- A warning is emitted if the ``isolation_level`` parameter is used

with :meth:`.Connection.execution_options` when a :class:`.Transaction`
is in play; DBAPIs and/or SQLAlchemy dialects such as psycopg2,
MySQLdb may implicitly rollback or commit the transaction, or
not change the setting til next transaction, so this is never safe.
- Added new parameter :paramref:`.Session.connection.execution_options`
which may be used to set up execution options on a :class:`.Connection`
when it is first checked out, before the transaction has begun.
This is used to set up options such as isolation level on the
connection before the transaction starts.
- added new documentation section
detailing best practices for setting transaction isolation with
sessions.
fixes #3296

1 files changed, 6 insertions, 0 deletions

diff --git a/lib/sqlalchemy/engine/default.py b/lib/sqlalchemy/engine/default.py
index f6c2263b3..17d2e2531 100644
--- a/lib/sqlalchemy/engine/default.py
+++ b/lib/sqlalchemy/engine/default.py
@@ -395,6 +395,12 @@ class DefaultDialect(interfaces.Dialect):
             self._set_connection_isolation(connection, opts['isolation_level'])
 
     def _set_connection_isolation(self, connection, level):
+        if connection.in_transaction():
+            util.warn(
+                "Connection is already established with a Transaction; "
+                "setting isolation_level may implicitly rollback or commit "
+                "the existing transaction, or have no effect until "
+                "next transaction")
         self.set_isolation_level(connection.connection, level)
         connection.connection._connection_record.\
             finalize_callback.append(self.reset_isolation_level)