Warn when unsupported isolation level is requested

dqlite only supports SERIALIZABLE isolation. Emit a warning instead of
silently ignoring when a different level is requested.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/sqlalchemydqlite/base.py

@@ -61,10 +61,17 @@ def get_isolation_level(self, dbapi_connection: DBAPIConnection) -> IsolationLev
     def set_isolation_level(self, dbapi_connection: DBAPIConnection, level: str | None) -> None:
         """Set isolation level.
 
-        dqlite doesn't support changing isolation levels via PRAGMA,
-        so this is a no-op. dqlite uses SERIALIZABLE isolation by default.
+        dqlite only supports SERIALIZABLE isolation. A warning is emitted
+        if a different level is requested.
         """
-        pass
+        if level is not None and level != "SERIALIZABLE":
+            import warnings
+
+            warnings.warn(
+                f"dqlite only supports SERIALIZABLE isolation. "
+                f"Requested level {level!r} is ignored.",
+                stacklevel=2,
+            )
 
     def do_rollback(self, dbapi_connection: DBAPIConnection) -> None:
         """Rollback the current transaction.

tests/test_dialect.py

@@ -280,6 +280,52 @@ def test_is_defined_on_dialect(self) -> None:
         )
 
 
+class TestIsolationLevel:
+    def test_set_isolation_level_warns_on_unsupported(self) -> None:
+        """set_isolation_level should warn when a non-SERIALIZABLE level is requested."""
+        import warnings
+        from unittest.mock import MagicMock
+
+        dialect = DqliteDialect()
+        mock_conn = MagicMock()
+
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
+            dialect.set_isolation_level(mock_conn, "READ UNCOMMITTED")
+
+        assert len(w) == 1
+        assert "SERIALIZABLE" in str(w[0].message)
+        assert "READ UNCOMMITTED" in str(w[0].message)
+
+    def test_set_isolation_level_silent_for_serializable(self) -> None:
+        """set_isolation_level should not warn for SERIALIZABLE."""
+        import warnings
+        from unittest.mock import MagicMock
+
+        dialect = DqliteDialect()
+        mock_conn = MagicMock()
+
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
+            dialect.set_isolation_level(mock_conn, "SERIALIZABLE")
+
+        assert len(w) == 0
+
+    def test_set_isolation_level_silent_for_none(self) -> None:
+        """set_isolation_level should not warn when level is None."""
+        import warnings
+        from unittest.mock import MagicMock
+
+        dialect = DqliteDialect()
+        mock_conn = MagicMock()
+
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
+            dialect.set_isolation_level(mock_conn, None)
+
+        assert len(w) == 0
+
+
 class TestURLParsing:
     def test_parse_basic_url(self) -> None:
         url = URL.create("dqlite", host="localhost", port=9001, database="test")