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, 4 insertions, 13 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/asyncpg.py b/lib/sqlalchemy/dialects/postgresql/asyncpg.py
index 28374ed60..fe1f9fd5a 100644
--- a/lib/sqlalchemy/dialects/postgresql/asyncpg.py
+++ b/lib/sqlalchemy/dialects/postgresql/asyncpg.py
@@ -933,20 +933,11 @@ class PGDialect_asyncpg(PGDialect):
             "SERIALIZABLE": "serializable",
         }
 
-    def set_isolation_level(self, connection, level):
-        try:
-            level = self._isolation_lookup[level.replace("_", " ")]
-        except KeyError as err:
-            util.raise_(
-                exc.ArgumentError(
-                    "Invalid value '%s' for isolation_level. "
-                    "Valid isolation levels for %s are %s"
-                    % (level, self.name, ", ".join(self._isolation_lookup))
-                ),
-                replace_context=err,
-            )
+    def get_isolation_level_values(self, dbapi_conn):
+        return list(self._isolation_lookup)
 
-        connection.set_isolation_level(level)
+    def set_isolation_level(self, connection, level):
+        connection.set_isolation_level(self._isolation_lookup[level])
 
     def set_readonly(self, connection, value):
         connection.readonly = value