Implement `_from_iterable` on `builtins.set` and`builtins.frozenset`

[randolf-scholz]

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 runtime

Alternatives

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?

python/typeshed#15298

Links to previous discussion of this feature:

No response

[picnixz]

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.