Gracefully skip isolation level if no row returned

Fixed issue in MySQL dialect when connecting to a psuedo-MySQL database
such as that provided by ProxySQL, the up front check for isolation level
when it returns no row will not prevent the dialect from continuing to
connect. A warning is emitted that the isolation level could not be
detected.

Fixes: #5239
Change-Id: I4a240386a0d38bd90733819495ce50e37fe2234c

1 files changed, 8 insertions, 1 deletions

diff --git a/lib/sqlalchemy/dialects/mysql/base.py b/lib/sqlalchemy/dialects/mysql/base.py
index 26d751faa..54a13b550 100644
--- a/lib/sqlalchemy/dialects/mysql/base.py
+++ b/lib/sqlalchemy/dialects/mysql/base.py
@@ -2297,7 +2297,14 @@ class MySQLDialect(default.DefaultDialect):
             cursor.execute("SELECT @@transaction_isolation")
         else:
             cursor.execute("SELECT @@tx_isolation")
-        val = cursor.fetchone()[0]
+        row = cursor.fetchone()
+        if row is None:
+            util.warn(
+                "Could not retrieve transaction isolation level for MySQL "
+                "connection."
+            )
+            raise NotImplementedError()
+        val = row[0]
         cursor.close()
         if util.py3k and isinstance(val, bytes):
             val = val.decode()