Document the special frozendict.fromkeys() behavior

Call the type constructor with a frozendict for all frozendict
subclasses.

Author: vstinner

Doc/library/stdtypes.rst

@@ -5757,6 +5757,11 @@ Frozen dictionaries
    Like dictionaries, frozendicts are :ref:`generic <generics>` over two types,
    signifying (respectively) the types of the frozendict's keys and values.
 
+   .. classmethod:: fromkeys(iterable, value=None, /)
+
+      Similar to :meth:`dict.fromkeys`, but call the type constructor with a
+      :class:`frozendict` if the type is a :class:`frozendict` subclass.
+
    .. versionadded:: 3.15
 
 

Lib/test/test_dict.py

@@ -1955,7 +1955,8 @@ def __new__(cls, *args, **kwargs):
         self.assertEqual(type(fd), FrozenDictSubclass)
         self.assertEqual(created, frozendict(x=1))
 
-        # Dict subclass which overrides the constructor
+        # Dict subclass with a constructor which returns a frozendict
+        # by default
         class DictSubclass(dict):
             def __new__(cls, *args, **kwargs):
                 if args or kwargs:

Objects/dictobject.c

@@ -3419,12 +3419,9 @@ _PyDict_FromKeys(PyObject *cls, PyObject *iterable, PyObject *value)
     PyObject *d;
     int need_copy = 0;
 
-    PyTypeObject *cls_type = _PyType_CAST(cls);
-    if (PyObject_IsSubclass(cls, (PyObject*)&PyFrozenDict_Type)
-        && cls_type->tp_new == frozendict_new)
-    {
+    if (cls == (PyObject*)&PyFrozenDict_Type) {
         // gh-151722: Create a frozendict which is not tracked by the GC.
-        d = frozendict_new_untracked(cls_type);
+        d = frozendict_new_untracked(&PyFrozenDict_Type);
     }
     else {
         // Dict subclass, or frozendict subclass which overrides