) == __element:
                self.col.discard(member)
                return
        raise KeyError(__element)

    def pop(self) -> _T:
        if not self.col:
            raise KeyError("pop from an empty set")
        member = self.col.pop()
        return self._get(member)

    def update(self, *s: Iterable[_T]) -> None:
        for iterable in s:
            for value in iterable:
                self.add(value)

    def _bulk_replace(self, assoc_proxy: Any, values: Iterable[_T]) -> None:
        existing = set(self)
        constants = existing.intersection(values or ())
        additions = set(values or ()).difference(constants)
        removals = existing.difference(constants)

        appender = self.add
        remover = self.remove

        for member in values or ():
            if member in additions:
                appender(member)
            elif member in constants:
                appender(member)

        for member in removals:
            remover(member)

    def __ior__(  # type: ignore
        self, other: AbstractSet[_S]
    ) -> MutableSet[Union[_T, _S]]:
        if not collections._set_binops_check_strict(self, other):
            raise NotImplementedError()
        for value in other:
            self.add(value)
        return self

    def _set(self) -> Set[_T]:
        return set(iter(self))

    def union(self, *s: Iterable[_S]) -> MutableSet[Union[_T, _S]]:
        return set(self).union(*s)

    def __or__(self, __s: AbstractSet[_S]) -> MutableSet[Union[_T, _S]]:
        return self.union(__s)

    def difference(self, *s: Iterable[Any]) -> MutableSet[_T]:
        return set(self).difference(*s)

    def __sub__(self, s: AbstractSet[Any]) -> MutableSet[_T]:
        return self.difference(s)

    def difference_update(self, *s: Iterable[Any]) -> None:
        for other in s:
            for value in other:
                self.discard(value)

    def __isub__(self, s: AbstractSet[Any]) -> Self:
        if not collections._set_binops_check_strict(self, s):
            raise NotImplementedError()
        for value in s:
            self.discard(value)
        return self

    def intersection(self, *s: Iterable[Any]) -> MutableSet[_T]:
        return set(self).intersection(*s)

    def __and__(self, s: AbstractSet[Any]) -> MutableSet[_T]:
        return self.intersection(s)

    def intersection_update(self, *s: Iterable[Any]) -> None:
        for other in s:
            want, have = self.intersection(other), set(self)

            remove, add = have - want, want - have

            for value in remove:
                self.remove(value)
            for value in add:
                self.add(value)

    def __iand__(self, s: AbstractSet[Any]) -> Self:
        if not collections._set_binops_check_strict(self, s):
            raise NotImplementedError()
        want = self.intersection(s)
        have: Set[_T] = set(self)

        remove, add = have - want, want - have

        for value in remove:
            self.remove(value)
        for value in add:
            self.add(value)
        return self

    def symmetric_difference(self, __s: Iterable[_T]) -> MutableSet[_T]:
        return set(self).symmetric_difference(__s)

    def __xor__(self, s: AbstractSet[_S]) -> MutableSet[Union[_T, _S]]:
        return self.symmetric_difference(s)

    def symmetric_difference_update(self, other: Iterable[Any]) -> None:
        want, have = self.symmetric_difference(other), set(self)

        remove, add = have - want, want - have

        for value in remove:
            self.remove(value)
        for value in add:
            self.add(value)

    def __ixor__(self, other: AbstractSet[_S]) -> MutableSet[Union[_T, _S]]:  # type: ignore  # noqa: E501
        if not collections._set_binops_check_strict(self, other):
            raise NotImplementedError()

        self.symmetric_difference_update(other)
        return self

    def issubset(self, __s: Iterable[Any]) -> bool:
        return set(self).issubset(__s)

    def issuperset(self, __s: Iterable[Any]) -> bool:
        return set(self).issuperset(__s)

    def clear(self) -> None:
        self.col.clear()

    def copy(self) -> AbstractSet[_T]:
        return set(self)

    def __eq__(self, other: object) -> bool:
        return set(self) == other

    def __ne__(self, other: object) -> bool:
        return set(self) != other

    def __lt__(self, other: AbstractSet[Any]) -> bool:
        return set(self) < other

    def __le__(self, other: AbstractSet[Any]) -> bool:
        return set(self) <= other

    def __gt__(self, other: AbstractSet[Any]) -> bool:
        return set(self) > other

    def __ge__(self, other: AbstractSet[Any]) -> bool:
        return set(self) >= other

    def __repr__(self) -> str:
        return repr(set(self))

    def __hash__(self) -> NoReturn:
        raise TypeError("%s objects are unhashable" % type(self).__name__)

    if not typing.TYPE_CHECKING:
        for func_name, func in list(locals().items()):
            if (
                callable(func)
                and func.__name__ == func_name
                and not func.__doc__
                and hasattr(set, func_name)
            ):
                func.__doc__ = getattr(set, func_name).__doc__
        del func_name, func
