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

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

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@skirpichev skirpichev commented Sep 12, 2025 •
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This value indicating support the IEC 60559 floating-point standard (the Annex F of C99). If enabled, the float type characteristics matches the IEC 60559 double format and exceptional cases for the math's functions follow to the section F.10 of the C99 standard.

📚 Documentation preview 📚: https://cpython-previews--138811.org.readthedocs.build/

https://cpython-previews--138811.org.readthedocs.build/en/138811/library/sys.html#sys.float_info
https://cpython-previews--138811.org.readthedocs.build/en/138811/library/math.html#math.nan

@skirpichev
pythongh-138580: add sys.float_info.stdc_iec_559 boolean flag
cffad22 
This value indicating support the IEC 60559 floating-point standard (the
Annex F of C99).  If enabled, the float type characteristics matches the
IEC 60559 double format and exceptional cases for the math's functions
follow to the section F.10 of the C99 standard.
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@skirpichev skirpichev mentioned this pull request Sep 12, 2025
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picnixz commented Sep 13, 2025

I think it would be better if the field was in uppercase but I don't know if we have many other cases like that.

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I think it would be better if the field was in uppercase

This will be inconsistent wrt the rest (other field names are also come from macros).

vstinner
vstinner reviewed Sep 13, 2025
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@skirpichev skirpichev requested a review from vstinner September 13, 2025 12:59
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LGTM

@skirpichev @vstinner
Update Objects/floatobject.c
4e6ba85 
Co-authored-by: Victor Stinner <[email protected]>
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I'm thinking if it does make sense to mention this flag in cmath docs.

We could say that:

  1. Functions selection mostly follows to the Annex G.6 section (modulo conversions and classifications functions). Currently we just mention that functions set is different wrt the math module.
  2. Functions behavior follows to the Annex G.6 in exceptional cases (well, with some exceptions, see gh-133895: correct values of cmath.cosh/sinh in case of overflows #135603, which aren't user-visible right now), if sys.float_info.stdc_iec_559 is true.

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@picnixz @serhiy-storchaka: What do you think of this change?

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I am not sure what is the best place for this constant. What do @mdickinson and @tim-one think about this? Other options are discussed in the issue.

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picnixz commented Sep 17, 2025

I'm not really comfortable with float_info but I don't really know whether a module-wide constant is better. I would be more comfortable with a new struct, say stdc_info or features_info which contains other feature for the C standard.

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