Implement rudimentary asyncio support w/ asyncpg

Using the approach introduced at
https://gist.github.com/zzzeek/6287e28054d3baddc07fa21a7227904e

We can now create asyncio endpoints that are then handled
in "implicit IO" form within the majority of the Core internals.
Then coroutines are re-exposed at the point at which we call
into asyncpg methods.

Patch includes:

* asyncpg dialect

* asyncio package

* engine, result, ORM session classes

* new test fixtures, tests

* some work with pep-484 and a short plugin for the
  pyannotate package, which seems to have so-so results

Change-Id: Idbcc0eff72c4cad572914acdd6f40ddb1aef1a7d
Fixes: #3414

1 files changed, 14 insertions, 1 deletions

diff --git a/lib/sqlalchemy/orm/loading.py b/lib/sqlalchemy/orm/loading.py
index 2eb3e1368..fd3e92055 100644
--- a/lib/sqlalchemy/orm/loading.py
+++ b/lib/sqlalchemy/orm/loading.py
@@ -125,8 +125,21 @@ def instances(cursor, context):
             if not yield_per:
                 break
 
+    if context.execution_options.get("prebuffer_rows", False):
+        # this is a bit of a hack at the moment.
+        # I would rather have some option in the result to pre-buffer
+        # internally.
+        _prebuffered = list(chunks(None))
+
+        def chunks(size):
+            return iter(_prebuffered)
+
     result = ChunkedIteratorResult(
-        row_metadata, chunks, source_supports_scalars=single_entity, raw=cursor
+        row_metadata,
+        chunks,
+        source_supports_scalars=single_entity,
+        raw=cursor,
+        dynamic_yield_per=cursor.context._is_server_side,
     )
 
     result._attributes = result._attributes.union(