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, 29 insertions, 0 deletions

diff --git a/lib/sqlalchemy/ext/asyncio/events.py b/lib/sqlalchemy/ext/asyncio/events.py
new file mode 100644
index 000000000..a8daefc4b
--- /dev/null
+++ b/lib/sqlalchemy/ext/asyncio/events.py
@@ -0,0 +1,29 @@
+from .engine import AsyncConnectable
+from .session import AsyncSession
+from ...engine import events as engine_event
+from ...orm import events as orm_event
+
+
+class AsyncConnectionEvents(engine_event.ConnectionEvents):
+    _target_class_doc = "SomeEngine"
+    _dispatch_target = AsyncConnectable
+
+    @classmethod
+    def _listen(cls, event_key, retval=False):
+        raise NotImplementedError(
+            "asynchronous events are not implemented at this time.  Apply "
+            "synchronous listeners to the AsyncEngine.sync_engine or "
+            "AsyncConnection.sync_connection attributes."
+        )
+
+
+class AsyncSessionEvents(orm_event.SessionEvents):
+    _target_class_doc = "SomeSession"
+    _dispatch_target = AsyncSession
+
+    @classmethod
+    def _listen(cls, event_key, retval=False):
+        raise NotImplementedError(
+            "asynchronous events are not implemented at this time.  Apply "
+            "synchronous listeners to the AsyncSession.sync_session."
+        )