add isolation_level option

Author: michiya

README.rst

@@ -128,6 +128,16 @@ Dictionary. Current available keys are:
    See http://msdn.microsoft.com/en-us/library/ms130892.aspx. Default is
    ``"SQL Server"`` on Windows and ``"FreeTDS"`` on other platforms.
 
+-  isolation_level
+
+   String. Sets `transaction isolation level
+   <https://docs.microsoft.com/en-us/sql/t-sql/statements/set-transaction-isolation-level-transact-sql>`__
+   for each database session. Valid values for this entry are
+   ``READ UNCOMMITTED``, ``READ COMMITTED``, ``REPEATABLE READ``,
+   ``SNAPSHOT``, and ``SERIALIZABLE``. Default is ``None`` which means
+   no isolation levei is set to a database session and SQL Server default
+   will be used.
+
 -  dsn
 
    String. A named DSN can be used instead of ``HOST``.

sql_server/pyodbc/base.py

@@ -361,11 +361,15 @@ def init_connection_state(self):
         settings_dict = self.settings_dict
         cursor = self.create_cursor()
 
+        options = settings_dict.get('OPTIONS', {})
+        isolation_level = options.get('isolation_level', None)
+        if isolation_level:
+            cursor.execute('SET TRANSACTION ISOLATION LEVEL %s' % isolation_level)
+
         # Set date format for the connection. Also, make sure Sunday is
         # considered the first day of the week (to be consistent with the
         # Django convention for the 'week_day' Django lookup) if the user
         # hasn't told us otherwise
-        options = settings_dict.get('OPTIONS', {})
         datefirst = options.get('datefirst', 7)
         cursor.execute('SET DATEFORMAT ymd; SET DATEFIRST %s' % datefirst)