Strip trailing whitespace in yaml

Author: emersonknapp

polymath_code_standard/yaml_format.py

@@ -301,6 +301,19 @@ def apply_matrices(source: str, matrices: dict[str, int]) -> str:
 # ---------------------------------------------------------------------------
 
 
+def _strip_trailing_whitespace(text: str) -> str:
+    """Strip trailing whitespace from each line, preserving newlines.
+
+    Why: yamlfix emits an empty list item as ``- `` (dash + trailing space).
+    The pre-commit ``trailing-whitespace`` hook would then fight this hook
+    in a loop. Stripping line-trailing whitespace here makes our output a
+    fixed point for both hooks. Safe in YAML: literal/folded block scalars
+    have their per-line trailing whitespace stripped by the parser anyway,
+    and other contexts never carry meaningful trailing spaces.
+    """
+    return '\n'.join(line.rstrip() for line in text.split('\n'))
+
+
 def format_yaml(source: str) -> str:
     """Format YAML source, preserving matrix-structured flow sequences.
 
@@ -312,7 +325,7 @@ def format_yaml(source: str) -> str:
     fixed = fix_code(source, _yamlfix_config())
     if matrices:
         fixed = apply_matrices(fixed, matrices)
-    return fixed
+    return _strip_trailing_whitespace(fixed)
 
 
 def format_yaml_files(files: list[str]) -> list[tuple[str, bool, str]]:

tests/test_yaml_format.py

@@ -116,6 +116,21 @@ def test_single_line_sequence_preserved(self):
         result = format_yaml('tags: [a, b, c]\n')
         assert '[a, b, c]' in result
 
+    def test_empty_list_item_has_no_trailing_space(self):
+        # yamlfix emits empty list items as '- ' (dash + space). The
+        # trailing-whitespace hook would then undo it, leading to a hook
+        # fight. format_yaml must produce a fixed point: bare '-'.
+        source = textwrap.dedent("""\
+            items:
+              -
+              -
+              - x
+        """)
+        result = format_yaml(source)
+        for i, line in enumerate(result.splitlines()):
+            assert line == line.rstrip(), f'trailing whitespace on line {i}: {line!r}'
+        assert '\n  -\n  -\n  - x\n' in result
+
 
 # ---------------------------------------------------------------------------
 # format_yaml — matrix preservation