Make PySet_Add() work with frozensets. Works like PyTuple_SetItem() to build-up values in a brand new frozenset.

Doc/c-api/set.rst

@@ -112,9 +112,6 @@ or :class:`frozenset` or instances of their subtypes.
    the *key* is unhashable. Raise :exc:`PyExc_SystemError` if *anyset* is not a
    :class:`set`, :class:`frozenset`, or an instance of a subtype.
 
-The following functions are available for instances of :class:`set` or its
-subtypes but not for instances of :class:`frozenset` or its subtypes.
-
 
 .. cfunction:: int PySet_Add(PyObject *set, PyObject *key)
 
@@ -124,6 +121,14 @@ subtypes but not for instances of :class:`frozenset` or its subtypes.
    Raise a :exc:`SystemError` if *set* is an not an instance of :class:`set` or its
    subtype.
 
+   .. versionchanged:: 2.6
+      Now works with instances of :class:`frozenset` or its subtypes.
+      Like :cfunc:`PyTuple_SetItem` in that it can be used to fill-in the
+      values of brand new frozensets before they are exposed to other code.
+
+The following functions are available for instances of :class:`set` or its
+subtypes but not for instances of :class:`frozenset` or its subtypes.
+
 
 .. cfunction:: int PySet_Discard(PyObject *set, PyObject *key)
 

Objects/setobject.c

@@ -2198,10 +2198,6 @@ PySet_Discard(PyObject *set, PyObject *key)
 int
 PySet_Add(PyObject *set, PyObject *key)
 {
-	if (!PyType_IsSubtype(Py_TYPE(set), &PySet_Type)) {
-		PyErr_BadInternalCall();
-		return -1;
-	}
 	return set_add_key((PySetObject *)set, key);
 }