gh-141510: Support frozendict in plistlib

[hugovk]

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

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📚 Documentation preview 📚: https://cpython-previews--145590.org.readthedocs.build/

[vstinner]

LGTM.

@serhiy-storchaka@ronaldoussoren: Do you want to double check this plistlib change?

[serhiy-storchaka]

Please add a NEWS entry. Note that support of frozendict is half-hearted -- only for serialization. dict_type=frozendict does not work in load(). I am not sure that it is worth to add support for deserialization.

[vstinner]

Oh, can you also add plistlib to https://docs.python.org/dev/whatsnew/3.15.html#whatsnew315-frozendict?

[vstinner]

Please add a NEWS entry. Note that support of frozendict is half-hearted -- only for serialization. dict_type=frozendict does not work in load(). I am not sure that it is worth to add support for deserialization.

I saw that and hesitated to propose a follow-up change to add frozendict support for deserialization. It should be possible to create a (mutable) dict, populate the dictionary, and then convert the dict to a frozendict.

[serhiy-storchaka]

It should be possible to create a (mutable) dict, populate the dictionary, and then convert the dict to a frozendict.

This can add an overhead and this would not work in case of cyclic references in binary Plist.

Originally, dict_type was only added to support OrderedDict. Now, when dict is ordered, this parameter is mostly useless. If there was other use cases for this feature, we would need also an option to override the list type.

So, there is very little value in such feature, and it is very difficult to do without breaking corner cases. But this should be explicitly stated that frozendicts are supported only in one way (like tuple).

[vstinner]

LGTM