generalize scoped_session proxying and apply to asyncio elements

Reworked the proxy creation used by scoped_session() to be
based on fully copied code with augmented docstrings and
moved it into langhelpers.  asyncio session, engine,
connection can now take
advantage of it so that all non-async methods are availble.

Overall implementation of most important accessors / methods
on AsyncConnection, etc. , including awaitable versions
of invalidate, execution_options, etc.

In order to support an event dispatcher on the async
classes while still allowing them to hold __slots__,
make some adjustments to the event system to allow
that to be present, at least rudimentally.

Fixes: #5628
Change-Id: I5eb6929fc1e4fdac99e4b767dcfd49672d56e2b2

1 files changed, 33 insertions, 2 deletions

diff --git a/lib/sqlalchemy/event/base.py b/lib/sqlalchemy/event/base.py
index daa6f9aea..1ba88f3d2 100644
--- a/lib/sqlalchemy/event/base.py
+++ b/lib/sqlalchemy/event/base.py
@@ -195,7 +195,14 @@ def _create_dispatcher_class(cls, classname, bases, dict_):
                 dispatch_cls._event_names.append(ls.name)
 
     if getattr(cls, "_dispatch_target", None):
-        cls._dispatch_target.dispatch = dispatcher(cls)
+        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):
@@ -304,5 +311,29 @@ class dispatcher(object):
     def __get__(self, obj, cls):
         if obj is None:
             return self.dispatch
-        obj.__dict__["dispatch"] = disp = self.dispatch._for_instance(obj)
+
+        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