fix: respect Python indentation significance in Apply diff pipeline

core/diff/streamDiff.ts

@@ -14,6 +14,7 @@ import { LineStream, matchLine } from "./util.js";
 export async function* streamDiff(
   oldLines: string[],
   newLines: LineStream,
+  filename?: string,
 ): AsyncGenerator<DiffLine> {
   const oldLinesCopy = [...oldLines];
 
@@ -27,6 +28,7 @@ export async function* streamDiff(
       newLineResult.value,
       oldLinesCopy,
       seenIndentationMistake,
+      filename,
     );
 
     if (!seenIndentationMistake && newLineResult.value !== newLine) {

core/diff/util.ts

@@ -49,6 +49,7 @@ export function matchLine(
   newLine: string,
   oldLines: string[],
   permissiveAboutIndentation = false,
+  filename?: string,
 ): MatchLineResult {
   // Only match empty lines if it's the next one:
   if (newLine.trim() === "" && oldLines[0]?.trim() === "") {
@@ -77,9 +78,14 @@ export function matchLine(
     }
     if (linesMatch(newLineTrimmed, oldLineTrimmed, i)) {
       // This is a way to fix indentation, but only for sufficiently long lines to avoid matching whitespace or short lines
+      // For Python files, indentation is syntax and should never be permissive
+      const isPythonFile = filename?.endsWith(".py");
+      const shouldBePermissive =
+        !isPythonFile &&
+        (permissiveAboutIndentation || newLine.trim().length > 8);
       if (
         newLineTrimmed.trimStart() === oldLineTrimmed.trimStart() &&
-        (permissiveAboutIndentation || newLine.trim().length > 8)
+        shouldBePermissive
       ) {
         return {
           matchIndex: i,

core/edit/lazy/deterministic-python.vitest.ts

@@ -0,0 +1,50 @@
+import { describe, expect, test } from "vitest";
+
+import { DiffLine } from "../..";
+import { streamDiff } from "../../diff/streamDiff";
+
+async function* toLineStream(
+  lines: string[],
+): AsyncGenerator<string, void, unknown> {
+  for (const line of lines) {
+    yield line;
+  }
+}
+
+async function collectStreamDiff(
+  oldText: string,
+  newText: string,
+  filename: string,
+): Promise<DiffLine[]> {
+  const oldLines = oldText.split("\n");
+  const newLines = toLineStream(newText.split("\n"));
+  const diffs: DiffLine[] = [];
+  for await (const line of streamDiff(oldLines, newLines, filename)) {
+    diffs.push(line);
+  }
+  return diffs;
+}
+
+describe("streamDiff python indentation awareness", () => {
+  test("treats indentation changes as real diffs for .py files", async () => {
+    const oldText = 'def hello():\n    print("world")';
+    const newText = 'def hello():\n  print("world")';
+
+    const diffs = await collectStreamDiff(oldText, newText, "example.py");
+
+    const newLines = diffs.filter((d) => d.type === "new").map((d) => d.line);
+    const oldLines = diffs.filter((d) => d.type === "old").map((d) => d.line);
+    expect(newLines).toContain('  print("world")');
+    expect(oldLines).toContain('    print("world")');
+  });
+
+  test("still treats indentation as cosmetic for non-python files", async () => {
+    const oldText = 'function hello() {\n    console.log("world");\n}';
+    const newText = 'function hello() {\n  console.log("world");\n}';
+
+    const diffs = await collectStreamDiff(oldText, newText, "example.js");
+
+    const allSame = diffs.every((d) => d.type === "same");
+    expect(allSame).toBe(true);
+  });
+});

core/edit/lazy/streamLazyApply.ts

@@ -62,7 +62,7 @@ export async function* streamLazyApply(
 
   // Convert output to diff
   const oldLines = oldCode.split(/\r?\n/);
-  let diffLines = streamDiff(oldLines, lines);
+  let diffLines = streamDiff(oldLines, lines, filename);
   diffLines = filterLeadingAndTrailingNewLineInsertion(diffLines);
   for await (const diffLine of diffLines) {
     yield diffLine;

core/edit/streamDiffLines.ts

@@ -176,7 +176,7 @@ export async function* streamDiffLines(
     lines = filterEnglishLinesAtEnd(lines);
   }
 
-  let diffLines = streamDiff(oldLines, lines);
+  let diffLines = streamDiff(oldLines, lines, options.fileUri);
   diffLines = filterLeadingAndTrailingNewLineInsertion(diffLines);
   if (highlighted.length === 0) {
     const line = prefix.split("\n").slice(-1)[0];