68 lines
2.5 KiB
Python
68 lines
2.5 KiB
Python
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"""Structural accessibility checks for rendered pages.
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Not a substitute for axe or a screen reader, but cheap enough to run on every page in unit tests.
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"""
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from html.parser import HTMLParser
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_UNLABELLED_INPUT_TYPES = {"hidden", "submit", "button", "reset", "image"}
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class _Collector(HTMLParser):
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def __init__(self):
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super().__init__()
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self.html_lang = None
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self.headings = []
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self.ids = []
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self.label_fors = set()
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self.controls = []
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self.images_without_alt = 0
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self._label_depth = 0
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def handle_starttag(self, tag, attrs):
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attrs = dict(attrs)
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if tag == "html":
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self.html_lang = attrs.get("lang")
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if "id" in attrs:
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self.ids.append(attrs["id"])
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if tag in {"h1", "h2", "h3", "h4", "h5", "h6"}:
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self.headings.append(int(tag[1]))
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if tag == "label":
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self._label_depth += 1
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if attrs.get("for"):
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self.label_fors.add(attrs["for"])
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if tag == "img" and "alt" not in attrs:
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self.images_without_alt += 1
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if tag in {"input", "select", "textarea"}:
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if tag == "input" and attrs.get("type", "text") in _UNLABELLED_INPUT_TYPES:
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return
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wrapped = self._label_depth > 0
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named = bool(attrs.get("aria-label") or attrs.get("aria-labelledby"))
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self.controls.append((attrs.get("id"), attrs.get("name"), wrapped or named))
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def handle_endtag(self, tag):
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if tag == "label":
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self._label_depth -= 1
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def assert_accessible_html(html: str) -> None:
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page = _Collector()
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page.feed(html)
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problems = []
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if not page.html_lang:
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problems.append("<html> has no lang")
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if page.headings.count(1) != 1:
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problems.append(f"expected one <h1>, found {page.headings.count(1)}")
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for before, after in zip(page.headings, page.headings[1:], strict=False):
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if after > before + 1:
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problems.append(f"heading level skips from h{before} to h{after}")
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duplicates = {i for i in page.ids if page.ids.count(i) > 1}
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if duplicates:
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problems.append(f"duplicate ids: {sorted(duplicates)}")
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for control_id, name, labelled in page.controls:
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if not (labelled or control_id in page.label_fors):
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problems.append(f"form control without a label: id={control_id} name={name}")
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if page.images_without_alt:
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problems.append(f"{page.images_without_alt} <img> without alt")
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assert not problems, "\n".join(problems)
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