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