feat: add a simple HTML check pre-commit hook

pre_commit_hooks/check_html.py

@@ -0,0 +1,74 @@
+from __future__ import annotations
+
+import argparse
+import collections
+from html.parser import HTMLParser
+from typing import Sequence
+
+
+class ValidationException(Exception):
+    pass
+
+
+class ValidatingHTMLParser(HTMLParser):
+    def __init__(self) -> None:
+        super(HTMLParser, self).__init__()
+        self.stack: collections.deque[str] = collections.deque()
+
+    def handle_starttag(
+        self, tag: str,
+        attrs: list[tuple[str, str | None]],
+    ) -> None:
+        self.stack.append(tag)
+
+    def handle_endtag(self, tag: str) -> None:
+        if not self.stack:
+            raise ValidationException(f"no opening tag for </{tag}>")
+        opening_tag = self.stack.pop()
+        if opening_tag != tag:
+            stack = '/'.join(self.stack)
+            raise ValidationException(
+                f"attempt to close {opening_tag} with {tag} at /{stack}",
+            )
+
+    def handle_startendtag(
+        self, tag: str,
+        attrs: list[tuple[str, str | None]],
+    ) -> None:
+        # append and immediately pop stack
+        pass
+
+    def close(self) -> None:
+        super().close()
+        if self.stack:
+            opening_tag = self.stack.pop()
+            stack = '/'.join(self.stack)
+            raise ValidationException(
+                f"EOF reached while {opening_tag} at /{stack} not closed",
+            )
+
+
+def main(argv: Sequence[str] | None = None) -> int:
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        'filenames',
+        nargs='*',
+        help='HTML filenames to check.',
+    )
+    args = parser.parse_args(argv)
+
+    retval = 0
+    for filename in args.filenames:
+        try:
+            with open(filename, 'rb') as html_file:
+                html_parser = ValidatingHTMLParser()
+                html_parser.feed(html_file.read().decode('ascii', 'ignore'))
+                html_parser.close()
+        except ValidationException as exc:
+            print(f'{filename}: Failed to parse HTML: ({exc})')
+            retval = 1
+    return retval
+
+
+if __name__ == '__main__':
+    raise SystemExit(main())