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, 54 insertions, 0 deletions

diff --git a/lib/sqlalchemy/testing/requirements.py b/lib/sqlalchemy/testing/requirements.py
index 4cc431bb7..a3d277c50 100644
--- a/lib/sqlalchemy/testing/requirements.py
+++ b/lib/sqlalchemy/testing/requirements.py
@@ -20,6 +20,7 @@ import sys
 
 from . import exclusions
 from . import only_on
+from .. import create_engine
 from .. import util
 from ..pool import QueuePool
 
@@ -873,6 +874,59 @@ class SuiteRequirements(Requirements):
                 ]
             }
         """
+        with config.db.connect() as conn:
+
+            try:
+                supported = conn.dialect.get_isolation_level_values(
+                    conn.connection.dbapi_connection
+                )
+            except NotImplementedError:
+                return None
+            else:
+                return {
+                    "default": conn.dialect.default_isolation_level,
+                    "supported": supported,
+                }
+
+    @property
+    def get_isolation_level_values(self):
+        """target dialect supports the
+        :meth:`_engine.Dialect.get_isolation_level_values`
+        method added in SQLAlchemy 2.0.
+
+        """
+
+        def go(config):
+            with config.db.connect() as conn:
+                try:
+                    conn.dialect.get_isolation_level_values(
+                        conn.connection.dbapi_connection
+                    )
+                except NotImplementedError:
+                    return False
+                else:
+                    return True
+
+        return exclusions.only_if(go)
+
+    @property
+    def dialect_level_isolation_level_param(self):
+        """test that the dialect allows the 'isolation_level' argument
+        to be handled by DefaultDialect"""
+
+        def go(config):
+            try:
+                e = create_engine(
+                    config.db.url, isolation_level="READ COMMITTED"
+                )
+            except:
+                return False
+            else:
+                return (
+                    e.dialect._on_connect_isolation_level == "READ COMMITTED"
+                )
+
+        return exclusions.only_if(go)
 
     @property
     def json_type(self):