Graceful handling of broken lists when cuddled-lists extra is enabled

lib/markdown2.py

@@ -2181,8 +2181,13 @@ def _form_paragraphs(self, text):
                     ):
                         start = li.start()
                         cuddled_list = self._do_lists(graf[start:]).rstrip("\n")
-                        assert re.match(r'^<(?:ul|ol).*?>', cuddled_list)
-                        graf = graf[:start]
+                        if re.match(r'^<(?:ul|ol).*?>', cuddled_list):
+                            graf = graf[:start]
+                        else:
+                            # Not quite a cuddled list. (See not_quite_a_list_cuddled_lists test case)
+                            # Store as a simple paragraph.
+                            graf = cuddled_list
+                            cuddled_list = None
 
                 # Wrap <p> tags.
                 graf = self._run_span_gamut(graf)

test/tm-cases/not_quite_a_list_cuddled_lists.html

@@ -0,0 +1,26 @@
+<p>So:</p>
+
+<ul>
+<li>This</li>
+<li>is</li>
+<li>a list.</li>
+</ul>
+
+<p>And:</p>
+
+<ul>
+<li><em>This</em> is</li>
+<li>a list</li>
+<li>too.</li>
+</ul>
+
+<p>However, because ASCII art can have long dash/asterisk lines let ensure that:</p>
+
+<p>- - - - - - - This isn't a list.</p>
+
+<p>* * Nor is this a list.
+-   -   - Or this.</p>
+
+<p>- - - - - - - - - - +
+ hi there ascii art
+- - - - - - - - - - +</p>

test/tm-cases/not_quite_a_list_cuddled_lists.opts

@@ -0,0 +1 @@
+{"extras": ["cuddled-lists"]}

test/tm-cases/not_quite_a_list_cuddled_lists.text

@@ -0,0 +1,21 @@
+So:
+- This
+- is
+- a list.
+
+And:
+* *This* is
+* a list
+* too.
+
+However, because ASCII art can have long dash/asterisk lines let ensure that:
+
+- - - - - - - This isn't a list.
+
+* * Nor is this a list.
+-   -   - Or this.
+
+
+- - - - - - - - - - +
+ hi there ascii art
+- - - - - - - - - - +