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diff --git a/lib/sqlalchemy/event/base.py b/lib/sqlalchemy/event/base.py
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+# event/base.py
+# Copyright (C) 2005-2022 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
+
+"""Base implementation classes.
+
+The public-facing ``Events`` serves as the base class for an event interface;
+its public attributes represent different kinds of events.   These attributes
+are mirrored onto a ``_Dispatch`` class, which serves as a container for
+collections of listener functions.   These collections are represented both
+at the class level of a particular ``_Dispatch`` class as well as within
+instances of ``_Dispatch``.
+
+"""
+from __future__ import absolute_import
+
+import weakref
+
+from .attr import _ClsLevelDispatch
+from .attr import _EmptyListener
+from .attr import _JoinedListener
+from .. import util
+
+
+_registrars = util.defaultdict(list)
+
+
+def _is_event_name(name):
+    # _sa_event prefix is special to support internal-only event names.
+    # most event names are just plain method names that aren't
+    # underscored.
+
+    return (
+        not name.startswith("_") and name != "dispatch"
+    ) or name.startswith("_sa_event")
+
+
+class _UnpickleDispatch(object):
+    """Serializable callable that re-generates an instance of
+    :class:`_Dispatch` given a particular :class:`.Events` subclass.
+
+    """
+
+    def __call__(self, _instance_cls):
+        for cls in _instance_cls.__mro__:
+            if "dispatch" in cls.__dict__:
+                return cls.__dict__["dispatch"].dispatch._for_class(
+                    _instance_cls
+                )
+        else:
+            raise AttributeError("No class with a 'dispatch' member present.")
+
+
+class _Dispatch(object):
+    """Mirror the event listening definitions of an Events class with
+    listener collections.
+
+    Classes which define a "dispatch" member will return a
+    non-instantiated :class:`._Dispatch` subclass when the member
+    is accessed at the class level.  When the "dispatch" member is
+    accessed at the instance level of its owner, an instance
+    of the :class:`._Dispatch` class is returned.
+
+    A :class:`._Dispatch` class is generated for each :class:`.Events`
+    class defined, by the :func:`._create_dispatcher_class` function.
+    The original :class:`.Events` classes remain untouched.
+    This decouples the construction of :class:`.Events` subclasses from
+    the implementation used by the event internals, and allows
+    inspecting tools like Sphinx to work in an unsurprising
+    way against the public API.
+
+    """
+
+    # In one ORM edge case, an attribute is added to _Dispatch,
+    # so __dict__ is used in just that case and potentially others.
+    __slots__ = "_parent", "_instance_cls", "__dict__", "_empty_listeners"
+
+    _empty_listener_reg = weakref.WeakKeyDictionary()
+
+    def __init__(self, parent, instance_cls=None):
+        self._parent = parent
+        self._instance_cls = instance_cls
+
+        if instance_cls:
+            try:
+                self._empty_listeners = self._empty_listener_reg[instance_cls]
+            except KeyError:
+                self._empty_listeners = self._empty_listener_reg[
+                    instance_cls
+                ] = {
+                    ls.name: _EmptyListener(ls, instance_cls)
+                    for ls in parent._event_descriptors
+                }
+        else:
+            self._empty_listeners = {}
+
+    def __getattr__(self, name):
+        # Assign EmptyListeners as attributes on demand
+        # to reduce startup time for new dispatch objects.
+        try:
+            ls = self._empty_listeners[name]
+        except KeyError:
+            raise AttributeError(name)
+        else:
+            setattr(self, ls.name, ls)
+            return ls
+
+    @property
+    def _event_descriptors(self):
+        for k in self._event_names:
+            # Yield _ClsLevelDispatch related
+            # to relevant event name.
+            yield getattr(self, k)
+
+    @property
+    def _listen(self):
+        return self._events._listen
+
+    def _for_class(self, instance_cls):
+        return self.__class__(self, instance_cls)
+
+    def _for_instance(self, instance):
+        instance_cls = instance.__class__
+        return self._for_class(instance_cls)
+
+    def _join(self, other):
+        """Create a 'join' of this :class:`._Dispatch` and another.
+
+        This new dispatcher will dispatch events to both
+        :class:`._Dispatch` objects.
+
+        """
+        if "_joined_dispatch_cls" not in self.__class__.__dict__:
+            cls = type(
+                "Joined%s" % self.__class__.__name__,
+                (_JoinedDispatcher,),
+                {"__slots__": self._event_names},
+            )
+
+            self.__class__._joined_dispatch_cls = cls
+        return self._joined_dispatch_cls(self, other)
+
+    def __reduce__(self):
+        return _UnpickleDispatch(), (self._instance_cls,)
+
+    def _update(self, other, only_propagate=True):
+        """Populate from the listeners in another :class:`_Dispatch`
+        object."""
+        for ls in other._event_descriptors:
+            if isinstance(ls, _EmptyListener):
+                continue
+            getattr(self, ls.name).for_modify(self)._update(
+                ls, only_propagate=only_propagate
+            )
+
+    def _clear(self):
+        for ls in self._event_descriptors:
+            ls.for_modify(self).clear()
+
+
+class _EventMeta(type):
+    """Intercept new Event subclasses and create
+    associated _Dispatch classes."""
+
+    def __init__(cls, classname, bases, dict_):
+        _create_dispatcher_class(cls, classname, bases, dict_)
+        type.__init__(cls, classname, bases, dict_)
+
+
+def _create_dispatcher_class(cls, classname, bases, dict_):
+    """Create a :class:`._Dispatch` class corresponding to an
+    :class:`.Events` class."""
+
+    # there's all kinds of ways to do this,
+    # i.e. make a Dispatch class that shares the '_listen' method
+    # of the Event class, this is the straight monkeypatch.
+    if hasattr(cls, "dispatch"):
+        dispatch_base = cls.dispatch.__class__
+    else:
+        dispatch_base = _Dispatch
+
+    event_names = [k for k in dict_ if _is_event_name(k)]
+    dispatch_cls = type(
+        "%sDispatch" % classname, (dispatch_base,), {"__slots__": event_names}
+    )
+
+    dispatch_cls._event_names = event_names
+
+    dispatch_inst = cls._set_dispatch(cls, dispatch_cls)
+    for k in dispatch_cls._event_names:
+        setattr(dispatch_inst, k, _ClsLevelDispatch(cls, dict_[k]))
+        _registrars[k].append(cls)
+
+    for super_ in dispatch_cls.__bases__:
+        if issubclass(super_, _Dispatch) and super_ is not _Dispatch:
+            for ls in super_._events.dispatch._event_descriptors:
+                setattr(dispatch_inst, ls.name, ls)
+                dispatch_cls._event_names.append(ls.name)
+
+    if getattr(cls, "_dispatch_target", None):
+        the_cls = cls._dispatch_target
+        if (
+            hasattr(the_cls, "__slots__")
+            and "_slots_dispatch" in the_cls.__slots__
+        ):
+            cls._dispatch_target.dispatch = slots_dispatcher(cls)
+        else:
+            cls._dispatch_target.dispatch = dispatcher(cls)
+
+
+def _remove_dispatcher(cls):
+    for k in cls.dispatch._event_names:
+        _registrars[k].remove(cls)
+        if not _registrars[k]:
+            del _registrars[k]
+
+
+class Events(util.with_metaclass(_EventMeta, object)):
+    """Define event listening functions for a particular target type."""
+
+    @staticmethod
+    def _set_dispatch(cls, dispatch_cls):
+        # This allows an Events subclass to define additional utility
+        # methods made available to the target via
+        # "self.dispatch._events.<utilitymethod>"
+        # @staticmethod to allow easy "super" calls while in a metaclass
+        # constructor.
+        cls.dispatch = dispatch_cls(None)
+        dispatch_cls._events = cls
+        return cls.dispatch
+
+    @classmethod
+    def _accept_with(cls, target):
+        def dispatch_is(*types):
+            return all(isinstance(target.dispatch, t) for t in types)
+
+        def dispatch_parent_is(t):
+            return isinstance(target.dispatch.parent, t)
+
+        # Mapper, ClassManager, Session override this to
+        # also accept classes, scoped_sessions, sessionmakers, etc.
+        if hasattr(target, "dispatch"):
+            if (
+                dispatch_is(cls.dispatch.__class__)
+                or dispatch_is(type, cls.dispatch.__class__)
+                or (
+                    dispatch_is(_JoinedDispatcher)
+                    and dispatch_parent_is(cls.dispatch.__class__)
+                )
+            ):
+                return target
+
+    @classmethod
+    def _listen(
+        cls,
+        event_key,
+        propagate=False,
+        insert=False,
+        named=False,
+        asyncio=False,
+    ):
+        event_key.base_listen(
+            propagate=propagate, insert=insert, named=named, asyncio=asyncio
+        )
+
+    @classmethod
+    def _remove(cls, event_key):
+        event_key.remove()
+
+    @classmethod
+    def _clear(cls):
+        cls.dispatch._clear()
+
+
+class _JoinedDispatcher(object):
+    """Represent a connection between two _Dispatch objects."""
+
+    __slots__ = "local", "parent", "_instance_cls"
+
+    def __init__(self, local, parent):
+        self.local = local
+        self.parent = parent
+        self._instance_cls = self.local._instance_cls
+
+    def __getattr__(self, name):
+        # Assign _JoinedListeners as attributes on demand
+        # to reduce startup time for new dispatch objects.
+        ls = getattr(self.local, name)
+        jl = _JoinedListener(self.parent, ls.name, ls)
+        setattr(self, ls.name, jl)
+        return jl
+
+    @property
+    def _listen(self):
+        return self.parent._listen
+
+    @property
+    def _events(self):
+        return self.parent._events
+
+
+class dispatcher(object):
+    """Descriptor used by target classes to
+    deliver the _Dispatch class at the class level
+    and produce new _Dispatch instances for target
+    instances.
+
+    """
+
+    def __init__(self, events):
+        self.dispatch = events.dispatch
+        self.events = events
+
+    def __get__(self, obj, cls):
+        if obj is None:
+            return self.dispatch
+
+        disp = self.dispatch._for_instance(obj)
+        try:
+            obj.__dict__["dispatch"] = disp
+        except AttributeError as ae:
+            util.raise_(
+                TypeError(
+                    "target %r doesn't have __dict__, should it be "
+                    "defining _slots_dispatch?" % (obj,)
+                ),
+                replace_context=ae,
+            )
+        return disp
+
+
+class slots_dispatcher(dispatcher):
+    def __get__(self, obj, cls):
+        if obj is None:
+            return self.dispatch
+
+        if hasattr(obj, "_slots_dispatch"):
+            return obj._slots_dispatch
+
+        disp = self.dispatch._for_instance(obj)
+        obj._slots_dispatch = disp
+        return disp