From 7232a5b132975cbbdca682ddd787f1c91e0f4c3c Mon Sep 17 00:00:00 2001
From: Anders Kaseorg <andersk@mit.edu>
Date: Mon, 5 Oct 2026 22:06:59 +0000
Subject: [PATCH] Reject assigning a string to an attribute index

AttrsList.__setitem__ with an integer index treated a string value as
a (key, value) pair, so attrs[0] = "confirm" renamed the first
attribute to "c" and set its value to "o".

Raise TypeError instead, and describe the accepted combinations with
overloads, so that type checkers report it too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
---
 python/markupever/dom.py | 18 +++++++++++++++++-
 python/tests/test_dom.py |  7 +++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/python/markupever/dom.py b/python/markupever/dom.py
index 17ca309..e684c2d 100644
--- a/python/markupever/dom.py
+++ b/python/markupever/dom.py
@@ -852,6 +852,16 @@ def __delitem__(self, index: int | str | _rustlib.QualName) -> None:
 
         self.__raw.remove(index)
 
+    @typing.overload
+    def __setitem__(self, index: int, val: tuple[_rustlib.QualName | str, str]) -> None: ...
+
+    @typing.overload
+    def __setitem__(
+        self,
+        index: str | _rustlib.QualName,
+        val: str | tuple[_rustlib.QualName | str, str],
+    ) -> None: ...
+
     def __setitem__(
         self,
         index: int | str | _rustlib.QualName,
@@ -865,7 +875,10 @@ def __setitem__(
         a new attribute is pushed. If the key exists, the attribute is updated.
 
         - index: An integer index or a string/QualName key to set.
-        - val: A value to set, either as a string or a (key, value) tuple.
+        - val: A value to set, either as a string or a (key, value) tuple. With an integer
+          index, it must be a (key, value) tuple.
+
+        Raises `TypeError` if the index is an integer and the value is a string.
         """
         if not isinstance(index, int):
             key, value = (index, val) if isinstance(val, str) else val
@@ -877,6 +890,9 @@ def __setitem__(
 
             val = (key, value)
 
+        if isinstance(val, str):
+            raise TypeError("an attribute index must be assigned a (key, value) tuple, not a str")
+
         self.__raw.update_item(index, val[0], val[1])
 
     @typing.overload
diff --git a/python/tests/test_dom.py b/python/tests/test_dom.py
index c38725a..87b506c 100644
--- a/python/tests/test_dom.py
+++ b/python/tests/test_dom.py
@@ -406,6 +406,13 @@ def test_element():
     html.attrs.insert(0, "onclick", "alert")
     assert html.attrs[0] == (markupever.dom.QualName("onclick"), "alert")
 
+    # An attribute index must be assigned a (key, value) tuple.
+    with pytest.raises(TypeError):
+        html.attrs[0] = "confirm"  # type: ignore[call-overload]
+    assert html.attrs[0] == (markupever.dom.QualName("onclick"), "alert")
+    html.attrs[0] = ("onload", "init")
+    assert html.attrs[0] == (markupever.dom.QualName("onload"), "init")
+
     html.attrs = [("id", "id1"), ("id", "id2"), ("data-role", "button")]
     assert html.id == "id1"