Override get_isolation_level_values to advertise SERIALIZABLE only

The previous ``supported_isolation_levels`` class attribute was a
bespoke name that SQLAlchemy never consults. The real introspection
surface is ``get_isolation_level_values(dbapi_connection)``, which
parent ``SQLiteDialect`` overrides to return both READ UNCOMMITTED and
SERIALIZABLE. dqlite runs every statement through Raft consensus and
cannot honour READ UNCOMMITTED, so a caller passing
``isolation_level="READ UNCOMMITTED"`` to ``create_engine`` was
silently accepted and routed into pysqlite's PRAGMA read_uncommitted
code path with no effect. Override the method to return
``["SERIALIZABLE"]`` only and drop the bespoke attribute.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/sqlalchemydqlite/base.py

@@ -5,7 +5,7 @@
 import math
 import types
 import warnings
-from collections.abc import Callable
+from collections.abc import Callable, Sequence
 from typing import Any
 
 from sqlalchemy import types as sqltypes
@@ -119,11 +119,6 @@ class DqliteDialect(SQLiteDialect):
     # SQLAlchemy release decouples the two.
     non_native_boolean_check_constraint = False
 
-    # dqlite runs every statement through Raft consensus; there is no
-    # exposed way to weaken isolation. Declaring this explicitly lets
-    # applications introspect via ``engine.dialect.supported_isolation_levels``.
-    supported_isolation_levels: tuple[str, ...] = ("SERIALIZABLE",)
-
     # Since isolation is always SERIALIZABLE and cannot be weakened, the
     # reported isolation level is trustworthy across transactions. SQLAlchemy
     # skips defensive isolation-level resets when this is True.
@@ -237,6 +232,21 @@ def create_connect_args(self, url: URL) -> tuple[list[Any], dict[str, Any]]:
 
         return [], kwargs
 
+    def get_isolation_level_values(
+        self, dbapi_connection: DBAPIConnection
+    ) -> Sequence[IsolationLevel]:
+        """Return the isolation levels dqlite accepts.
+
+        The parent ``SQLiteDialect`` advertises ``["READ UNCOMMITTED",
+        "SERIALIZABLE"]`` because stdlib sqlite3 implements
+        ``READ UNCOMMITTED`` via ``PRAGMA read_uncommitted`` in
+        shared-cache mode. dqlite runs every statement through Raft
+        consensus and has no mechanism to weaken isolation, so advertise
+        only what we can honour — ``set_isolation_level`` below rejects
+        anything else explicitly.
+        """
+        return ["SERIALIZABLE"]
+
     def get_isolation_level(self, dbapi_connection: DBAPIConnection) -> IsolationLevel:
         """Return the isolation level.
 

tests/test_dialect_dialect_config.py

@@ -6,7 +6,7 @@
 - ``create_connect_args`` plumbs the ``timeout`` URL query
   through and rejects typos.
 - ``set_isolation_level`` explicitly rejects AUTOCOMMIT.
-- ``supported_isolation_levels`` is declared.
+- ``get_isolation_level_values`` advertises only SERIALIZABLE.
 """
 
 from unittest.mock import MagicMock
@@ -20,9 +20,26 @@
 from sqlalchemydqlite.base import DqliteDialect
 
 
-class TestSupportedIsolationLevels:
-    def test_declared(self) -> None:
-        assert DqliteDialect.supported_isolation_levels == ("SERIALIZABLE",)
+class TestGetIsolationLevelValues:
+    def test_only_serializable(self) -> None:
+        """dqlite accepts SERIALIZABLE only; inheriting SQLiteDialect's
+        ``["READ UNCOMMITTED", "SERIALIZABLE"]`` would let callers set a
+        level we cannot honour (PRAGMA read_uncommitted has no effect
+        server-side).
+        """
+        dialect = DqliteDialect()
+        assert dialect.get_isolation_level_values(MagicMock()) == ["SERIALIZABLE"]
+
+    def test_read_uncommitted_not_advertised(self) -> None:
+        dialect = DqliteDialect()
+        assert "READ UNCOMMITTED" not in dialect.get_isolation_level_values(MagicMock())
+
+    def test_defined_locally(self) -> None:
+        """Must be overridden on DqliteDialect itself; inheriting
+        SQLiteDialect's version would silently re-introduce
+        READ UNCOMMITTED.
+        """
+        assert "get_isolation_level_values" in DqliteDialect.__dict__
 
 
 class TestSetIsolationLevel: