gh-141510: Fix frozendict.fromkeys() for subclasses

[vstinner]

Don't call the constructor of the subclass, but frozendict constructor instead.

• Issue: PEP 814: Add built-in frozendict type #141510

[corona10]

I think we should discuss this before taking any action here.
Do we really need to handle cases where users shoot themselves in the foot? It doesn’t cause a crash at the moment, right?

[vstinner]

Currently, frozendict.fromkeys() allows modifying an immutable frozendict using a special subclass.

Example:

classSpecialDict(frozendict):
def__new__(self):
returnfrozendict(x=None)
print(SpecialDict.fromkeys("y"))

Current output:

frozendict({'x': None, 'y': None})

That's bad: the immutable frozendict is mutated!

[corona10]

How about copy whole object at

cpython/Objects/dictobject.c

Line 3319 in dd64e42

elseif (PyFrozenDict_CheckExact(d)) {

not to mutate original frozendict simply?

It follows the original intention of immutable data structures: copy instead of mutating.

[vstinner]

How about copy whole object

Ok, good idea: I wrote #144952 to copy the frozendict.

[vstinner]

I close this PR in favor of #144952 fix.