Sequence-like subclass __radd__ fallback regression

Bug report

Bug description:

In 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.

class A:
    def __radd__(self, other): ...

class B(tuple): ...

() + A()  # No error
B() + A() # TypeError: can only concatenate tuple (not "A") to tuple

I 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.

Works in 3.14.3. git bisect'd to e6e376a, 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).

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

gh-146587: fix type slot assignment incase of multiple slots for same name #146593

@sergey-miryanov

cc @sergey-miryanov as the bisect'd PR author

Will take a look soon

@JelleZijlstra

cc @JelleZijlstra, @vstinner

I'm working on a fix.

PR #146593 fixes this.

Merge branch 'fix-pythongh-146587' of https://github.com/kumaraditya3…

gh-146587: fix type slot assignment incase of multiple slots for same…

Merge remote-tracking branch 'upstream/main' into tachyon-ndjson-kole…

pythongh-146587: fix type slot assignment incase of multiple slots fo…

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Sequence-like subclass `radd` fallback regression #146587

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Operating systems tested on:

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Sequence-like subclass __radd__ fallback regression #146587

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Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

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inventshah commented on Mar 29, 2026

sergey-miryanov commented on Mar 29, 2026

serhiy-storchaka commented on Mar 29, 2026

kumaraditya303 commented on Mar 29, 2026

kumaraditya303 commented on Mar 29, 2026

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Sequence-like subclass `radd` fallback regression #146587