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GeezerVison is showing github.com — gh-161635: Add PyList_AsTupleAndClear() function by vstinner · Pull Request #161637 · python/cpython

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@vstinner

@vstinner vstinner commented Nov 8, 2026 •
edited by bedevere-app Bot
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  • Rename _PyList_AsTupleAndClear() to PyList_AsTupleAndClear().
  • The function now raises TypeError if the argument is not a list.
  • Add documentation and tests.
  • Issue: [C API] Add PyList_AsTupleAndClear() function #161635

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@vstinner
pythongh-161635: Add PyList_AsTupleAndClear() function
0823270 
* Rename _PyList_AsTupleAndClear() to PyList_AsTupleAndClear().
* The function now raises TypeError if the argument is not a list.
* Add documentation and tests.
@vstinner
vstinner requested a review from ZeroIntensity as a code owner November 8, 2026 18:44
@bedevere-app bedevere-app Bot added awaiting core review type-feature A feature request or enhancement labels Nov 8, 2026
@bedevere-app bedevere-app Bot mentioned this pull request Nov 8, 2026
[C API] Add PyList_AsTupleAndClear() function #161635
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@read-the-docs-community

read-the-docs-community Bot commented Nov 8, 2026 •
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Documentation build overview

📚 cpython-previews | 🛠️ Build #35738480 | 📁 Comparing fa0dea3 against main (c8eabcc)

  🔍 Preview build  

3 files changed
± c-api/list.html
± whatsnew/3.16.html
± whatsnew/changelog.html
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sobolevn
sobolevn reviewed Nov 8, 2026
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@sobolevn sobolevn left a comment

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Great stuff! This would be useful for us in msgspec's type convertion :)

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Comment thread Lib/test/test_capi/test_list.py Resolved

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Comment thread Lib/test/test_capi/test_list.py
self.assertEqual(lst, [])

lst = [] # empty list
self.assertIs(list_as_tuple_and_clear(lst), ())

@sobolevn sobolevn Nov 8, 2026

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question: is () being a singleton a contract or implementation detail? if the former, it would be better to compare the equality.

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@vstinner vstinner Nov 8, 2026

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Ah, that's a tricky question. It's more an implementation detail. But I would like to test this implementation detail, to check that we don't loose this optimization in a future refatoring.

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vstinner added 2 commits November 8, 2026 20:28
@vstinner
Test also list subclass
c8b5153
@vstinner
Add What's New and Changelog entries
fa0dea3
@vstinner
vstinner requested a review from AA-Turner as a code owner November 8, 2026 19:49
@sobolevn

sobolevn commented Nov 8, 2026

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@vstinner should we do the same with PySet_AsFrozenSetAndClear()?
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@vstinner

vstinner commented Nov 8, 2026

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@vstinner should we do the same with PySet_AsFrozenSetAndClear()?

I found a few functions calling PyFrozenSet_New(iterable) and then discards iterable:

  • _PyCode_ConstantKey(): from tuple
  • make_union(): from set
  • load_frozenset(): from tuple

So only make_union() with its unionbuilder API may benefit from adding a PySet_AsFrozenSetAndClear() function. Maybe we can start by adding a private function? Do you want to add such private function?

I'm not sure that it's worth it to add a public PySet_AsFrozenSetAndClear() function right now.

... Also, I'm working on PyFrozenSetWriter API which should cover more use cases: #160154 :-)
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@sobolevn

sobolevn commented Nov 8, 2026

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Do you want to add such private function?

No, I would prefer to have a well-designed public API :)
PyFrozenSetWriter seems promising! 🎉
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@sobolevn sobolevn sobolevn left review comments
@ZeroIntensity ZeroIntensity Awaiting requested review from ZeroIntensity ZeroIntensity is a code owner
@AA-Turner AA-Turner Awaiting requested review from AA-Turner AA-Turner is a code owner

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@vstinner @sobolevn
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