Floating-point/double is used, i.e. "optional" IEEE in C[11]

[PallHaraldsson]

It may be unclear to many but C does not specify floating point. I.e. what float and double mean exactly. But Python's floats use double/Float64.

So that means at least one optional C feature is used, Annex F (__STDC_IEC_559__)?

This is sort of a nitpick/question.

But another issue with this PEP is "XP" unexplained. I believe it means "extreme programming", and maybe it's best to link it. Not all may be familiar so maybe it's NOT better to (simply) spell it out.

https://stackoverflow.com/questions/31181897/status-of-stdc-iec-559-with-modern-c-compilers

https://softwarerecs.stackexchange.com/questions/78793/is-there-any-c-compiler-which-defines-both-stdc-and-stdc-iec-559-to-1

https://gcc.gnu.org/onlinedocs/gcc/Floating-point-implementation.html

The accuracy of the floating-point operations and of the library functions in <math.h> and <complex.h> that return floating-point results (C90, C99 and C11 5.2.4.2.2).
The accuracy is unknown.

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[gvanrossum]

I feel that the addition, as proposed, does not make the PEP clearer for most people. And IEEE floating point is what’s provided by all supported platforms.

[PallHaraldsson]

I think at least https://en.wikipedia.org/wiki/Extreme_programming or other link for XP could be added.

I know you can by now expect IEEE floating point, and in all C compilers. Maybe write "C11 with IEEE floating-point, and otherwise no optional features"?

I forget does Python use 64-bit floats on all platforms, or 80-bit (registers) on x86 (with 64-bit for storage format)?

[encukou]

Would it be better to list the optional features? Or remove the note entirely, and leave the necessary standards extensions unspecified?

In addition to the Annex F floats, CPython 3.13 now also needs C11 atomics (or MSVC).