Description

neutrinoceros

opened on Mar 29, 2026

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Bug report

Bug description:

I'm seeing a regression in how an + operation is resolved between a NamedTuple and an arbitrary type with a __radd__ implementation, which looks like an LSP violation since builtin tuple instance still behave the same as with Python 3.14

from typing import NamedTuple

class T(NamedTuple):
     x: int

class A:
    def __init__(self, *args):
       self.lst = list(args)
    def __radd__(self, other):
        return A(*self.lst, other)
 
print(((1,)+A()).lst) # [(1,)]
print((T(x=1)+A()).lst) # raises

Traceback (most recent call last):
  File "/Users/clm/dev/gh/astropy/astropy/t.py", line 13, in <module>
    print((T(x=1)+A()).lst) # raises
           ~~~~~~^~~~
TypeError: can only concatenate tuple (not "A") to tuple

The __radd__ implementation on the right operand appears to be entirely skipped, and we only see the error from the left operand's __add__ method.

This was originally discovered in astropy's test suite.

xref numpy/numpy#31088
xref astropy/astropy#19503

CPython versions tested on:

3.15

Operating systems tested on:

Linux

Reacted by Joren Hammudoglu

Activity

1. neutrinoceros

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Mar 29, 2026
2. serhiy-storchaka commented on Mar 29, 2026

   

   serhiy-storchaka

   on Mar 29, 2026

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   This looks like a duplicate of #146587 opened three hours ago.

   Did you guys get this from the same source?
3. neutrinoceros commented on Mar 29, 2026

   

   neutrinoceros

   on Mar 29, 2026

   ContributorAuthor

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   Did you guys get this from the same source?

   Not that I'm aware. This looks entirely coincidental to me, but of course I'm okay with closing this one as a dupe. Thanks for pointing it out !
4. neutrinoceros

   closed this as a duplicateof Sequence-like subclass __radd__ fallback regression #146587 on Mar 29, 2026
5. neutrinoceros commented on Mar 29, 2026

   

   neutrinoceros

   on Mar 29, 2026

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   Not that I'm aware.

   to be clear what I meant here: I discovered the problem independently about 13h ago and reported it in astropy, then numpy.
   I don't know wether #146587 was an entirely independent discovery or if it built from my early (misplaced) reports.

   Reacted by Serhiy Storchaka
6. serhiy-storchaka commented on Mar 29, 2026

   

   serhiy-storchaka

   on Mar 29, 2026

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   Most likely.
7. inventshah commented on Mar 29, 2026

   

   inventshah

   on Mar 29, 2026

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   Coincidental, but glad there is an established use case! I was only on the beta to play with Tachyon, and couldn't pass up an opportunity to git bisect.

   Reacted by Clément Robert

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