Behaviour change in defaultdict.__missing__ between 3.13.11 and 3.13.12

Bug report

Bug description:

I have observed a subtle change in behavior of defaultdict.__missing__ between Python 3.13.11 and 3.13.12. Consider the following output:

3.13.11:

$ uv run --python 3.13.11 python -c 'from collections import defaultdict; print(defaultdict(lambda: "A", {None: "B"}).__missing__(None))'
Using CPython 3.13.11
Creating virtual environment at: .venv
      Built pdonnx @ file:///Users/christian.bourjau/repos/pdonnx
Installed 1 package in 1ms
A

3.13.12:

$ uv run --python 3.13.12 python -c 'from collections import defaultdict; print(defaultdict(lambda: "A", {None: "B"}).__missing__(None))'
Using CPython 3.13.12
Removed virtual environment at: .venv
Creating virtual environment at: .venv
Installed 1 package in 1ms
B

The same change in behavior can be observed between 3.14.2 and 3.14.3.

This might be related to #142495 and https://github.com/python/cpython/pull/142668

Was this an intentional fix?

CPython versions tested on:

3.13

Operating systems tested on:

macOS

Linked PRs

• gh-148880
• gh-148881

Inizia riproducendo il comportamento segnalato di defaultdict.missing(None) su Python 3.13.11, 3.13.12, 3.14.2 e 3.14.3, quindi leggi il contesto nelle issues #142495 e pull request #142668. Esamina i PRs collegati gh-148880 e gh-148881; il lavoro è completato quando il comportamento previsto è stato stabilito e l’issue ha una conclusione chiara o un follow-up.