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Simplify Placeholder with the new sentinel() class #161031

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rhettinger wants to merge 2 commits into from
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Simplify Placeholder with the new sentinel() class #161031

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b03a69e
Simplify Placeholder with the new sentinel() class
rhettinger Oct 31, 2026
4f77a8c
Remove unused import
rhettinger Oct 31, 2026
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26 changes: 2 additions & 24 deletions Lib/functools.py
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Original file line number Diff line number Diff line change
Expand Up @@ -275,29 +275,7 @@ def reduce(function, sequence, /, initial=_initial_missing):
################################################################################


class _PlaceholderType:
"""The type of the Placeholder singleton.

Used as a placeholder for partial arguments.
"""
__instance = None
__slots__ = ()

def __init_subclass__(cls, *args, **kwargs):
raise TypeError(f"type '{cls.__name__}' is not an acceptable base type")

def __new__(cls):
if cls.__instance is None:
cls.__instance = object.__new__(cls)
return cls.__instance

def __repr__(self):
return 'Placeholder'

def __reduce__(self):
return 'Placeholder'

Placeholder = _PlaceholderType()
Placeholder = sentinel('Placeholder') # _PlaceholderType()

@picnixz picnixz Oct 31, 2026

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I think you want to retain pickability but I'm not sure that way works (I think you need to add the "functools." part for the module).

cc @JelleZijlstra

@rhettinger rhettinger Oct 31, 2026

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Something like this? Placeholder = sentinel('functools.Placeholder', repr='Placeholder')

My understanding from PEP 661 is that this is already handled by the sentinel's __module__ attribute.

@JelleZijlstra JelleZijlstra Oct 31, 2026

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I believe this should be fine, though you can always add a new test.

I made this same change in #149088, along with the corresponding change to the C implementation. I didn't push forward on it because of pushback around hypothetical code relying on type(Placeholder).

@rhettinger rhettinger Oct 31, 2026

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I'm going to revive Jelle's PR which also handles the C implementation.


def _partial_prepare_merger(args):
if not args:
Expand Down Expand Up @@ -440,7 +418,7 @@ def __setstate__(self, state):


try:
from _functools import partial, Placeholder, _PlaceholderType
from _functools import partial, Placeholder
except ImportError:
pass

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7 changes: 0 additions & 7 deletions Lib/test/test_functools.py
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Original file line number Diff line number Diff line change
Expand Up @@ -279,13 +279,6 @@ def test_placeholders_kw_restriction(self):
self.assertEqual(len(actual_kwds), 1)
self.assertIs(actual_kwds['a'], ALWAYS_EQ)

def test_construct_placeholder_singleton(self):
PH = self.module.Placeholder
tp = type(PH)
self.assertIs(tp(), PH)
self.assertRaises(TypeError, tp, 1, 2)
self.assertRaises(TypeError, tp, a=1, b=2)

def test_repr(self):
args = (object(), object())
args_repr = ', '.join(repr(a) for a in args)
Expand Down
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