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diff --git a/lib/sqlalchemy/event/attr.py b/lib/sqlalchemy/event/attr.py
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+# event/attr.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
+
+"""Attribute implementation for _Dispatch classes.
+
+The various listener targets for a particular event class are represented
+as attributes, which refer to collections of listeners to be fired off.
+These collections can exist at the class level as well as at the instance
+level.  An event is fired off using code like this::
+
+    some_object.dispatch.first_connect(arg1, arg2)
+
+Above, ``some_object.dispatch`` would be an instance of ``_Dispatch`` and
+``first_connect`` is typically an instance of ``_ListenerCollection``
+if event listeners are present, or ``_EmptyListener`` if none are present.
+
+The attribute mechanics here spend effort trying to ensure listener functions
+are available with a minimum of function call overhead, that unnecessary
+objects aren't created (i.e. many empty per-instance listener collections),
+as well as that everything is garbage collectable when owning references are
+lost.  Other features such as "propagation" of listener functions across
+many ``_Dispatch`` instances, "joining" of multiple ``_Dispatch`` instances,
+as well as support for subclass propagation (e.g. events assigned to
+``Pool`` vs. ``QueuePool``) are all implemented here.
+
+"""
+
+from __future__ import absolute_import
+from __future__ import with_statement
+
+import collections
+from itertools import chain
+import weakref
+
+from . import legacy
+from . import registry
+from .. import exc
+from .. import util
+from ..util import threading
+from ..util.concurrency import AsyncAdaptedLock
+
+
+class RefCollection(util.MemoizedSlots):
+    __slots__ = ("ref",)
+
+    def _memoized_attr_ref(self):
+        return weakref.ref(self, registry._collection_gced)
+
+
+class _empty_collection(object):
+    def append(self, element):
+        pass
+
+    def extend(self, other):
+        pass
+
+    def remove(self, element):
+        pass
+
+    def __iter__(self):
+        return iter([])
+
+    def clear(self):
+        pass
+
+
+class _ClsLevelDispatch(RefCollection):
+    """Class-level events on :class:`._Dispatch` classes."""
+
+    __slots__ = (
+        "clsname",
+        "name",
+        "arg_names",
+        "has_kw",
+        "legacy_signatures",
+        "_clslevel",
+        "__weakref__",
+    )
+
+    def __init__(self, parent_dispatch_cls, fn):
+        self.name = fn.__name__
+        self.clsname = parent_dispatch_cls.__name__
+        argspec = util.inspect_getfullargspec(fn)
+        self.arg_names = argspec.args[1:]
+        self.has_kw = bool(argspec.varkw)
+        self.legacy_signatures = list(
+            reversed(
+                sorted(
+                    getattr(fn, "_legacy_signatures", []), key=lambda s: s[0]
+                )
+            )
+        )
+        fn.__doc__ = legacy._augment_fn_docs(self, parent_dispatch_cls, fn)
+
+        self._clslevel = weakref.WeakKeyDictionary()
+
+    def _adjust_fn_spec(self, fn, named):
+        if named:
+            fn = self._wrap_fn_for_kw(fn)
+        if self.legacy_signatures:
+            try:
+                argspec = util.get_callable_argspec(fn, no_self=True)
+            except TypeError:
+                pass
+            else:
+                fn = legacy._wrap_fn_for_legacy(self, fn, argspec)
+        return fn
+
+    def _wrap_fn_for_kw(self, fn):
+        def wrap_kw(*args, **kw):
+            argdict = dict(zip(self.arg_names, args))
+            argdict.update(kw)
+            return fn(**argdict)
+
+        return wrap_kw
+
+    def insert(self, event_key, propagate):
+        target = event_key.dispatch_target
+        assert isinstance(
+            target, type
+        ), "Class-level Event targets must be classes."
+        if not getattr(target, "_sa_propagate_class_events", True):
+            raise exc.InvalidRequestError(
+                "Can't assign an event directly to the %s class" % target
+            )
+
+        for cls in util.walk_subclasses(target):
+            if cls is not target and cls not in self._clslevel:
+                self.update_subclass(cls)
+            else:
+                if cls not in self._clslevel:
+                    self._assign_cls_collection(cls)
+                self._clslevel[cls].appendleft(event_key._listen_fn)
+        registry._stored_in_collection(event_key, self)
+
+    def append(self, event_key, propagate):
+        target = event_key.dispatch_target
+        assert isinstance(
+            target, type
+        ), "Class-level Event targets must be classes."
+        if not getattr(target, "_sa_propagate_class_events", True):
+            raise exc.InvalidRequestError(
+                "Can't assign an event directly to the %s class" % target
+            )
+        for cls in util.walk_subclasses(target):
+            if cls is not target and cls not in self._clslevel:
+                self.update_subclass(cls)
+            else:
+                if cls not in self._clslevel:
+                    self._assign_cls_collection(cls)
+                self._clslevel[cls].append(event_key._listen_fn)
+        registry._stored_in_collection(event_key, self)
+
+    def _assign_cls_collection(self, target):
+        if getattr(target, "_sa_propagate_class_events", True):
+            self._clslevel[target] = collections.deque()
+        else:
+            self._clslevel[target] = _empty_collection()
+
+    def update_subclass(self, target):
+        if target not in self._clslevel:
+            self._assign_cls_collection(target)
+        clslevel = self._clslevel[target]
+        for cls in target.__mro__[1:]:
+            if cls in self._clslevel:
+                clslevel.extend(
+                    [fn for fn in self._clslevel[cls] if fn not in clslevel]
+                )
+
+    def remove(self, event_key):
+        target = event_key.dispatch_target
+        for cls in util.walk_subclasses(target):
+            if cls in self._clslevel:
+                self._clslevel[cls].remove(event_key._listen_fn)
+        registry._removed_from_collection(event_key, self)
+
+    def clear(self):
+        """Clear all class level listeners"""
+
+        to_clear = set()
+        for dispatcher in self._clslevel.values():
+            to_clear.update(dispatcher)
+            dispatcher.clear()
+        registry._clear(self, to_clear)
+
+    def for_modify(self, obj):
+        """Return an event collection which can be modified.
+
+        For _ClsLevelDispatch at the class level of
+        a dispatcher, this returns self.
+
+        """
+        return self
+
+
+class _InstanceLevelDispatch(RefCollection):
+    __slots__ = ()
+
+    def _adjust_fn_spec(self, fn, named):
+        return self.parent._adjust_fn_spec(fn, named)
+
+
+class _EmptyListener(_InstanceLevelDispatch):
+    """Serves as a proxy interface to the events
+    served by a _ClsLevelDispatch, when there are no
+    instance-level events present.
+
+    Is replaced by _ListenerCollection when instance-level
+    events are added.
+
+    """
+
+    propagate = frozenset()
+    listeners = ()
+
+    __slots__ = "parent", "parent_listeners", "name"
+
+    def __init__(self, parent, target_cls):
+        if target_cls not in parent._clslevel:
+            parent.update_subclass(target_cls)
+        self.parent = parent  # _ClsLevelDispatch
+        self.parent_listeners = parent._clslevel[target_cls]
+        self.name = parent.name
+
+    def for_modify(self, obj):
+        """Return an event collection which can be modified.
+
+        For _EmptyListener at the instance level of
+        a dispatcher, this generates a new
+        _ListenerCollection, applies it to the instance,
+        and returns it.
+
+        """
+        result = _ListenerCollection(self.parent, obj._instance_cls)
+        if getattr(obj, self.name) is self:
+            setattr(obj, self.name, result)
+        else:
+            assert isinstance(getattr(obj, self.name), _JoinedListener)
+        return result
+
+    def _needs_modify(self, *args, **kw):
+        raise NotImplementedError("need to call for_modify()")
+
+    exec_once = (
+        exec_once_unless_exception
+    ) = insert = append = remove = clear = _needs_modify
+
+    def __call__(self, *args, **kw):
+        """Execute this event."""
+
+        for fn in self.parent_listeners:
+            fn(*args, **kw)
+
+    def __len__(self):
+        return len(self.parent_listeners)
+
+    def __iter__(self):
+        return iter(self.parent_listeners)
+
+    def __bool__(self):
+        return bool(self.parent_listeners)
+
+    __nonzero__ = __bool__
+
+
+class _CompoundListener(_InstanceLevelDispatch):
+    __slots__ = "_exec_once_mutex", "_exec_once", "_exec_w_sync_once"
+
+    def _set_asyncio(self):
+        self._exec_once_mutex = AsyncAdaptedLock()
+
+    def _memoized_attr__exec_once_mutex(self):
+        return threading.Lock()
+
+    def _exec_once_impl(self, retry_on_exception, *args, **kw):
+        with self._exec_once_mutex:
+            if not self._exec_once:
+                try:
+                    self(*args, **kw)
+                    exception = False
+                except:
+                    exception = True
+                    raise
+                finally:
+                    if not exception or not retry_on_exception:
+                        self._exec_once = True
+
+    def exec_once(self, *args, **kw):
+        """Execute this event, but only if it has not been
+        executed already for this collection."""
+
+        if not self._exec_once:
+            self._exec_once_impl(False, *args, **kw)
+
+    def exec_once_unless_exception(self, *args, **kw):
+        """Execute this event, but only if it has not been
+        executed already for this collection, or was called
+        by a previous exec_once_unless_exception call and
+        raised an exception.
+
+        If exec_once was already called, then this method will never run
+        the callable regardless of whether it raised or not.
+
+        .. versionadded:: 1.3.8
+
+        """
+        if not self._exec_once:
+            self._exec_once_impl(True, *args, **kw)
+
+    def _exec_w_sync_on_first_run(self, *args, **kw):
+        """Execute this event, and use a mutex if it has not been
+        executed already for this collection, or was called
+        by a previous _exec_w_sync_on_first_run call and
+        raised an exception.
+
+        If _exec_w_sync_on_first_run was already called and didn't raise an
+        exception, then a mutex is not used.
+
+        .. versionadded:: 1.4.11
+
+        """
+        if not self._exec_w_sync_once:
+            with self._exec_once_mutex:
+                try:
+                    self(*args, **kw)
+                except:
+                    raise
+                else:
+                    self._exec_w_sync_once = True
+        else:
+            self(*args, **kw)
+
+    def __call__(self, *args, **kw):
+        """Execute this event."""
+
+        for fn in self.parent_listeners:
+            fn(*args, **kw)
+        for fn in self.listeners:
+            fn(*args, **kw)
+
+    def __len__(self):
+        return len(self.parent_listeners) + len(self.listeners)
+
+    def __iter__(self):
+        return chain(self.parent_listeners, self.listeners)
+
+    def __bool__(self):
+        return bool(self.listeners or self.parent_listeners)
+
+    __nonzero__ = __bool__
+
+
+class _ListenerCollection(_CompoundListener):
+    """Instance-level attributes on instances of :class:`._Dispatch`.
+
+    Represents a collection of listeners.
+
+    As of 0.7.9, _ListenerCollection is only first
+    created via the _EmptyListener.for_modify() method.
+
+    """
+
+    __slots__ = (
+        "parent_listeners",
+        "parent",
+        "name",
+        "listeners",
+        "propagate",
+        "__weakref__",
+    )
+
+    def __init__(self, parent, target_cls):
+        if target_cls not in parent._clslevel:
+            parent.update_subclass(target_cls)
+        self._exec_once = False
+        self._exec_w_sync_once = False
+        self.parent_listeners = parent._clslevel[target_cls]
+        self.parent = parent
+        self.name = parent.name
+        self.listeners = collections.deque()
+        self.propagate = set()
+
+    def for_modify(self, obj):
+        """Return an event collection which can be modified.
+
+        For _ListenerCollection at the instance level of
+        a dispatcher, this returns self.
+
+        """
+        return self
+
+    def _update(self, other, only_propagate=True):
+        """Populate from the listeners in another :class:`_Dispatch`
+        object."""
+
+        existing_listeners = self.listeners
+        existing_listener_set = set(existing_listeners)
+        self.propagate.update(other.propagate)
+        other_listeners = [
+            l
+            for l in other.listeners
+            if l not in existing_listener_set
+            and not only_propagate
+            or l in self.propagate
+        ]
+
+        existing_listeners.extend(other_listeners)
+
+        to_associate = other.propagate.union(other_listeners)
+        registry._stored_in_collection_multi(self, other, to_associate)
+
+    def insert(self, event_key, propagate):
+        if event_key.prepend_to_list(self, self.listeners):
+            if propagate:
+                self.propagate.add(event_key._listen_fn)
+
+    def append(self, event_key, propagate):
+        if event_key.append_to_list(self, self.listeners):
+            if propagate:
+                self.propagate.add(event_key._listen_fn)
+
+    def remove(self, event_key):
+        self.listeners.remove(event_key._listen_fn)
+        self.propagate.discard(event_key._listen_fn)
+        registry._removed_from_collection(event_key, self)
+
+    def clear(self):
+        registry._clear(self, self.listeners)
+        self.propagate.clear()
+        self.listeners.clear()
+
+
+class _JoinedListener(_CompoundListener):
+    __slots__ = "parent", "name", "local", "parent_listeners"
+
+    def __init__(self, parent, name, local):
+        self._exec_once = False
+        self.parent = parent
+        self.name = name
+        self.local = local
+        self.parent_listeners = self.local
+
+    @property
+    def listeners(self):
+        return getattr(self.parent, self.name)
+
+    def _adjust_fn_spec(self, fn, named):
+        return self.local._adjust_fn_spec(fn, named)
+
+    def for_modify(self, obj):
+        self.local = self.parent_listeners = self.local.for_modify(obj)
+        return self
+
+    def insert(self, event_key, propagate):
+        self.local.insert(event_key, propagate)
+
+    def append(self, event_key, propagate):
+        self.local.append(event_key, propagate)
+
+    def remove(self, event_key):
+        self.local.remove(event_key)
+
+    def clear(self):
+        raise NotImplementedError()