gh-141510: Always return a dict in PyDict_Copy()

[vstinner]

• PyDict_Copy() now also returns a dict if the argument is a frozendict.
• Remove _PyDict_CopyAsDict() function.
• Fix frozendict.items() ^ frozendict.items(). Add non-regression test.

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

---

📚 Documentation preview 📚: https://cpython-previews--145517.org.readthedocs.build/

[vstinner]

I propose changing PyDict_Copy() API to always return a dict. Previously, it returned a frozendict if it was called on a frozendict. IMO it's less surprising and more convenient that PyDict_Copy() always return a copy (as a dict).

Example in typeobject.c:

static PyTypeObject*
type_new_init(type_new_ctx *ctx)
{   
    PyObject *dict = PyDict_Copy(ctx->orig_dict);
    if (dict == NULL) {
        goto error;
    }
    
    ...
    set_tp_dict(type, dict);
    ...
}

The function accepts dict or frozendict but the type dictionary (dict variable) must be a dict.

Example in Modules/_elementtree.c:

static PyObject*
get_attrib_from_keywords(PyObject *kwds)
{
    ...
    if (attrib) {
        ...
        Py_SETREF(attrib, PyDict_Copy(attrib));
    }
    else {
        attrib = PyDict_New();
    }

    if (attrib != NULL && PyDict_Update(attrib, kwds) < 0) {
        Py_DECREF(attrib);
        return NULL;
    }
    return attrib;
}

(With my pending change PR gh-145508), the function accepts dict or frozendict but the result must be a dict (for example, we call PyDict_Update() on it).

cc @corona10 @ZeroIntensity @eendebakpt

[vstinner]

Currently, PyDict_Copy(frozendict) returns a frozendict copy which is something used internally to modify an immutable frozendict! But I don't think that we should leak such implementation details to the public C API.

Existing C extensions calling PyDict_Copy() expect it to return a dict. If it can return a frozendict on Python 3.15, it can trigger bugs.

Always returning dict avoids these issues.