Fix toc crash on a heading with a valueless id attribute

docs/changelog.md

@@ -16,6 +16,10 @@ See the [Contributing Guide](contributing.md) for details.
 
 ### Fixed
 
+* The `toc` extension no longer raises `TypeError` for a heading whose `id`
+  attribute is present but valueless, as the `attr_list` extension's
+  bare-attribute syntax (`{id}`) writes it: like a heading with no `id` at
+  all, it now gets a generated slug (#1705).
 * Fix for complex cases in new emphasis handling where text could be unexpectedly
   dropped (#1694).
 * Fix rule of 3 handling in new emphasis handling where an opener skipped by the rule could still be paired

markdown/extensions/toc.py

@@ -400,7 +400,10 @@ def run(self, doc: etree.Element) -> None:
                 name = strip_tags(innerhtml)
 
                 # Do not override pre-existing ids
-                if "id" not in el.attrib:
+                if not el.attrib.get("id"):
+                    # The `attr_list` extension writes bare attributes with a
+                    # value of None. A valueless id is no id at all, so a slug
+                    # is generated for it like for any other id-less heading.
                     el.attrib["id"] = unique(self.slugify(html.unescape(name), self.sep), used_ids, id_counters)
 
                 data_toc_label = ''

tests/test_syntax/extensions/test_toc.py

@@ -22,6 +22,7 @@
 from markdown.test_tools import TestCase
 from markdown import Markdown
 from markdown.extensions.toc import TocExtension, strip_tags, unique
+from markdown.extensions.attr_list import AttrListExtension
 from markdown.extensions.nl2br import Nl2BrExtension
 
 
@@ -1574,3 +1575,39 @@ def testDontStripHTMLEntities(self):
             strip_tags('foo < & < bar'),
             'foo < & < bar'
         )
+
+
+class TestTOCWithAttrList(TestCase):
+    """The `toc` extension together with `attr_list` (#1705)."""
+
+    maxDiff = None
+    default_kwargs = {
+        'extensions': [TocExtension(), AttrListExtension()]
+    }
+
+    def testBareIdAttribute(self):
+        # The `attr_list` extension writes bare attributes with a value of
+        # None. A valueless `id` is no id at all, so `toc` generates a slug
+        # for it like for any other id-less heading instead of raising a
+        # `TypeError`.
+        self.assertMarkdownRenders(
+            '#d {id}',
+            '<h1 id="d">d</h1>'
+        )
+
+    def testBareIdAttributeWithTOCMarker(self):
+        self.assertMarkdownRenders(
+            self.dedent(
+                '''
+                [TOC]
+
+                # Heading One {id}
+                '''
+            ),
+            '<div class="toc">\n'
+              '<ul>\n'                                                 # noqa
+                '<li><a href="#heading-one">Heading One</a></li>\n'   # noqa
+              '</ul>\n'                                                # noqa
+            '</div>\n'
+            '<h1 id="heading-one">Heading One</h1>'
+        )