fully support isolation_level parameter in base dialect

Generalized the :paramref:`_sa.create_engine.isolation_level` parameter to
the base dialect so that it is no longer dependent on individual dialects
to be present. This parameter sets up the "isolation level" setting to
occur for all new database connections as soon as they are created by the
connection pool, where the value then stays set without being reset on
every checkin.

The :paramref:`_sa.create_engine.isolation_level` parameter is essentially
equivalent in functionality to using the
:paramref:`_engine.Engine.execution_options.isolation_level` parameter via
:meth:`_engine.Engine.execution_options` for an engine-wide setting. The
difference is in that the former setting assigns the isolation level just
once when a connection is created, the latter sets and resets the given
level on each connection checkout.

Fixes: #6342
Change-Id: Id81d6b1c1a94371d901ada728a610696e09e9741

1 files changed, 5 insertions, 35 deletions

diff --git a/lib/sqlalchemy/dialects/mysql/base.py b/lib/sqlalchemy/dialects/mysql/base.py
index 5382e00db..af2d7bdbb 100644
--- a/lib/sqlalchemy/dialects/mysql/base.py
+++ b/lib/sqlalchemy/dialects/mysql/base.py
@@ -2385,7 +2385,6 @@ class MySQLDialect(default.DefaultDialect):
 
     def __init__(
         self,
-        isolation_level=None,
         json_serializer=None,
         json_deserializer=None,
         is_mariadb=None,
@@ -2393,49 +2392,20 @@ class MySQLDialect(default.DefaultDialect):
     ):
         kwargs.pop("use_ansiquotes", None)  # legacy
         default.DefaultDialect.__init__(self, **kwargs)
-        self.isolation_level = isolation_level
         self._json_serializer = json_serializer
         self._json_deserializer = json_deserializer
         self._set_mariadb(is_mariadb, None)
 
-    def on_connect(self):
-        if self.isolation_level is not None:
-
-            def connect(conn):
-                self.set_isolation_level(conn, self.isolation_level)
-
-            return connect
-        else:
-            return None
-
-    _isolation_lookup = set(
-        [
+    def get_isolation_level_values(self, dbapi_conn):
+        return (
             "SERIALIZABLE",
             "READ UNCOMMITTED",
             "READ COMMITTED",
             "REPEATABLE READ",
-        ]
-    )
-
-    def set_isolation_level(self, connection, level):
-        level = level.replace("_", " ")
-
-        # adjust for ConnectionFairy being present
-        # allows attribute set e.g. "connection.autocommit = True"
-        # to work properly
-        if hasattr(connection, "dbapi_connection"):
-            connection = connection.dbapi_connection
-
-        self._set_isolation_level(connection, level)
+        )
 
-    def _set_isolation_level(self, connection, level):
-        if level not in self._isolation_lookup:
-            raise exc.ArgumentError(
-                "Invalid value '%s' for isolation_level. "
-                "Valid isolation levels for %s are %s"
-                % (level, self.name, ", ".join(self._isolation_lookup))
-            )
-        cursor = connection.cursor()
+    def set_isolation_level(self, dbapi_conn, level):
+        cursor = dbapi_conn.cursor()
         cursor.execute("SET SESSION TRANSACTION ISOLATION LEVEL %s" % level)
         cursor.execute("COMMIT")
         cursor.close()