gh-145633: deprecate float.__getformat__() class method

[skirpichev]

• Issue: Remove "unknown_format" code path in PyFloat_Pack/Unpack*() #145633

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📚 Documentation preview 📚: https://cpython-previews--146400.org.readthedocs.build/

[skirpichev]

CC @serhiy-storchaka

[vstinner]

LGTM

[skirpichev]

@vstinner, please don't merge. I would appreciate a second review.

[serhiy-storchaka]

in general, I like removing private, unsystematic, undocumented methods which only serve internal debugging purpose. There some things in bytearray and set. But first we need to look at their history and ask authors who added them. Do they still need them? I think that special testing module would be better place for such things.

[skirpichev]

But first we need to look at their history and ask authors who added them. Do they still need them?

The __getformat__() class method, together with removed __setformat__() was added by ba283e2. CC @mwhudson

I think that special testing module would be better place for such things.

This private API now is helpful only for alternative implementations, which run the CPython test suite. I don't see how a special testing module would help here.

See #145633 (comment) for some usage examples.

[mwhudson]

But first we need to look at their history and ask authors who added them. Do they still need them?

The __getformat__() class method, together with removed __setformat__() was added by ba283e2. CC @mwhudson

Oh dear me that was nearly 21 years ago! Unsurprisingly the details have faded a bit but I'm pretty sure this was around making struct.{pack,unpack} work with NaNs and infs and such. If Python requires IEEE 754 to build then I think it can all be regarded as historical fluff and deprecated then deleted.

[skirpichev]

If Python requires IEEE 754 to build then I think it can all be regarded as historical fluff and deprecated then deleted.

The problem is that the CPython test suite used in alternative implementations, like PyPy. In principle they may not require IEEE-compatible formats for floats.

That's why @mdickinson suggested to keep requires_IEEE_754 decorator. Which, in turn, needs something to check that this requirement is satisfied. New helper does same checks as before, if ctypes module is available.

[serhiy-storchaka]

Would not be better to first add a field to sys.version_info? Then, few versions late, we can deprecate float.__getformat__().

[skirpichev]

Would not be better to first add a field to sys.version_info?

@serhiy-storchaka, not sure I get this part of you proposal. Could you elaborate?

[vstinner]

Can you modify collect_support() to log support.HAVE_IEEE_754?

[skirpichev]

Why do you think this make sense? For CPython builds this flag is True unconditionally.

[vstinner]

To help debugging issues if HAVE_IEEE_754 is false for whatever reasons.

[skirpichev]

I would consider case when sys.implementation.name != 'cpython' for CPython as very unlikely scenario.

[vstinner]

Would not be better to first add a field to sys.version_info?

Do you mean sys.float_info? Which members do you want to add?

new code