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

2 files changed, 19 insertions, 1 deletions

diff --git a/lib/sqlalchemy/future/__init__.py b/lib/sqlalchemy/future/__init__.py
index 37ce46e47..b07b9b040 100644
--- a/lib/sqlalchemy/future/__init__.py
+++ b/lib/sqlalchemy/future/__init__.py
@@ -14,4 +14,5 @@ from .engine import Engine  # noqa
 from ..sql.selectable import Select  # noqa
 from ..util.langhelpers import public_factory
 
+
 select = public_factory(Select._create_future_select, ".future.select")
diff --git a/lib/sqlalchemy/future/engine.py b/lib/sqlalchemy/future/engine.py
index d5922daa3..dd72360ed 100644
--- a/lib/sqlalchemy/future/engine.py
+++ b/lib/sqlalchemy/future/engine.py
@@ -359,6 +359,22 @@ class Engine(_LegacyEngine):
             execution_options=legacy_engine._execution_options,
         )
 
+    class _trans_ctx(object):
+        def __init__(self, conn):
+            self.conn = conn
+
+        def __enter__(self):
+            self.transaction = self.conn.begin()
+            return self.conn
+
+        def __exit__(self, type_, value, traceback):
+            if type_ is not None:
+                self.transaction.rollback()
+            else:
+                if self.transaction.is_active:
+                    self.transaction.commit()
+            self.conn.close()
+
     def begin(self):
         """Return a :class:`_future.Connection` object with a transaction
         begun.
@@ -381,7 +397,8 @@ class Engine(_LegacyEngine):
             :meth:`_future.Connection.begin`
 
         """
-        return super(Engine, self).begin()
+        conn = self.connect()
+        return self._trans_ctx(conn)
 
     def connect(self):
         """Return a new :class:`_future.Connection` object.