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, 6 insertions, 1 deletions

diff --git a/lib/sqlalchemy/engine/base.py b/lib/sqlalchemy/engine/base.py
index d60f14f31..34bf720b7 100644
--- a/lib/sqlalchemy/engine/base.py
+++ b/lib/sqlalchemy/engine/base.py
@@ -89,6 +89,7 @@ class Connection(Connectable):
                 if connection is not None
                 else engine.raw_connection()
             )
+
             self._transaction = self._nested_transaction = None
             self.__savepoint_seq = 0
             self.__in_begin = False
@@ -623,6 +624,9 @@ class Connection(Connectable):
 
         self._dbapi_connection.detach()
 
+    def _autobegin(self):
+        self.begin()
+
     def begin(self):
         """Begin a transaction and return a transaction handle.
 
@@ -1433,7 +1437,7 @@ class Connection(Connectable):
             self._invalid_transaction()
 
         if self._is_future and self._transaction is None:
-            self.begin()
+            self._autobegin()
 
         context.pre_exec()
 
@@ -2592,6 +2596,7 @@ class Engine(Connectable, log.Identified):
             return self.conn
 
         def __exit__(self, type_, value, traceback):
+
             if type_ is not None:
                 self.transaction.rollback()
             else: