Proposed code changes for [PEP XXX](link).

Signed-off-by: Matthew A Johnson <matjoh@microsoft.com>

Author: matajoh

.gitignore

@@ -156,3 +156,6 @@ Python/frozen_modules/MANIFEST
 # Ignore ./python binary on Unix but still look into ./Python/ directory.
 /python
 !/Python/
+
+# Ignore the build directory.
+build*

Doc/conf.py

@@ -215,6 +215,7 @@
     ('c:data', 'PyExc_UnicodeError'),
     ('c:data', 'PyExc_UnicodeTranslateError'),
     ('c:data', 'PyExc_ValueError'),
+    ('c:data', 'PyExc_NotWriteableError'),
     ('c:data', 'PyExc_ZeroDivisionError'),
     # C API: Standard Python warning classes
     ('c:data', 'PyExc_BytesWarning'),

Doc/data/stable_abi.dat

@@ -168,6 +168,13 @@ PyAPI_FUNC(void) _Py_NO_RETURN _Py_FatalErrorFormat(
     const char *format,
     ...);
 
+PyAPI_FUNC(PyObject *) _PyErr_WriteToImmutable(const char* file, int line, PyObject *obj);
+#define PyErr_WriteToImmutable(obj) _PyErr_WriteToImmutable(__FILE__, __LINE__, _PyObject_CAST(obj))
+
+PyAPI_FUNC(PyObject *) _PyErr_WriteToImmutableKey(const char* file, int line, PyObject *key);
+#define PyErr_WriteToImmutableKey(key) _PyErr_WriteToImmutableKey(__FILE__, __LINE__, _PyObject_CAST(key))
+
+
 extern PyObject *_PyErr_SetImportErrorWithNameFrom(
         PyObject *,
         PyObject *,

Include/cpython/pyerrors.h

@@ -24,14 +24,24 @@ typedef struct {
     /* Cached hash code of me_key. */
     Py_hash_t me_hash;
     PyObject *me_key;
-    PyObject *me_value; /* This field is only meaningful for combined tables */
+    PyObject *_me_value; /* This field is only meaningful for combined tables */
 } PyDictKeyEntry;
 
 typedef struct {
     PyObject *me_key;   /* The key must be Unicode and have hash. */
-    PyObject *me_value; /* This field is only meaningful for combined tables */
+    PyObject *_me_value; /* This field is only meaningful for combined tables */
 } PyDictUnicodeEntry;
 
+#define _PyDictEntry_IsImmutable(entry) (((uintptr_t)((entry)->_me_value)) & 0x1)
+#define _PyDictEntry_SetImmutable(entry) ((entry)->_me_value = (PyObject*)((uintptr_t)(entry)->_me_value | 0x1))
+#define _PyDictEntry_Hash(entry) ((entry)->me_hash)
+#define _PyDictEntry_Key(entry) ((entry)->me_key)
+#define _PyDictEntry_Value(entry) ((PyObject*)((((uintptr_t)((entry)->_me_value)) >> 1) << 1))
+#define _PyDictEntry_SetValue(entry, value) ((entry)->_me_value = value)
+#define _PyDictEntry_IsEmpty(entry) ((entry)->_me_value == NULL)
+
+extern PyObject *_PyDict_IsKeyImmutable(PyObject* op, PyObject* key);
+
 extern PyDictKeysObject *_PyDict_NewKeysForClass(void);
 extern PyObject *_PyDict_FromKeys(PyObject *, PyObject *, PyObject *);
 
@@ -50,6 +60,7 @@ extern Py_ssize_t _Py_dict_lookup(PyDictObject *mp, PyObject *key, Py_hash_t has
 extern Py_ssize_t _PyDict_LookupIndex(PyDictObject *, PyObject *);
 extern Py_ssize_t _PyDictKeys_StringLookup(PyDictKeysObject* dictkeys, PyObject *key);
 extern PyObject *_PyDict_LoadGlobal(PyDictObject *, PyDictObject *, PyObject *);
+extern PyObject *_PyDict_SetKeyImmutable(PyDictObject *mp, PyObject *key);
 
 /* Consumes references to key and value */
 extern int _PyDict_SetItem_Take2(PyDictObject *op, PyObject *key, PyObject *value);

Include/internal/pycore_dict.h

@@ -198,7 +198,7 @@ _PyFrame_MakeAndSetFrameObject(_PyInterpreterFrame *frame);
 
 /* Gets the PyFrameObject for this frame, lazily
  * creating it if necessary.
- * Returns a borrowed referennce */
+ * Returns a borrowed reference */
 static inline PyFrameObject *
 _PyFrame_GetFrameObject(_PyInterpreterFrame *frame)
 {

Include/internal/pycore_frame.h

@@ -48,10 +48,12 @@ struct _Py_global_strings {
         STRUCT_FOR_STR(json_decoder, "json.decoder")
         STRUCT_FOR_STR(kwdefaults, ".kwdefaults")
         STRUCT_FOR_STR(list_err, "list index out of range")
+        STRUCT_FOR_STR(namedtuple, "namedtuple")
         STRUCT_FOR_STR(newline, "\n")
         STRUCT_FOR_STR(open_br, "{")
         STRUCT_FOR_STR(percent, "%")
         STRUCT_FOR_STR(shim_name, "<shim>")
+        STRUCT_FOR_STR(startswith, "startswith")
         STRUCT_FOR_STR(type_params, ".type_params")
         STRUCT_FOR_STR(utf_8, "utf-8")
     } literals;

Include/internal/pycore_global_objects_fini_generated.h

@@ -0,0 +1,18 @@
+#ifndef Py_FREEZE_H
+#define Py_FREEZE_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+PyObject* _Py_Freeze(PyObject*);
+#define Py_Freeze(op) _Py_Freeze(_PyObject_CAST(op))
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_FREEZE_H */

Include/internal/pycore_global_strings.h

@@ -71,7 +71,7 @@ static inline void _Py_RefcntAdd(PyObject* op, Py_ssize_t n)
 static inline void _Py_SetImmortal(PyObject *op)
 {
     if (op) {
-        op->ob_refcnt = _Py_IMMORTAL_REFCNT;
+        op->ob_refcnt = (op->ob_refcnt & _Py_IMMUTABLE_MASK) | _Py_IMMORTAL_REFCNT;
     }
 }
 #define _Py_SetImmortal(op) _Py_SetImmortal(_PyObject_CAST(op))
@@ -80,7 +80,8 @@ static inline void _Py_SetImmortal(PyObject *op)
 static inline void _Py_ClearImmortal(PyObject *op)
 {
     if (op) {
-        assert(op->ob_refcnt == _Py_IMMORTAL_REFCNT);
+        assert((op->ob_refcnt & _Py_REFCNT_MASK) == _Py_IMMORTAL_REFCNT);
+        // note this also clears the _Py_IMMUTABLE_FLAG, if set
         op->ob_refcnt = 1;
         Py_DECREF(op);
     }
@@ -91,6 +92,17 @@ static inline void _Py_ClearImmortal(PyObject *op)
         op = NULL; \
     } while (0)
 
+static inline void _Py_SetImmutable(PyObject *op)
+{
+    if(op) {
+        op->ob_refcnt |= _Py_IMMUTABLE_FLAG;
+    }
+}
+#define _Py_SetImmutable(op) _Py_SetImmutable(_PyObject_CAST(op))
+
+#define Py_CHECKWRITE(op) ((op) && (!_Py_IsImmutable(op) || _Py_IsFinalizing()))
+#define Py_REQUIREWRITE(op, msg) {if (Py_CHECKWRITE(op)) { _PyObject_ASSERT_FAILED_MSG(op, msg); }}
+
 static inline void
 _Py_DECREF_SPECIALIZED(PyObject *op, const destructor destruct)
 {
@@ -101,7 +113,8 @@ _Py_DECREF_SPECIALIZED(PyObject *op, const destructor destruct)
 #ifdef Py_REF_DEBUG
     _Py_DEC_REFTOTAL(_PyInterpreterState_GET());
 #endif
-    if (--op->ob_refcnt != 0) {
+    op->ob_refcnt -= 1;
+    if ((op->ob_refcnt & _Py_REFCNT_MASK) != 0) {
         assert(op->ob_refcnt > 0);
     }
     else {