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, 0 deletions

diff --git a/lib/sqlalchemy/ext/asyncio/exc.py b/lib/sqlalchemy/ext/asyncio/exc.py
new file mode 100644
index 000000000..6137bf6df
--- /dev/null
+++ b/lib/sqlalchemy/ext/asyncio/exc.py
@@ -0,0 +1,14 @@
+from ... import exc
+
+
+class AsyncMethodRequired(exc.InvalidRequestError):
+    """an API can't be used because its result would not be
+    compatible with async"""
+
+
+class AsyncContextNotStarted(exc.InvalidRequestError):
+    """a startable context manager has not been started."""
+
+
+class AsyncContextAlreadyStarted(exc.InvalidRequestError):
+    """a startable context manager is already started."""