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gh-149083: Convert some private and simple sentinels to PEP 661 by JelleZijlstra · Pull Request #149084 · python/cpython · GitHub

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gh-149083: Convert some private and simple sentinels to PEP 661 - #149084

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JelleZijlstra:more-sentinels
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gh-149083: Convert some private and simple sentinels to PEP 661#149084
JelleZijlstra wants to merge 9 commits into
python:mainfrom
JelleZijlstra:more-sentinels

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JelleZijlstra commented Apr 28, 2026 •
edited by bedevere-app Bot
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bedevere-app Bot mentioned this pull request Apr 28, 2026
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picnixz reviewed Apr 28, 2026
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

There is a sentinel in hashlib.py to detect usage of arguments and raise appropriate warnings/errors so could you also update it? it's a hacky code because I needed to mirror the C behavior in some sense.

picnixz reviewed Apr 28, 2026
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

In dataclasses, there is also _FIELD, _FIELD_CLASSVAR and _FIELD_INITVAR that you could update I think?

JelleZijlstra commented Apr 28, 2026

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I was going to leave those dataclasses objects alone since they're instances of a common class, and I wouldn't be able to preserve that behavior.

jonathandung commented Apr 30, 2026

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base64._NOT_SPECIFIED is also a candidate (I don't know why, but it's currently a list of one string)

JelleZijlstra commented May 4, 2026
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Comment thread Lib/configparser.py Outdated

read-the-docs-community Bot commented May 6, 2026

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Documentation build overview

📚 cpython-previews | 🛠️ Build #32562194 | 📁 Comparing 0ebd184 against main (70e365c)

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71 files changed · ± 70 modified · - 1 deleted

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sobolevn mentioned this pull request May 8, 2026
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JelleZijlstra added the needs backport to 3.15 pre-release feature fixes, bugs and security fixes label May 9, 2026
serhiy-storchaka reviewed May 9, 2026
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

I think that using names like "<not specified>", "<omitted>", etc for sentinels shown in function signatures will more clearly show that the behavior when the argument is not specified differs from the behavior when the argument is specified with any value.

Currently, for functions implemented in Argument Clinic it is shown as <unrepresentable>, but <not specified> would be better in most cases.

serhiy-storchaka reviewed Jun 6, 2026
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

No need to use sentinel() for sentinels not visible to users.

Comment thread Lib/_collections_abc.py
raise KeyError

__marker = object()
__marker = sentinel("__marker", repr="<marker>")

serhiy-storchaka Jun 6, 2026

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Would not it be better to stick to some standard representation like <not specified> for sentinels only exposed in function signatures?

pop(self, key, default=<not specified>)

And why do we need a name for private sentinels? They are not supposed to be pickleable.

Comment thread Lib/configparser.py


UNNAMED_SECTION = _UnnamedSection()
UNNAMED_SECTION = sentinel("UNNAMED_SECTION", repr="<UNNAMED_SECTION>")

serhiy-storchaka Jun 6, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

UNNAMED_SECTION is in the public API, it should not have obscure repr.

JelleZijlstra Jun 6, 2026

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What would you prefer?

Comment thread Lib/functools.py
################################################################################

_NOT_FOUND = object()
_NOT_FOUND = sentinel("_NOT_FOUND", repr="<not found>")

serhiy-storchaka Jun 6, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

It is not visible to user. It can remain object().

JelleZijlstra Jun 6, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

I think it's still cleaner to use sentinel. Also, users inspecting the cache for debugging purposes can see this object.

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