feat(python): support Ruff formatting and config directories (#1573)

Ruff is now an external formatter for Python next to the Ruff language server: Settings › Languages & Frameworks › Ruff has the executable path, a configuration directory (or file), the provider preference, enabled languages, run-on-save, and Reformat with Ruff. Formatting runs on the unsaved document through stdin (with Ruff's --range for selections, in 1-based code-point columns) and never writes the file first. The configuration directory is shared with Ruff LSP, which can then start without a root marker. Tools that can't format a language are rejected even when requested.

Co-authored-by: muutot <[email protected]>
Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/editor/lsp/actions.ts

@@ -124,12 +124,12 @@ function editorAction(
   return { id, title, category, when: EDITOR_WHEN, run: withEditor(run), ...extra };
 }
 
-/** Whether saving a `language` file formats it: Format on save, or Run on save of Prettier or oxfmt. */
+/** Whether saving a `language` file formats it: Format on save, or Run on save of an external formatter. */
 export function formatsOnSave(settings: Settings | null, language: string): boolean {
-  const { prettier, oxfmt } = validateLanguages(settings?.languages);
+  const { prettier, oxfmt, ruff } = validateLanguages(settings?.languages);
   return (
     !!settings?.editor?.formatOnSave ||
-    [prettier, oxfmt].some((formatter) => formatter.runOnSave && formatter.languages.includes(language))
+    [prettier, oxfmt, ruff].some((formatter) => formatter.runOnSave && formatter.languages.includes(language))
   );
 }
 
@@ -206,6 +206,13 @@ export function registerLspActions(): () => void {
       (v) => formatDocument(v, { explicit: true, tool: "oxfmt" }),
       { keywords: ["format", "oxfmt", "oxc"] },
     ),
+    editorAction(
+      "code.reformatWithRuff",
+      "Reformat with Ruff",
+      "Code",
+      (v) => formatDocument(v, { explicit: true, tool: "ruff" }),
+      { keywords: ["format", "ruff", "python"] },
+    ),
     editorAction("code.reformatDialog", "Reformat File…", "Code", (v) =>
       reformatDialog(v, (view) => applyKindSilently(view, "source.organizeImports")),
     ),

src/editor/lsp/format.test.ts

@@ -177,6 +177,8 @@ describe("format on save provider isolation", () => {
     expect(formatsOnSave(oxfmt, "typescript")).toBe(true);
     expect(formatsOnSave(oxfmt, "markdown")).toBe(false);
     expect(formatsOnSave(settings({ prettier: { runOnSave: true } }), "markdown")).toBe(true);
+    expect(formatsOnSave(settings({ ruff: { runOnSave: true } }), "python")).toBe(true);
+    expect(formatsOnSave(settings({ ruff: { runOnSave: true } }), "typescript")).toBe(false);
     expect(formatsOnSave(settings({}), "typescript")).toBe(false);
     expect(formatsOnSave(settings({}, true), "rust")).toBe(true);
     expect(formatsOnSave(null, "typescript")).toBe(false);

src/editor/lsp/format.ts

@@ -136,20 +136,26 @@ async function format(view: EditorView, opts: FormatDocumentOptions): Promise<Fo
       commands.lspFormat(doc.doc, range, { ...options, onSave: opts.onSave, tool: opts.tool }),
     );
   } catch (e) {
-    if (e instanceof IpcError && e.data?.type === "tool" && (e.data.tool === "prettier" || e.data.tool === "oxfmt")) {
+    const formatter =
+      e instanceof IpcError && e.data?.type === "tool"
+        ? {
+            prettier: { failed: "Prettier formatting failed", timedOut: "Prettier formatting timed out" },
+            oxfmt: { failed: "oxfmt formatting failed", timedOut: "oxfmt formatting timed out" },
+            ruff: { failed: "Ruff formatting failed", timedOut: "Ruff formatting timed out" },
+          }[e.data.tool]
+        : undefined;
+    if (e instanceof IpcError && e.data?.type === "tool" && formatter) {
       const tool = e.data.tool;
       if (e.kind === "notFound" && e.data.missing) {
-        await executeAction("tools.promptInstall", { name: tool, sticky: !opts.onSave });
+        await executeAction("tools.promptInstall", { name: tool, sticky: !opts.onSave, formatter: true });
       } else if (e.kind !== "cancelled") {
         notifications().notify({
           group: "tools",
           level: "error",
-          title: tool === "oxfmt" ? tx("oxfmt formatting failed") : tx("Prettier formatting failed"),
+          title: tx(formatter.failed),
           body:
             e.kind === "timedOut"
-              ? tool === "oxfmt"
-                ? tx("oxfmt formatting timed out")
-                : tx("Prettier formatting timed out")
+              ? tx(formatter.timedOut)
               : createElement("span", { className: styles.diagnostics }, e.message),
           key: `${tool}-format-error:${doc.path}`,
           sticky: false,

src/editor/lsp/formatErrors.test.tsx

@@ -44,9 +44,9 @@ afterEach(() => {
   _resetI18nForTests();
 });
 
-describe.each(["prettier", "oxfmt"])("%s formatter errors", (name) => {
+describe.each(["prettier", "oxfmt", "ruff"])("%s formatter errors", (name) => {
   const tool = { type: "tool", tool: name, missing: false } as const;
-  const display = name === "prettier" ? "Prettier" : name;
+  const display = name === "prettier" ? "Prettier" : name === "ruff" ? "Ruff" : name;
   it("uses structured missing-tool information to request the shared install prompt", async () => {
     const prompt = vi.fn();
     const dispose = registerAction({
@@ -57,9 +57,9 @@ describe.each(["prettier", "oxfmt"])("%s formatter errors", (name) => {
     try {
       failWith({ kind: "notFound", message: "unrelated diagnostic", data: { ...tool, missing: true } });
       await formatDocument(viewFor());
-      expect(prompt).toHaveBeenCalledWith({ name, sticky: true });
+      expect(prompt).toHaveBeenCalledWith({ name, sticky: true, formatter: true });
       await formatDocument(viewFor(), { explicit: false, onSave: true });
-      expect(prompt).toHaveBeenLastCalledWith({ name, sticky: false });
+      expect(prompt).toHaveBeenLastCalledWith({ name, sticky: false, formatter: true });
       expect(useNotifications.getState().items).toHaveLength(0);
     } finally {
       dispose();
@@ -126,10 +126,11 @@ describe.each(["prettier", "oxfmt"])("%s formatter errors", (name) => {
   });
 });
 
-describe("Reformat with Prettier / oxfmt", () => {
+describe("Reformat with an external formatter", () => {
   it.each([
     ["code.reformatWithPrettier", "prettier"],
     ["code.reformatWithOxfmt", "oxfmt"],
+    ["code.reformatWithRuff", "ruff"],
   ])("%s asks the backend for %s whatever the settings select", async (id, tool) => {
     vi.spyOn(lsp, "lspRequest").mockImplementation(async (request) => {
       await request();