serialization issues addressed

Lib/functools.py

@@ -280,6 +280,7 @@ class __PlaceholderTypeBase:
     Used as a placeholder for partial arguments.
     """
     __instance = None
+    __slots__ = ()
 
     def __new__(cls):
         if cls.__instance is None:
@@ -295,8 +296,14 @@ def __repr__(self):
     def __reduce__(self):
         return 'Placeholder'
 
+def __placeholder_init_subclass__(cls, *args, **kwargs):
+    raise TypeError(f"type '{cls.__name__}' is not an acceptable base type")
 
-Placeholder = type('PlaceholderType', (__PlaceholderTypeBase,), {})()
+Placeholder = type(
+    'PlaceholderType',
+    (__PlaceholderTypeBase,),
+    {'__slots__': (), '__init_subclass__': __placeholder_init_subclass__}
+)()
 
 
 def _partial_prepare_new(cls, func, args, keywords):

Lib/pickle.py

@@ -27,7 +27,7 @@
 from copyreg import dispatch_table
 from copyreg import _extension_registry, _inverted_registry, _extension_cache
 from itertools import islice
-from functools import partial
+from functools import partial, Placeholder
 import sys
 from sys import maxsize
 from struct import pack, unpack
@@ -1120,6 +1120,8 @@ def save_type(self, obj):
             return self.save_reduce(type, (NotImplemented,), obj=obj)
         elif obj is type(...):
             return self.save_reduce(type, (...,), obj=obj)
+        elif obj is type(Placeholder):
+            return self.save_reduce(type, (Placeholder,), obj=obj)
         return self.save_global(obj)
 
     dispatch[FunctionType] = save_global

Modules/_functoolsmodule.c

@@ -63,6 +63,17 @@ placeholder_repr(PyObject *op)
     return PyUnicode_FromString("Placeholder");
 }
 
+static PyObject *
+placeholder_reduce(PyObject *op, PyObject *Py_UNUSED(ignored))
+{
+    return PyUnicode_FromString("Placeholder");
+}
+
+static PyMethodDef placeholder_methods[] = {
+    {"__reduce__", placeholder_reduce, METH_NOARGS, NULL},
+    {NULL, NULL}
+};
+
 static void
 placeholder_dealloc(PyObject* placeholder)
 {
@@ -98,6 +109,7 @@ static PyType_Slot placeholder_type_slots[] = {
     {Py_tp_dealloc, placeholder_dealloc},
     {Py_tp_repr, placeholder_repr},
     {Py_tp_doc, (void *)placeholder_doc},
+    {Py_tp_methods, placeholder_methods},
     {Py_tp_new, placeholder_new},
     // Number protocol
     {Py_nb_bool, placeholder_bool},

Modules/_pickle.c

@@ -198,6 +198,7 @@ typedef struct {
     /* functools.partial, used for implementing __newobj_ex__ with protocols
        2 and 3 */
     PyObject *partial;
+    PyObject *placeholder;
 
     /* Types */
     PyTypeObject *Pickler_Type;
@@ -253,6 +254,7 @@ _Pickle_ClearState(PickleState *st)
     Py_CLEAR(st->codecs_encode);
     Py_CLEAR(st->getattr);
     Py_CLEAR(st->partial);
+    Py_CLEAR(st->placeholder);
     Py_CLEAR(st->Pickler_Type);
     Py_CLEAR(st->Unpickler_Type);
     Py_CLEAR(st->Pdata_Type);
@@ -375,6 +377,9 @@ _Pickle_InitState(PickleState *st)
     st->partial = _PyImport_GetModuleAttrString("functools", "partial");
     if (!st->partial)
         goto error;
+    st->placeholder = _PyImport_GetModuleAttrString("functools", "Placeholder");
+    if (!st->placeholder)
+        goto error;
 
     return 0;
 
@@ -3861,6 +3866,9 @@ save_type(PickleState *state, PicklerObject *self, PyObject *obj)
     else if (obj == (PyObject *)&_PyNotImplemented_Type) {
         return save_singleton_type(state, self, obj, Py_NotImplemented);
     }
+    else if (obj == (PyObject *)Py_TYPE(state->placeholder)) {
+        return save_singleton_type(state, self, obj, state->placeholder);
+    }
     return save_global(state, self, obj, NULL);
 }
 
@@ -7791,6 +7799,7 @@ pickle_traverse(PyObject *m, visitproc visit, void *arg)
     Py_VISIT(st->codecs_encode);
     Py_VISIT(st->getattr);
     Py_VISIT(st->partial);
+    Py_VISIT(st->placeholder);
     Py_VISIT(st->Pickler_Type);
     Py_VISIT(st->Unpickler_Type);
     Py_VISIT(st->Pdata_Type);