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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Sequence-like subclass `__radd__` fallback regression","articleBody":"# Bug report\n\n### Bug description:\n\nIn the 3.15 alphas, the `+` operator does not dispatch to `obj.__radd__` when the left hand object inherits from a sequence like class (`list`, `tuple`, ...), but does if it is a base instance. \n\n```python\nclass A:\n    def __radd__(self, other): ...\n\nclass B(tuple): ...\n\n() + A()  # No error\nB() + A() # TypeError: can only concatenate tuple (not \"A\") to tuple\n```\n\nI originally ran into the issue with a `NamedTuple`, but looks like subclasses of `array.array`, `bytearray`, `bytes`, `collections.deque`, `list`, `str`, and `tuple` all raise the error. \n\nWorks in 3.14.3. git bisect'd to e6e376a760b7a3b267e89244a2c89e7ddfba9c38, so something in the resolution between `nb_add` and `sq_concat` may have changed while optimizing (`*` has a similar problem with `__rmul__` mapping to `nb_multiply` and `sq_repeat`).\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nmacOS\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-146593\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/inventshah","@type":"Person","name":"inventshah"},"datePublished":"2026-03-29T06:58:19.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":5},"url":"https://github.com/146587/cpython/issues/146587"}