Fix exception when using reformatting on scalars

.github/workflows/tox.yml

@@ -59,7 +59,7 @@ jobs:
       WSLENV: FORCE_COLOR:PYTEST_REQPASS:TOXENV:GITHUB_STEP_SUMMARY
       # Number of expected test passes, safety measure for accidental skip of
       # tests. Update value if you add/remove tests.
-      PYTEST_REQPASS: 796
+      PYTEST_REQPASS: 797
     steps:
       - name: Activate WSL1
         if: "contains(matrix.shell, 'wsl')"

.pre-commit-config.yaml

@@ -21,6 +21,8 @@ exclude: >
     examples/broken/encoding.j2|
     examples/broken/yaml-with-tabs/invalid-due-tabs.yaml|
     examples/playbooks/collections/.*|
+    examples/playbooks/vars/empty.transformed.yml|
+    examples/playbooks/vars/empty.yml|
     src/ansiblelint/schemas/rulebook.json|
     test/schemas/data/licenses.json|
     test/schemas/negative_test|
@@ -40,17 +42,19 @@ repos:
           (?x)^(
             .*\.md$|
             examples/other/some.j2.yaml|
+            examples/playbooks/collections/.*|
             examples/playbooks/example.yml|
             examples/playbooks/multiline-brackets.*|
             examples/playbooks/templates/not-valid.yaml|
-            examples/playbooks/with-umlaut-.*|
+            examples/playbooks/vars/empty.transformed.yml|
+            examples/playbooks/vars/empty.yml|
             examples/playbooks/with-skip-tag-id.yml|
+            examples/playbooks/with-umlaut-.*|
             examples/yamllint/.*|
-            examples/playbooks/collections/.*|
+            src/ansiblelint/schemas/(molecule|tasks|playbook|rulebook).json|
             test/fixtures/formatting-before/.*|
-            test/schemas/data/.*|
             test/schemas/(negative_test|test)/.*\.md|
-            src/ansiblelint/schemas/(molecule|tasks|playbook|rulebook).json|
+            test/schemas/data/.*|
             src/ansiblelint/schemas/ansible-navigator-config.json
           )$
         always_run: true

examples/playbooks/vars/empty.transformed.yml

@@ -0,0 +1,3 @@
+
+---
+# broken vars files due to spacing at the start of the file.

examples/playbooks/vars/empty.yml

@@ -0,0 +1,3 @@
+
+---
+# broken vars files due to spacing at the start of the file.

src/ansiblelint/file_utils.py

@@ -248,10 +248,14 @@ def __init__(
 
     def _guess_kind(self) -> None:
         if self.kind == "yaml":
-            if isinstance(self.data, list) and (
-                "hosts" in self.data[0]
-                or "import_playbook" in self.data[0]
-                or "ansible.builtin.import_playbook" in self.data[0]
+            if (
+                isinstance(self.data, list)
+                and len(self.data) > 0
+                and (
+                    "hosts" in self.data[0]
+                    or "import_playbook" in self.data[0]
+                    or "ansible.builtin.import_playbook" in self.data[0]
+                )
             ):
                 if "rules" not in self.data[0]:
                     self.kind = "playbook"

src/ansiblelint/transformer.py

@@ -100,6 +100,10 @@ def run(self) -> None:
                     # This is an empty vars file or similar which loads as None.
                     # It is not safe to write this file or data-loss is likely.
                     # Only maps and sequences can preserve comments. Skip it.
+                    _logger.debug(
+                        "Ignored reformatting %s because current implementation in ruamel.yaml would drop comments. See https://sourceforge.net/p/ruamel-yaml/tickets/460/",
+                        file,
+                    )
                     continue
 
             if self.write_set != {"none"}:

src/ansiblelint/yaml_utils.py

@@ -924,10 +924,20 @@ def loads(self, stream: str) -> Any:
         if not isinstance(stream, str):
             msg = f"expected a str but got {type(stream)}"
             raise NotImplementedError(msg)
+        # As ruamel drops comments for any document that is not a mapping or sequence,
+        # we need to avoid using it to reformat those documents.
+        # https://sourceforge.net/p/ruamel-yaml/tickets/460/
+
         text, preamble_comment = self._pre_process_yaml(stream)
         data = self.load(stream=text)
-        if preamble_comment is not None:
-            data.preamble_comment = preamble_comment
+        if preamble_comment is not None and isinstance(
+            data,
+            (CommentedMap, CommentedSeq),
+        ):
+            data.preamble_comment = preamble_comment  # type: ignore[union-attr]
+        # Because data can validly also be None for empty documents, we cannot
+        # really annotate the return type here, so we need to remember to
+        # never save None or scalar data types when reformatting.
         return data
 
     def dumps(self, data: Any) -> str:

test/test_transformer.py

@@ -81,6 +81,7 @@ def fixture_runner_result(
             id="empty_vars",
         ),
         pytest.param("examples/playbooks/vars/strings.yml", 0, True, id="strings"),
+        pytest.param("examples/playbooks/vars/empty.yml", 1, False, id="empty"),
         pytest.param("examples/playbooks/name-case.yml", 1, True, id="name_case"),
         pytest.param("examples/playbooks/fqcn.yml", 3, True, id="fqcn"),
     ),