Guard move destinations against overwrite

desloppify/app/commands/move/apply.py

@@ -17,6 +17,11 @@
 from desloppify.base.output.terminal import colorize
 
 
+def _ensure_move_destination_absent(dest_abs: str) -> None:
+    if Path(dest_abs).exists():
+        raise FileExistsError(f"Destination already exists: {dest_abs}")
+
+
 def _rollback_written_files(written_files: dict[str, str]) -> None:
     failed = restore_files_best_effort(written_files, safe_write_text)
     for filepath in failed:
@@ -55,6 +60,7 @@ def apply_file_move(
     Path(dest_abs).parent.mkdir(parents=True, exist_ok=True)
     written_files: dict[str, str] = {}
     try:
+        _ensure_move_destination_absent(dest_abs)
         shutil.move(source_abs, dest_abs)
 
         if dest_abs in new_contents:
@@ -85,6 +91,7 @@ def apply_directory_move(
     Path(dest_abs).parent.mkdir(parents=True, exist_ok=True)
     written_files: dict[str, str] = {}
     try:
+        _ensure_move_destination_absent(dest_abs)
         shutil.move(source_abs, dest_abs)
 
         for src_file, changes in internal_changes.items():

desloppify/tests/commands/test_transitive_engine.py

@@ -753,6 +753,19 @@ def test_file_move_with_importer_changes(self, tmp_path):
         )
         assert importer.read_text() == "from b import thing"
 
+    def test_file_move_fails_if_destination_already_exists(self, tmp_path):
+        src = tmp_path / "a.py"
+        dest = tmp_path / "b.py"
+        src.write_text("source content")
+        dest.write_text("existing content")
+
+        with pytest.raises(FileExistsError, match="Destination already exists"):
+            move_apply_mod.apply_file_move(str(src), str(dest), {}, [])
+
+        assert src.exists()
+        assert src.read_text() == "source content"
+        assert dest.read_text() == "existing content"
+
     def test_file_move_rollback_on_write_error(self, tmp_path):
         """If writing an importer fails, the move is rolled back."""
         src = tmp_path / "a.py"
@@ -802,6 +815,22 @@ def test_directory_move_with_internal_changes(self, tmp_path):
         )
         assert (dest / "a.py").read_text() == "from new_pkg.b import f"
 
+    def test_directory_move_fails_if_destination_already_exists(self, tmp_path):
+        src = tmp_path / "pkg"
+        src.mkdir()
+        (src / "mod.py").write_text("source content")
+
+        dest = tmp_path / "new_pkg"
+        dest.mkdir()
+        (dest / "mod.py").write_text("existing content")
+
+        with pytest.raises(FileExistsError, match="Destination already exists"):
+            move_apply_mod.apply_directory_move(str(src), str(dest), src, {}, {})
+
+        assert src.exists()
+        assert (src / "mod.py").read_text() == "source content"
+        assert (dest / "mod.py").read_text() == "existing content"
+
 
 # =====================================================================
 # Module 5: shared_phases.py