Remove the float.__set_format__() method

[vstinner]

BPO	46852
Nosy	@tim-one, @rhettinger, @mdickinson, @vstinner
PRs	bpo-46852: Rename float.__set_format__() to float.__setformat__() #31558 [3.10] bpo-46852: Rename float.__set_format__() to float.__setformat__() (GH-31558) #31578 [3.9] bpo-46852: Rename float.__set_format__() to float.__setformat__() (GH-31558) #31581 bpo-46852: Remove the float.__set_format__() method #31585 bpo-46852: Building Python requires IEEE 754 support #31592 bpo-46852: Restore test_getformat() test #31601

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2022-03-03.23:39:31.420>
created_at = <Date 2022-02-25.00:07:57.345>
labels = ['interpreter-core', '3.11']
title = 'Remove the float.__set_format__() method'
updated_at = <Date 2022-03-03.23:39:31.419>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2022-03-03.23:39:31.419>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2022-03-03.23:39:31.420>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2022-02-25.00:07:57.345>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 46852
keywords = ['patch']
message_count = 17.0
messages = ['413943', '413946', '413947', '413948', '413949', '413950', '413953', '413997', '414005', '414011', '414018', '414050', '414131', '414150', '414152', '414158', '414493']
nosy_count = 4.0
nosy_names = ['tim.peters', 'rhettinger', 'mark.dickinson', 'vstinner']
pr_nums = ['31558', '31578', '31581', '31585', '31592', '31601']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue46852'
versions = ['Python 3.11']

[vstinner]

It has been decided to require IEEE 754 to build Python 3.11:
https://mail.python.org/archives/list/python-dev@python.org/thread/J5FSP6J4EITPY5C2UJI7HSL2GQCTCUWN/

At Python startup, _PyFloat_InitState() checks the IEEE 754 format at runtime. It can be changed using float.__get_format__() and float.__set_format__() methods.

These methods docstrings say that they only exist to test Python itself:

"You probably don't want to use this function. It exists mainly to be used in Python's test suite."

These methods are private and not documented.

I propose to remove them.

Once they will be removed, it will become possible to move the detection of the IEEE 754 format in the build step (./configure script) rather than doing the detection at runtime (slower). It would remove an "if" in _PyFloat_Pack4() and _PyFloat_Pack8(), and allow to specialize these functions for the detected format at build time. These functions are used by serialization formats: marshal, pickle and struct.

[rhettinger]

I would not miss these methods. Unless Mark says they are needed, +1 for removal.

[vstinner]

See also bpo-46656: "Remove the Py_NO_NAN macro: require NAN to build Python 3.11".

[vstinner]

Oh wait, I'm now confused by the method names. In Python 3.10, the correct names at:

• float.__getformat__() <= 4 underscores
• float.__set_format__() <= 5 underscores

It's even more confusing because the "set format" is only used in one place: test_float, and test_float uses... __setformat__() (4 underscores).

A typo a was introduced in Python 3.7 by:

commit b5c51d3
Author: Serhiy Storchaka <storchaka@gmail.com>
Date: Sat Mar 11 09:21:05 2017 +0200

bpo-20185: Convert float object implementation to Argument Clinic. (#543)

Based on patch by Vajrasky Kok.

Since Python 3.7, the 4 "set format" tests are simply skipped!

$ ./python -m test -v test_float
(...)
test_getformat (test.test_float.FormatFunctionsTestCase) ... skipped 'requires __setformat__'
test_setformat (test.test_float.FormatFunctionsTestCase) ... skipped 'requires __setformat__'
(...)
test_double_specials_dont_unpack (test.test_float.UnknownFormatTestCase) ... skipped 'requires __setformat__'
test_float_specials_dont_unpack (test.test_float.UnknownFormatTestCase) ... skipped 'requires __setformat__'
(...)

Moreover, unittest.mock supports mocking __setformat__() (4 underscores).

[vstinner]

I wrote #75739 to fix the typo in the method name.

[vstinner]

We can keep the float.__getformat__() method, it doesn't harm. I change the issue title to only propose to remove the float.__setformat__() method.

[vstinner]

New changeset 7d03c8b by Victor Stinner in branch 'main':
bpo-46852: Rename float.__set_format__() to float.__setformat__() (GH-31558)
7d03c8b

[vstinner]

New changeset 0848da1 by Victor Stinner in branch '3.10':
bpo-46852: Rename float.__set_format__() to float.__setformat__() (GH-31558) (GH-31578)
0848da1

[vstinner]

New changeset a549cd1 by Victor Stinner in branch '3.9':
bpo-46852: Rename float.__set_format__() to float.__setformat__() (GH-31558) (GH-31581)
a549cd1

[mdickinson]

I'd be happy to see float.__setformat__ go, if it's not still needed for Python's test suite (which was its entire raison d'être). If no-one noticed the accidental misnaming, then it's pretty clear no-one's been using it.

I'd like to bet that there are at least a few people out there using float.__getformat__, despite that its docstring says "You probably don't want to use this function".

Maybe we could consider moving the information contained in __getformat__ to somewhere more accessible (e.g., a new field in sys.float_info)?

[vstinner]

Mark Dickinson:

I'd be happy to see float.__setformat__ go, if it's not still needed for Python's test suite (which was its entire raison d'être). If no-one noticed the accidental misnaming, then it's pretty clear no-one's been using it.

Nobody noticed the since Python 3.7 (released in June 2018). Well, even test_float didn't use it :-D (I just fixed the typo yesterday.) So I expect that no one uses it.

I'd like to bet that there are at least a few people out there using float.__getformat__, despite that its docstring says "You probably don't want to use this function".

Yeah, I changed my mind and I prefer to leave it unchanged for now. It doesn't prevent me to optimize _PyFloat_Pack8().

[vstinner]

New changeset 5ab745f by Victor Stinner in branch 'main':
bpo-46852: Remove the float.__set_format__() method (GH-31585)
5ab745f

[vstinner]

New changeset 5a1c637 by Victor Stinner in branch 'main':
bpo-46852: Restore test_getformat() test (GH-31601)
5a1c637

[mdickinson]

Thanks, Victor. I think this can be closed now.

[vstinner]

I reopen the issue for the second part of my plan:

"Once they will be removed, it will become possible to move the detection of the IEEE 754 format in the build step (./configure script) rather than doing the detection at runtime (slower). It would remove an "if" in _PyFloat_Pack4() and _PyFloat_Pack8(), and allow to specialize these functions for the detected format at build time. These functions are used by serialization formats: marshal, pickle and struct."

[mdickinson]

I reopen the issue for the second part of my plan

Hmm. That sounds like it should be a separate issue, or at the least, this issue should be retitled. It's helpful to keep issue titles accurate.

[vstinner]

I created a follow-up issue: bpo-46917 "Require IEEE 754 floating point to build Python 3.11". I close this one: float.__set_format__() has been removed.