I think this is a great idea. For clarity, do we already run similar checks for actual symbols? (For example, it should be impossible to add a new function to our headers that isn't prefixed with Py.)

We have two other scripts with somewhat similar functionality to check things against ignore lists, and they both also verify the opposite: that the ignore file has no stale entries.

If a stale entry is found, the script also fails. This makes sure we keep it clean and relevant.

Let's add a similar check here, it would find the unnecessary FVC_ALT_REPR. And can also identify the generated ones from opcode_ids.h.

For reference:

• cpython/Doc/tools/check-warnings.py

  Lines 237 to 251 in 659e262

  	def fail_if_improved(
  	files_with_expected_nits: set[str], files_with_nits: set[str]
  	) -> int:
  	"""
  	We may have fixed warnings in some files so that the files are now completely clean.
  	Good news! Let's add them to .nitignore to prevent regression.
  	"""
  	files_with_no_nits = files_with_expected_nits - files_with_nits
  	if files_with_no_nits:
  	print("\nCongratulations! You improved:\n")
  	for filename in sorted(files_with_no_nits):
  	print(filename)
  	print("\nPlease remove from Doc/tools/.nitignore\n")
  	return -1
  	return 0

• cpython/Tools/build/check_warnings.py

  Lines 195 to 225 in 659e262

  	def get_unexpected_improvements(
  	ignore_rules: set[IgnoreRule],
  	files_with_warnings: dict[str, list[CompileWarning]],
  	) -> int:
  	"""
  	Returns failure status if the number of warnings for a file is greater
  	than the expected number of warnings for that file based on the ignore
  	rules
  	"""
  	unexpected_improvements = []
  	for rule in ignore_rules:
  	if (
  	not rule.ignore_all
  	and rule.file_path not in files_with_warnings.keys()
  	):
  	if rule.file_path not in files_with_warnings.keys():
  	unexpected_improvements.append((rule.file_path, rule.count, 0))
  	elif len(files_with_warnings[rule.file_path]) < rule.count:
  	unexpected_improvements.append((
  	rule.file_path,
  	rule.count,
  	len(files_with_warnings[rule.file_path]),
  	))
  	if unexpected_improvements:
  	print("Unexpected improvements:")
  	for file in unexpected_improvements:
  	print(f"{file[0]} expected {file[1]} warnings, found {file[2]}")
  	return 1
  	return 0

I also added a check to make sure that the ignore list is sorted. Is it overkill?

I think it's worth it, it's easier to manually search a sorted list, and it's a simple check.

(prek/pre-commit have a built-in sorter lint (file-contents-sorter) but that would require moving the ignore file's header into the .py)

Thanks! Looks good, can always exclude generated ones later if we decide to do so.