diff --git a/Objects/setobject.c b/Objects/setobject.c
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -2345,6 +2345,45 @@
     return set_update_internal((PySetObject *)set, iterable);
 }
 
+PyObject *
+_PySet_Intern(PyObject *set, PyObject *key)
+{
+    PySetObject *so;
+    setentry *entry;
+    Py_hash_t hash;
+
+    if (!PySet_Check(set)) {
+        PyErr_BadInternalCall();
+        return NULL;
+    }
+    so = (PySetObject *)set;
+    if (!PyUnicode_CheckExact(key) ||
+        (hash = ((PyASCIIObject *) key)->hash) == -1) {
+        hash = PyObject_Hash(key);
+        if (hash == -1)
+            return NULL;
+    }
+    entry = so->lookup(so, key, hash);
+    if (entry == NULL)
+        return NULL;
+    if (entry->key == NULL) {
+        so->fill++;
+    } else if (entry->key != dummy) {
+        key = entry->key;
+        Py_INCREF(key);
+        return key;
+    }
+    Py_INCREF(key);
+    entry->key = key;
+    entry->hash = hash;
+    so->used++;
+    if (so->fill*3 < (so->mask+1)*2)
+        return key;
+    if (set_table_resize(so, so->used>50000 ? so->used*2 : so->used*4))
+        return NULL;
+    return key;
+}
+
 #ifdef Py_DEBUG
 
 /* Test code to be called with any three element set.