gh-145119: Allow frozendict to be assigned to instance __dict__

[mohsinm-dev]

Allow frozendict (and its subclasses) to be assigned to an instance's __dict__, enabling immutable instances.

Changes:

• _PyObject_SetDict: accept frozendict via PyAnyDict_Check
• _PyDict_SetItem_LockHeld: raise TypeError on set/delete instead of SystemError
• object___dir___impl: convert frozendict to a mutable dict for merging

• Issue: Consider allowing frozendicts to be assigned to instance dicts #145119

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid frozendict dict on objects mutating their own dict

Allowing _PyObject_SetDict() to accept PyAnyDict_Check here makes frozendict assignable to every PyObject_GenericSetDict user, but some of those types still perform internal writes with PyDict_SetItem/PyDict_DelItem (for example descriptor_get_wrapped_attribute and descriptor_set_wrapped_attribute in Objects/funcobject.c for classmethod/staticmethod annotation caching). PyDict_SetItem still rejects frozendict with PyErr_BadInternalCall, so after obj.__dict__ = frozendict(...), operations like reading or updating __annotations__ can now raise SystemError instead of behaving normally or reporting an intentional immutability error.

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[skirpichev]

In order to keep the commit history intact, please avoid squashing or amending history and then force-pushing to the PR. Reviewers often want to look at individual commits. When the PR is merged, everything will be squashed into a single commit.

[vstinner]

Please wait for #145517. If PyDict_Copy() is modified to always return a dict, you can reuse the PyDict_Check() branch for frozendict.

[vstinner]

For now, you can use _PyDict_CopyAsDict() from Include/internal/pycore_dict.h.

[vstinner]

Suggested change

	"__dict__ must be set to a dictionary, "
	"not a '%.200s'", Py_TYPE(value)->tp_name);
	"__dict__ must be set to a dict or frozendict, "
	"not a %T", value);

[vstinner]

_PyDict_SetItem_LockHeld: raise TypeError on set/delete instead of SystemError

I extracted these changes from your PR and created a new PR based on it: PR gh-145564.

[StanFromIreland]

The attached issue was closed.