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, 43 insertions, 0 deletions

diff --git a/lib/sqlalchemy/testing/suite/test_dialect.py b/lib/sqlalchemy/testing/suite/test_dialect.py
index b3e43aad0..28fd99876 100644
--- a/lib/sqlalchemy/testing/suite/test_dialect.py
+++ b/lib/sqlalchemy/testing/suite/test_dialect.py
@@ -8,6 +8,7 @@ from .. import eq_
 from .. import fixtures
 from .. import ne_
 from .. import provide_metadata
+from ..assertions import expect_raises_message
 from ..config import requirements
 from ..provision import set_default_schema_on_connection
 from ..schema import Column
@@ -140,6 +141,48 @@ class IsolationLevelTest(fixtures.TestBase):
                     levels["default"],
                 )
 
+    @testing.requires.get_isolation_level_values
+    def test_invalid_level_execution_option(self, connection_no_trans):
+        """test for the new get_isolation_level_values() method"""
+
+        connection = connection_no_trans
+        with expect_raises_message(
+            exc.ArgumentError,
+            "Invalid value '%s' for isolation_level. "
+            "Valid isolation levels for '%s' are %s"
+            % (
+                "FOO",
+                connection.dialect.name,
+                ", ".join(
+                    requirements.get_isolation_levels(config)["supported"]
+                ),
+            ),
+        ):
+            connection.execution_options(isolation_level="FOO")
+
+    @testing.requires.get_isolation_level_values
+    @testing.requires.dialect_level_isolation_level_param
+    def test_invalid_level_engine_param(self, testing_engine):
+        """test for the new get_isolation_level_values() method
+        and support for the dialect-level 'isolation_level' parameter.
+
+        """
+
+        eng = testing_engine(options=dict(isolation_level="FOO"))
+        with expect_raises_message(
+            exc.ArgumentError,
+            "Invalid value '%s' for isolation_level. "
+            "Valid isolation levels for '%s' are %s"
+            % (
+                "FOO",
+                eng.dialect.name,
+                ", ".join(
+                    requirements.get_isolation_levels(config)["supported"]
+                ),
+            ),
+        ):
+            eng.connect()
+
 
 class AutocommitIsolationTest(fixtures.TablesTest):