Use `dict.setdefault()` in functools caching for better thread-safety

Feature or enhancement

Proposal:

Currently, functools.cache family of functions uses _PyDict_SetItem_KnownHash. But I am wondering if it shouldn't use SetDefault style access at basically no-cost?

That way, when working with many threads, while multiple evaluations are still possible, all threads are guaranteed to end up with the same object.
There may be great reasons why that isn't wanted, but on first sight it seems like a simple and practically free way to remove a pit-fall for threaded code.
(Of course building a custom version that does this is easy, but functools.cache is great and used quite a lot.)

Admittedly, an optional once= would also be a very cool feature (or maybe as a dedicated pattern).

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-154069

先从 issue 中对 functools.cache、_PyDict_SetItem_KnownHash 和 SetDefault 的描述开始，然后查看链接的 PR gh-154069。在进行任何更改之前，确定预期的线程安全性和对象身份语义；只有在设计获​​得接受并完成相应的实现和测试后，才能算作完成。