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, 28 deletions

diff --git a/lib/sqlalchemy/dialects/sqlite/base.py b/lib/sqlalchemy/dialects/sqlite/base.py
index dc8425859..6c22c8ef3 100644
--- a/lib/sqlalchemy/dialects/sqlite/base.py
+++ b/lib/sqlalchemy/dialects/sqlite/base.py
@@ -1815,7 +1815,6 @@ class SQLiteDialect(default.DefaultDialect):
     preparer = SQLiteIdentifierPreparer
     ischema_names = ischema_names
     colspecs = colspecs
-    isolation_level = None
 
     construct_arguments = [
         (
@@ -1856,7 +1855,6 @@ class SQLiteDialect(default.DefaultDialect):
     )
     def __init__(
         self,
-        isolation_level=None,
         native_datetime=False,
         json_serializer=None,
         json_deserializer=None,
@@ -1865,7 +1863,6 @@ class SQLiteDialect(default.DefaultDialect):
         **kwargs
     ):
         default.DefaultDialect.__init__(self, **kwargs)
-        self.isolation_level = isolation_level
 
         if _json_serializer:
             json_serializer = _json_serializer
@@ -1918,22 +1915,12 @@ class SQLiteDialect(default.DefaultDialect):
         {"READ UNCOMMITTED": 1, "SERIALIZABLE": 0}
     )
 
+    def get_isolation_level_values(self, dbapi_conn):
+        return list(self._isolation_lookup)
+
     def set_isolation_level(self, connection, level):
-        try:
-            isolation_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,
-            )
+        isolation_level = self._isolation_lookup[level]
+
         cursor = connection.cursor()
         cursor.execute("PRAGMA read_uncommitted = %d" % isolation_level)
         cursor.close()
@@ -1960,16 +1947,6 @@ class SQLiteDialect(default.DefaultDialect):
         else:
             assert False, "Unknown isolation level %s" % value
 
-    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
-
     @reflection.cache
     def get_schema_names(self, connection, **kw):
         s = "PRAGMA database_list"