provide connectionfairy on initialize

This is so that dialect methods that are called within init
can assume the same argument structure as when they are called
in other places; we can nail down the type of object as well.

This change seems to mostly impact the isolation level routines
in the dialects, as these are called during initialize()
as well as on established connections.  these methods can now
assume a non-proxied DBAPI connection object in all cases,
as it is commonly required that attributes like ".autocommit"
are set on the object which don't work well in a proxied
situation.

Other changes:

* adds an interface for the "connectionfairy" concept
  called PoolProxiedConnection.
* Removes ``Connectable`` superclass of Connection.
  ``Connectable`` was originally meant to provide for the
  "method which accepts connection or engine" theme.  As this
  pattern  is greatly reduced in 2.0 and Engine no longer extends
  from it, the ``Connectable`` superclass doesnt serve any real
  purpose.

Leading from that, to set this in I also applied pep 484 annotations
to the Dialect base, and then in the interests of seeing some
of the typing information show up in my IDE did a little bit for Engine,
Connection and others.  I hope that it's feasible that we can
add annotations to specific classes and attributes ahead of when we
actually try to mass-populate the whole library.  This was
the original spirit of pep-484 that we can apply annotations
gradually.  I do of course want to try to do a mass-populate
although i think even in that case we will end up doing a lot
of manual work anyway (in particular for the changes here which
are distinct from what the stubs have).

Fixes: #7122
Change-Id: I5dd7fbff8a7ae520a81c165091af12a6a68826db

1 files changed, 23 insertions, 12 deletions

diff --git a/lib/sqlalchemy/engine/base.py b/lib/sqlalchemy/engine/base.py
index bfadcbce6..fbd8fe7df 100644
--- a/lib/sqlalchemy/engine/base.py
+++ b/lib/sqlalchemy/engine/base.py
@@ -6,9 +6,13 @@
 # the MIT License: https://www.opensource.org/licenses/mit-license.php
 import contextlib
 import sys
+import typing
+from typing import Any
+from typing import Mapping
+from typing import Optional
+from typing import Union
 
 from .interfaces import BindTyping
-from .interfaces import Connectable
 from .interfaces import ConnectionEventsTarget
 from .interfaces import ExceptionContext
 from .util import _distill_params_20
@@ -20,6 +24,11 @@ from .. import util
 from ..sql import compiler
 from ..sql import util as sql_util
 
+if typing.TYPE_CHECKING:
+    from .interfaces import Dialect
+    from .url import URL
+    from ..pool import Pool
+    from ..pool import PoolProxiedConnection
 
 """Defines :class:`_engine.Connection` and :class:`_engine.Engine`.
 
@@ -29,7 +38,7 @@ _EMPTY_EXECUTION_OPTS = util.immutabledict()
 NO_OPTIONS = util.immutabledict()
 
 
-class Connection(Connectable):
+class Connection(ConnectionEventsTarget):
     """Provides high-level functionality for a wrapped DB-API connection.
 
     The :class:`_engine.Connection` object is procured by calling
@@ -364,7 +373,7 @@ class Connection(Connectable):
         return self._dbapi_connection is None and not self.closed
 
     @property
-    def connection(self):
+    def connection(self) -> "PoolProxiedConnection":
         """The underlying DB-API connection managed by this Connection.
 
         This is a SQLAlchemy connection-pool proxied connection
@@ -422,7 +431,9 @@ class Connection(Connectable):
 
         """
         try:
-            return self.dialect.get_isolation_level(self.connection)
+            return self.dialect.get_isolation_level(
+                self.connection.dbapi_connection
+            )
         except BaseException as e:
             self._handle_dbapi_exception(e, None, None, None, None)
 
@@ -2296,14 +2307,14 @@ class Engine(ConnectionEventsTarget, log.Identified):
 
     def __init__(
         self,
-        pool,
-        dialect,
-        url,
-        logging_name=None,
-        echo=None,
-        query_cache_size=500,
-        execution_options=None,
-        hide_parameters=False,
+        pool: "Pool",
+        dialect: "Dialect",
+        url: "URL",
+        logging_name: Optional[str] = None,
+        echo: Union[None, str, bool] = None,
+        query_cache_size: int = 500,
+        execution_options: Optional[Mapping[str, Any]] = None,
+        hide_parameters: bool = False,
     ):
         self.pool = pool
         self.url = url