Create connection characteristics API; implement postgresql flags

Added support for PostgreSQL "readonly" and "deferrable" flags for all of
psycopg2, asyncpg and pg8000 dialects.   This takes advantage of a newly
generalized version of the "isolation level" API to support other kinds of
session attributes set via execution options that are reliably reset
when connections are returned to the connection pool.

Fixes: #5549
Change-Id: I0ad6d7a095e49d331618274c40ce75c76afdc7dd

2 files changed, 9 insertions, 0 deletions

diff --git a/lib/sqlalchemy/util/__init__.py b/lib/sqlalchemy/util/__init__.py
index 1e3eb9a29..1d92084cc 100644
--- a/lib/sqlalchemy/util/__init__.py
+++ b/lib/sqlalchemy/util/__init__.py
@@ -44,6 +44,7 @@ from ._collections import UniqueAppender  # noqa
 from ._collections import update_copy  # noqa
 from ._collections import WeakPopulateDict  # noqa
 from ._collections import WeakSequence  # noqa
+from .compat import ABC  # noqa
 from .compat import arm  # noqa
 from .compat import b  # noqa
 from .compat import b64decode  # noqa
diff --git a/lib/sqlalchemy/util/compat.py b/lib/sqlalchemy/util/compat.py
index caa97f72b..e1d0e6444 100644
--- a/lib/sqlalchemy/util/compat.py
+++ b/lib/sqlalchemy/util/compat.py
@@ -193,6 +193,9 @@ if py3k:
 
     # Unused. Kept for backwards compatibility.
     callable = callable  # noqa
+
+    from abc import ABC
+
 else:
     import base64
     import ConfigParser as configparser  # noqa
@@ -208,6 +211,11 @@ else:
     from urllib import unquote_plus  # noqa
     from urlparse import parse_qsl  # noqa
 
+    from abc import ABCMeta
+
+    class ABC(object):
+        __metaclass__ = ABCMeta
+
     try:
         import cPickle as pickle
     except ImportError: