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Implement _from_iterable on builtins.set andbuiltins.frozenset #144557
Description
Proposal:
collections.abc.{Set, MutableSet} implement a _from_iterable method, documented here: https://docs.python.org/3/library/collections.abc.html
However, builtins.frozenset and builtins.set, which are marked as inherting from Set and MutableSet, do not implement this constructor, therefore typeshed is kind of blocked from adding these methods in the stubs, as it would lead to unsafe code:
def f(x: MutableSet) -> MutableSet:
# passes type checking if _from_iterable is added to MutableSet
return type(x)._from_iterable(x)
f( {1,2,3} ) # Error at runtimeAlternatives
typeshed could just decide to never add the _from_iterable method, people can still override it regardless to get the desired runtime behavior on types that use the Set and MutableSet mixins that rely on this method.
Has this already been discussed elsewhere?
Links to previous discussion of this feature:
No response
Activity
picnixz commentedon Feb 7, 2026
I am not entirely sure we can do anything here. Changing builtins requires a PEP. If this is for the sake of completeness with typing, I'm not sure it's worth.
randolf-scholz commentedon Feb 11, 2026
That's totally fair, but important feedback for
typeshedon whether to include this method in the stubs.