Provide access to the __STDC_IEC_559__ macro

[skirpichev]

Feature or enhancement

Proposal:

This e.g. can allow us filter out some tests on systems, which don't conform to the C Annex F. Example: #138573.

I'm not sure where to put this stuff. In order of my preference:

1. New boolean variable in the sys module?
2. sys.float_info field? Though, sphinx docs says "The values correspond to the various floating-point constants defined in the standard header file float.h for the ‘C’ programming language".
3. in the math module?

Edit:

• sysconfig variable as another place

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-138580: add sys.float_info.stdc_iec_559 boolean flag #138811

[picnixz]

I don't really think float_info is a good place as __STDC_IEC_559__ also inidicates how various math functions behave so it's not just about floating-point values themselves (float_info is really about the representation of floats, not how the functions acting on it would behave).

I believe we could also add a new namespace in sys for features macros in general. Something like sys.features_macros. However, that's only if we plan to add more macros there and apart from __STDC_IEC_60559_COMPLEX__ and __STDC_IEC_60559_TYPES__, I don't think we have much to add there.

Adding it to math could be ok but functions affected by __STDC_IEC_559__ are not all in math.h and I feel weird to do if math.<whatever_flags>. math is rather home for mathematical constants and functions rather than the standards they follow. So, I think it's better that we keep in sys.

[emmatyping]

I agree noting the C headers that are relevant to match behavior would be nice to do. Would it make sense to put the values in sysconfig however, since that is where platform configuration variables usually live?

[skirpichev]

I don't really think float_info is a good place as __STDC_IEC_559__ also inidicates how various math functions behave

Not only. F.10 is just one of 10 sections.

Adding it to math could be ok

For same reasons, math looks not very appropriate.

I believe we could also add a new namespace in sys for features macros in general. Something like sys.features_macros.

The problems is that IMO we don't have too much macros to expose. E.g. __STDC_IEC_60559_COMPLEX__ right now is in 99.9?% cases just a plain lie (or a compiler bug, whatever).

Would it make sense to put the values in sysconfig however, since that is where platform configuration variables usually live?

Maybe. Though, this information is not used to configure CPython.

CC @FFY00 per experts index

[skirpichev]

CC @vstinner

[vstinner]

If the goal is to skip some tests, sysconfig is a good place for such variable.

[skirpichev]

If the goal is to skip some tests

This is just an example.

Another one is documentation. Currently we have:

The math module consists mostly of thin wrappers around the platform C math library functions. Behavior in exceptional cases follows Annex F of the C99 standard where appropriate.

That's a lie, strictly speaking.

[vstinner]

The documentation should be updated in this case.

[skirpichev]

PR #138811 is ready.

After some thinking I chose sys.float_info over new entry in the sys namespace. I think that will improve discoverability of this flag and it's logically belongs to float_info (i.e. indicates that the float type matches a particular IEC 60559 format).

If it's ok, I'll also update the math's documentation in this pr.

[serhiy-storchaka]

What is the use of that flag in the user code?