Preserve task-list checkbox in markdown and rst renderers

The task_lists plugin rewrites a list_item into a task_list_item and moves
the [ ]/[x] marker into attrs, registering a renderer method only for the
html renderer. The markdown and rst renderers render list items through the
shared _render_list_item helper rather than a per-token method, so the
checkbox had nowhere to be emitted and was silently dropped: reformatting
'- [x] done' produced '- done', losing information on round-trip.

Re-emit the marker for task_list_item tokens in _render_list_item so the
checkbox survives. Adds round-trip tests covering ordered, nested,
multiline and mixed lists, plus the rst renderer.

Fixes #441
Fixes #401

Author: Labib-Bin-Salam

src/mistune/renderers/_list.py

@@ -30,6 +30,13 @@ def _render_list_item(
 ) -> str:
     leading = cast(str, parent["leading"])
     text = ""
+    # The task_lists plugin rewrites a list_item into a task_list_item and
+    # moves the "[ ] "/"[x] " marker into attrs. Unlike the html renderer, the
+    # markdown and rst renderers render list items here rather than through a
+    # per-token method, so re-emit the checkbox to keep it from being silently
+    # dropped on round-trip.
+    if item["type"] == "task_list_item":
+        text = "[x] " if item["attrs"]["checked"] else "[ ] "
     for tok in item["children"]:
         if tok["type"] == "list":
             tok["parent"] = parent

tests/test_renderers.py

@@ -65,3 +65,41 @@ def test_prose_not_over_escaped(self):
             "ratio a/b is fine\n",
         ):
             self.assert_round_trip(text)
+
+
+class TestTaskListRoundTrip(TestCase):
+    """The task_lists plugin rewrites a list_item into a task_list_item and
+    moves the checkbox marker into attrs. The markdown and rst renderers must
+    re-emit it instead of silently dropping it (issues #401 and #441)."""
+
+    to_html = create_markdown(escape=False, plugins=["task_lists"])
+    reformat = create_markdown(renderer=MarkdownRenderer(), plugins=["task_lists"])
+    to_rst = create_markdown(renderer=RSTRenderer(), plugins=["task_lists"])
+
+    def assert_round_trip(self, text):
+        self.assertEqual(
+            self.to_html(self.reformat(text)),
+            self.to_html(text),
+        )
+
+    def test_checkbox_preserved(self):
+        # the exact issue example: the checkbox used to disappear entirely
+        self.assertEqual(
+            self.reformat("- [x] done\n- [ ] todo\n"),
+            "- [x] done\n- [ ] todo\n",
+        )
+
+    def test_round_trip_variants(self):
+        for text in (
+            "- [x] done\n- [ ] todo\n",
+            "- [X] upper-x is checked\n",
+            "1. [ ] a\n2. [x] b\n",
+            "- [ ] outer\n  - [x] inner\n",
+            "- [ ] line one\n  line two\n",
+            "- plain item\n- [ ] task item\n",
+        ):
+            self.assert_round_trip(text)
+
+    def test_rst_preserves_checkbox(self):
+        self.assertIn("[x] done", self.to_rst("- [x] done\n"))
+        self.assertIn("[ ] todo", self.to_rst("- [ ] todo\n"))