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[typing] Add missing `_from_iterable` classmethod to `Set` ABCs by emmanuel-ferdman · Pull Request #15331 · python/typeshed · GitHub

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[typing] Add missing _from_iterable classmethod to Set ABCs - #15331

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srittau merged 4 commits into
python:mainfrom
emmanuel-ferdman:main
Feb 12, 2026
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[typing] Add missing _from_iterable classmethod to Set ABCs#15331
srittau merged 4 commits into
python:mainfrom
emmanuel-ferdman:main

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emmanuel-ferdman commented Jan 27, 2026 •
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PR Summary

This PR adds _from_iterable to AbstractSet (returning AbstractSet[_S] to preserve element types while allowing subclasses that return set), and to KeysView/ItemsView which override it to return set[_S]. MutableSet inherits from AbstractSet so no separate definition is needed.

Fixes #15298.

[typing] Add missing _from_iterable classmethod to Set ABCs
d6136ba 
Signed-off-by: Emmanuel Ferdman <emmanuelferdman@gmail.com>

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randolf-scholz suggested changes Feb 7, 2026
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Comment thread stdlib/typing.pyi Outdated
class ItemsView(MappingView, AbstractSet[tuple[_KT_co, _VT_co]], Generic[_KT_co, _VT_co]):
def __init__(self, mapping: SupportsGetItemViewable[_KT_co, _VT_co]) -> None: ... # undocumented
@classmethod
def _from_iterable(cls, it: Iterable[Any]) -> set[Any]: ...

randolf-scholz Feb 7, 2026

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Why not def [S](cls, it: Iterable[S]) -> set[S] ?

emmanuel-ferdman Feb 7, 2026

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Good call, updated to use Iterable[_S] -> set[_S]. Used the existing _S TypeVar since PEP 695 syntax isn't used elsewhere in typeshed.

Comment thread stdlib/typing.pyi Outdated
Comment thread stdlib/typing.pyi Outdated
def _hash(self) -> int: ...
# Mixin methods
@classmethod
def _from_iterable(cls, it: Iterable[Any]) -> AbstractSet[Any]: ...

randolf-scholz Feb 7, 2026

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This should return Self, not any AbstractSet, since the implementation is return cls(it) and other methods on MutableSet that rely on this are annotated to return Self.

emmanuel-ferdman Feb 7, 2026

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Updated to AbstractSet[_S] for type preservation, but can't use Self here - KeysView and ItemsView override this to return set, not Self. Using Self on the base would make those overrides invalid. Also MutableSet.__ior__/__iand__ etc. don't actually call _from_iterable, they mutate in-place directly.

randolf-scholz Feb 7, 2026

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Actually, the type we want here is more like def [S](cls, Iterable[S]) -> Self & AbstractSet[S], which is not expressible at the moment.

randolf-scholz Feb 7, 2026

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So -> AbstractSet[S] seems like the best approximation atm.

[typing] Add missing _from_iterable classmethod to Set ABCs
490e437 
Signed-off-by: Emmanuel Ferdman <emmanuelferdman@gmail.com>

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randolf-scholz commented Feb 7, 2026 •
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Btw. in the original issue I posted a comment that this is probably a bad idea at the moment -- sorry I didn't mention it here earlier, I didn't realize this was a different account than the OP of #15298

randolf-scholz commented Feb 12, 2026 •
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So, cpython will not support _from_iterable on builtins.set. (python/cpython#144557)

Therefore, the best we can do is put that definition into a if not TYPE_CHECKING block with a comment explaining the situation

randolf-scholz suggested changes Feb 12, 2026
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Comment thread stdlib/typing.pyi
Comment on lines +707 to +708
@classmethod
def _from_iterable(cls, it: Iterable[_S]) -> AbstractSet[_S]: ...

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Suggested change
@classmethod
def _from_iterable(cls, it: Iterable[_S]) -> AbstractSet[_S]: ...
if not TYPE_CHECKING:
# this method exists on collections.abc.Set, but since it is not
# supported by builtins.set, we exclude it from static typing
# see: cpython/issues/144557 and typeshed/issues/15298
@classmethod
def _from_iterable(cls, it: Iterable[_S]) -> AbstractSet[_S]: ...

Comment thread stdlib/typing.pyi
Comment on lines +741 to +742
@classmethod
def _from_iterable(cls, it: Iterable[_S]) -> set[_S]: ...

randolf-scholz Feb 12, 2026

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Suggested change
@classmethod
def _from_iterable(cls, it: Iterable[_S]) -> set[_S]: ...
if not TYPE_CHECKING:
# this method exists on collections.abc.ItemsView, but since it is not
# supported by dict_items, we exclude it from static typing
# see: cpython/issues/144557 and typeshed/issues/15298
@classmethod
def _from_iterable(cls, it: Iterable[_S]) -> set[_S]: ...

Comment thread stdlib/typing.pyi
Comment on lines +756 to +757
@classmethod
def _from_iterable(cls, it: Iterable[_S]) -> set[_S]: ...

randolf-scholz Feb 12, 2026

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Suggested change
@classmethod
def _from_iterable(cls, it: Iterable[_S]) -> set[_S]: ...
if not TYPE_CHECKING:
# this method exists on collections.abc.KeysView, but since it is not
# supported by dict_keys, we exclude it from static typing
# see: cpython/issues/144557 and typeshed/issues/15298
@classmethod
def _from_iterable(cls, it: Iterable[_S]) -> set[_S]: ...

srittau commented Feb 12, 2026

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Therefore, the best we can do is put that definition into a if not TYPE_CHECKING block with a comment explaining the situation

I don't think we should start putting TYPE_CHECKING checks into stub files. I'd rather have these methods not in the stubs than introducing such an idiom in stub files. But as I mentioned in #15298, I'd rather add these methods unconditionally.

github-actions Bot commented Feb 12, 2026

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Diff from mypy_primer, showing the effect of this PR on open source code:

discord.py (https://github.com/Rapptz/discord.py)
- ...typeshed_to_test/stdlib/typing.pyi:1049: note: "update" of "TypedDict" defined here
+ ...typeshed_to_test/stdlib/typing.pyi:1055: note: "update" of "TypedDict" defined here

srittau approved these changes Feb 12, 2026
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Thanks!

srittau merged commit a19c15d into python:main Feb 12, 2026
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collections.abc.Set, etc., missing _from_iterable() classmethod

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