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Following up on issue 25658, it was found that the current definition of Py_tss_NEEDS_INIT restricts its use to initialisers in C and cannot be used for arbitrary assignments. It is currently declared as follows:
 #define Py_tss_NEEDS_INIT {0}
which results in a C compiler error for assignments like "x = Py_tss_NEEDS_INIT".
I proposed to change this to
 #define Py_tss_NEEDS_INIT ((Py_tss_t) {0})
in compliance with GCC and C99, but that fails to compile in MSVC and probably other old C++-ish compilers.
I'm not sure how to improve this declaration, but given that it's a public header file, restricting its applicability seems really unfortunate.

Right, the cases we were aiming to cover were:
- C variable declarations ("static Py_tss_t tss_key = Py_tss_NEEDS_INIT;")
- dynamic allocation with PyThread_tss_alloc
- resetting a key back to the uninitialised state with PyThread_tss_delete
The problem we have is that the second field in Py_tss_t is platform dependent, and not all platforms define a safe "unused" value for their NATIVE_TSS_KEY_T, which means Py_tss_NEEDS_INIT ends up being only a partial initialiser (earlier versions of the PEP used a field initialiser, but we were asked to switch it to a partial initialiser in order to support more compilers).
We *could* offer a `PyThread_tss_reset` (to reset a key back to Py_tss_NEEDS_INIT), but that's confusingly similar to PyThread_tss_delete.
Another option would be to check for typed partial initialiser support in the configure script, and declare Py_tss_NEEDS_INIT accordingly. However, that wouldn't solve the problem for any clients that are themselves also attempting to write cross-platform code.

Or Py_tss_NEEDS_INIT is removed from the C API document to become a private feature; in other words, the Python interpreter will recommend that the static allocation for the TSS key shouldn't be used in the API client codes except for the interpreter core and built-in modules. (okay, I know it's an unfair suggestion)
BTW, it's my mistake which the word "default value" for Py_tss_NEEDS_INIT, actually it makes more sense "initializer". I'd like to fix the document and the PEP, would I open the PR with this issue?

For the wording fix, since it's just a trivial docs update, we can go ahead and fix it without an issue reference from the PRs.
As far as the API goes, I do want to support "static Py_tss_t my_tss_key = Py_tss_NEEDS_INIT;" in third party extension modules, which is what the current API gives us.
The question is whether there's a reasonable compiler independent way to also support struct *assignment*, in addition to initialisation.