Exposing ISOLATION_LEVEL_DEFAULT to Python

This is now the state that is returned to Python if nothing has been
explicitly set.

doc/src/extensions.rst

@@ -651,6 +651,15 @@ set to one of the following constants:
 
         .. __: http://www.postgresql.org/docs/current/static/transaction-iso.html#XACT-SERIALIZABLE
 
+.. data:: ISOLATION_LEVEL_DEFAULT
+
+    Whatever is defined by the server, either via server configuration or by
+    statements executed within the session outside Pyscopg control; Psycopg
+    will not force an isolation level of its own. If you want to know what the
+    value is you can use a query such as :sql:`show transaction_isolation` or
+    :sql:`show default_transaction_isolation`.
+
+    .. versionadded:: 2.7
 
 
 .. index::

lib/extensions.py

@@ -72,6 +72,7 @@ ISOLATION_LEVEL_READ_UNCOMMITTED = 4
 ISOLATION_LEVEL_READ_COMMITTED = 1
 ISOLATION_LEVEL_REPEATABLE_READ = 2
 ISOLATION_LEVEL_SERIALIZABLE = 3
+ISOLATION_LEVEL_DEFAULT = 5
 
 
 """psycopg connection status values."""

tests/test_connection.py

@@ -488,7 +488,7 @@ class IsolationLevelsTestCase(ConnectingTestCase):
         conn = self.connect()
         self.assertEqual(
             conn.isolation_level,
-            psycopg2.extensions.ISOLATION_LEVEL_READ_COMMITTED)
+            psycopg2.extensions.ISOLATION_LEVEL_DEFAULT)
 
     def test_encoding(self):
         conn = self.connect()