diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-04-23-21-34-34.bpo-43908.YSSVMZ.rst b/Misc/NEWS.d/next/Core and Builtins/2021-04-23-21-34-34.bpo-43908.YSSVMZ.rst
new file mode 100644
index 0000000000..bd82fe2f75
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2021-04-23-21-34-34.bpo-43908.YSSVMZ.rst	
@@ -0,0 +1 @@
+Apply :const:`Py_TPFLAGS_IMMUTABLETYPE` to all built-in types. Patch by Erlend E. Aasland.
diff --git a/Modules/_abc.c b/Modules/_abc.c
index 0ddc2abeee..e058983100 100644
--- a/Modules/_abc.c
+++ b/Modules/_abc.c
@@ -109,7 +109,7 @@ static PyType_Slot _abc_data_type_spec_slots[] = {
 static PyType_Spec _abc_data_type_spec = {
     .name = "_abc._abc_data",
     .basicsize = sizeof(_abc_data),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC,
     .slots = _abc_data_type_spec_slots,
 };
 
diff --git a/Modules/_bz2module.c b/Modules/_bz2module.c
index bfcdac6924..ad133529a0 100644
--- a/Modules/_bz2module.c
+++ b/Modules/_bz2module.c
@@ -404,7 +404,7 @@ static PyType_Spec bz2_compressor_type_spec = {
     // bz2_compressor_type_spec does not have Py_TPFLAGS_BASETYPE flag
     // which prevents to create a subclass.
     // So calling PyType_GetModuleState() in this file is always safe.
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = bz2_compressor_type_slots,
 };
 
@@ -755,7 +755,7 @@ static PyType_Spec bz2_decompressor_type_spec = {
     // bz2_decompressor_type_spec does not have Py_TPFLAGS_BASETYPE flag
     // which prevents to create a subclass.
     // So calling PyType_GetModuleState() in this file is always safe.
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = bz2_decompressor_type_slots,
 };
 
diff --git a/Modules/_curses_panel.c b/Modules/_curses_panel.c
index 94caf8c93b..b1bdaddf01 100644
--- a/Modules/_curses_panel.c
+++ b/Modules/_curses_panel.c
@@ -520,7 +520,7 @@ static PyType_Slot PyCursesPanel_Type_slots[] = {
 static PyType_Spec PyCursesPanel_Type_spec = {
     .name = "_curses_panel.panel",
     .basicsize = sizeof(PyCursesPanelObject),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = PyCursesPanel_Type_slots
 };
 
diff --git a/Modules/_dbmmodule.c b/Modules/_dbmmodule.c
index 97772a04d0..5184a88386 100644
--- a/Modules/_dbmmodule.c
+++ b/Modules/_dbmmodule.c
@@ -414,7 +414,7 @@ static PyType_Spec dbmtype_spec = {
     // dbmtype_spec does not have Py_TPFLAGS_BASETYPE flag
     // which prevents to create a subclass.
     // So calling PyType_GetModuleState() in this file is always safe.
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = dbmtype_spec_slots,
 };
 
diff --git a/Modules/_functoolsmodule.c b/Modules/_functoolsmodule.c
index eea542e18c..11e813a03f 100644
--- a/Modules/_functoolsmodule.c
+++ b/Modules/_functoolsmodule.c
@@ -480,7 +480,7 @@ static PyType_Slot partial_type_slots[] = {
 static PyType_Spec partial_type_spec = {
     .name = "functools.partial",
     .basicsize = sizeof(partialobject),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC |
              Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_VECTORCALL,
     .slots = partial_type_slots
 };
@@ -546,7 +546,7 @@ static PyType_Slot keyobject_type_slots[] = {
 static PyType_Spec keyobject_type_spec = {
     .name = "functools.KeyWrapper",
     .basicsize = sizeof(keyobject),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = keyobject_type_slots
 };
 
@@ -766,7 +766,7 @@ static PyType_Slot lru_list_elem_type_slots[] = {
 static PyType_Spec lru_list_elem_type_spec = {
     .name = "functools._lru_list_elem",
     .basicsize = sizeof(lru_list_elem),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = lru_list_elem_type_slots
 };
 
@@ -1399,7 +1399,7 @@ static PyType_Slot lru_cache_type_slots[] = {
 static PyType_Spec lru_cache_type_spec = {
     .name = "functools._lru_cache_wrapper",
     .basicsize = sizeof(lru_cache_object),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC |
              Py_TPFLAGS_METHOD_DESCRIPTOR,
     .slots = lru_cache_type_slots
 };
diff --git a/Modules/_gdbmmodule.c b/Modules/_gdbmmodule.c
index 9e843acbaa..8ac40df4cd 100644
--- a/Modules/_gdbmmodule.c
+++ b/Modules/_gdbmmodule.c
@@ -570,7 +570,7 @@ static PyType_Spec gdbmtype_spec = {
     // dbmtype_spec does not have Py_TPFLAGS_BASETYPE flag
     // which prevents to create a subclass.
     // So calling PyType_GetModuleState() in this file is always safe.
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = gdbmtype_spec_slots,
 };
 
diff --git a/Modules/_hashopenssl.c b/Modules/_hashopenssl.c
index 870ee89fda..555281dc29 100644
--- a/Modules/_hashopenssl.c
+++ b/Modules/_hashopenssl.c
@@ -598,7 +598,7 @@ static PyType_Spec EVPtype_spec = {
     "_hashlib.HASH",    /*tp_name*/
     sizeof(EVPobject),  /*tp_basicsize*/
     0,                  /*tp_itemsize*/
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE,
     EVPtype_slots
 };
 
@@ -748,7 +748,7 @@ static PyType_Spec EVPXOFtype_spec = {
     "_hashlib.HASHXOF",    /*tp_name*/
     sizeof(EVPobject),  /*tp_basicsize*/
     0,                  /*tp_itemsize*/
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE,
     EVPXOFtype_slots
 };
 
@@ -1741,7 +1741,7 @@ static PyType_Slot HMACtype_slots[] = {
 PyType_Spec HMACtype_spec = {
     "_hashlib.HMAC",    /* name */
     sizeof(HMACobject),     /* basicsize */
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = HMACtype_slots,
 };
 
diff --git a/Modules/_json.c b/Modules/_json.c
index e10f83c96c..4158288f45 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -1228,7 +1228,7 @@ static PyType_Spec PyScannerType_spec = {
     .name = "_json.Scanner",
     .basicsize = sizeof(PyScannerObject),
     .itemsize = 0,
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC,
     .slots = PyScannerType_slots,
 };
 
@@ -1787,7 +1787,7 @@ static PyType_Spec PyEncoderType_spec = {
     .name = "_json.Encoder",
     .basicsize = sizeof(PyEncoderObject),
     .itemsize = 0,
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC,
     .slots = PyEncoderType_slots
 };
 
diff --git a/Modules/_lsprof.c b/Modules/_lsprof.c
index c32699cb8a..8ba3b7c7dc 100644
--- a/Modules/_lsprof.c
+++ b/Modules/_lsprof.c
@@ -807,7 +807,7 @@ static PyType_Slot _lsprof_profiler_type_spec_slots[] = {
 static PyType_Spec _lsprof_profiler_type_spec = {
     .name = "_lsprof.Profiler",
     .basicsize = sizeof(ProfilerObject),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE,
     .slots = _lsprof_profiler_type_spec_slots,
 };
 
diff --git a/Modules/_lzmamodule.c b/Modules/_lzmamodule.c
index b01f630009..e91f27dc22 100644
--- a/Modules/_lzmamodule.c
+++ b/Modules/_lzmamodule.c
@@ -883,7 +883,7 @@ static PyType_Spec lzma_compressor_type_spec = {
     // lzma_compressor_type_spec does not have Py_TPFLAGS_BASETYPE flag
     // which prevents to create a subclass.
     // So calling PyType_GetModuleState() in this file is always safe.
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = lzma_compressor_type_slots,
 };
 
@@ -1341,7 +1341,7 @@ static PyType_Spec lzma_decompressor_type_spec = {
     // lzma_decompressor_type_spec does not have Py_TPFLAGS_BASETYPE flag
     // which prevents to create a subclass.
     // So calling PyType_GetModuleState() in this file is always safe.
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = lzma_decompressor_type_slots,
 };
 
diff --git a/Modules/_operator.c b/Modules/_operator.c
index f55c2f1d1f..b5f24a72e5 100644
--- a/Modules/_operator.c
+++ b/Modules/_operator.c
@@ -1124,7 +1124,7 @@ static PyType_Spec itemgetter_type_spec = {
     .name = "operator.itemgetter",
     .basicsize = sizeof(itemgetterobject),
     .itemsize = 0,
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC,
     .slots = itemgetter_type_slots,
 };
 
@@ -1446,7 +1446,7 @@ static PyType_Spec attrgetter_type_spec = {
     .name = "operator.attrgetter",
     .basicsize = sizeof(attrgetterobject),
     .itemsize = 0,
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC,
     .slots = attrgetter_type_slots,
 };
 
@@ -1691,7 +1691,7 @@ static PyType_Spec methodcaller_type_spec = {
     .name = "operator.methodcaller",
     .basicsize = sizeof(methodcallerobject),
     .itemsize = 0,
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC,
     .slots = methodcaller_type_slots,
 };
 
diff --git a/Modules/_queuemodule.c b/Modules/_queuemodule.c
index c27fb1a001..9f0529c159 100644
--- a/Modules/_queuemodule.c
+++ b/Modules/_queuemodule.c
@@ -371,7 +371,7 @@ static PyType_Slot simplequeue_slots[] = {
 static PyType_Spec simplequeue_spec = {
     .name = "_queue.SimpleQueue",
     .basicsize = sizeof(simplequeueobject),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
     .slots = simplequeue_slots,
 };
 
diff --git a/Modules/_randommodule.c b/Modules/_randommodule.c
index cae49a009c..828ffc9a5b 100644
--- a/Modules/_randommodule.c
+++ b/Modules/_randommodule.c
@@ -575,7 +575,7 @@ static PyType_Spec Random_Type_spec = {
     "_random.Random",
     sizeof(RandomObject),
     0,
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE,
     Random_Type_slots
 };
 
diff --git a/Modules/_sqlite/cache.c b/Modules/_sqlite/cache.c
index 8da0a64a3f..e762c0c298 100644
--- a/Modules/_sqlite/cache.c
+++ b/Modules/_sqlite/cache.c
@@ -267,7 +267,7 @@ static PyType_Slot node_slots[] = {
 static PyType_Spec node_spec = {
     .name = MODULE_NAME ".Node",
     .basicsize = sizeof(pysqlite_Node),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE,
     .slots = node_slots,
 };
 PyTypeObject *pysqlite_NodeType = NULL;
@@ -291,7 +291,7 @@ static PyType_Slot cache_slots[] = {
 static PyType_Spec cache_spec = {
     .name = MODULE_NAME ".Cache",
     .basicsize = sizeof(pysqlite_Cache),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE,
     .slots = cache_slots,
 };
 PyTypeObject *pysqlite_CacheType = NULL;
diff --git a/Modules/_sqlite/connection.c b/Modules/_sqlite/connection.c
index 150291cb72..7bd948445c 100644
--- a/Modules/_sqlite/connection.c
+++ b/Modules/_sqlite/connection.c
@@ -1906,7 +1906,7 @@ static PyType_Slot connection_slots[] = {
 static PyType_Spec connection_spec = {
     .name = MODULE_NAME ".Connection",
     .basicsize = sizeof(pysqlite_Connection),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE,
     .slots = connection_slots,
 };
 
diff --git a/Modules/_sqlite/cursor.c b/Modules/_sqlite/cursor.c
index 60dfc7dce8..70d2b18f10 100644
--- a/Modules/_sqlite/cursor.c
+++ b/Modules/_sqlite/cursor.c
@@ -1020,7 +1020,7 @@ static PyType_Slot cursor_slots[] = {
 static PyType_Spec cursor_spec = {
     .name = MODULE_NAME ".Cursor",
     .basicsize = sizeof(pysqlite_Cursor),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE,
     .slots = cursor_slots,
 };
 
diff --git a/Modules/_sqlite/prepare_protocol.c b/Modules/_sqlite/prepare_protocol.c
index 7d2d7ade59..0ff63a8587 100644
--- a/Modules/_sqlite/prepare_protocol.c
+++ b/Modules/_sqlite/prepare_protocol.c
@@ -49,7 +49,7 @@ static PyType_Slot type_slots[] = {
 static PyType_Spec type_spec = {
     .name = MODULE_NAME ".PrepareProtocol",
     .basicsize = sizeof(pysqlite_PrepareProtocol),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = type_slots,
 };
 
diff --git a/Modules/_sqlite/row.c b/Modules/_sqlite/row.c
index f9dfcbd5d6..d969d6a746 100644
--- a/Modules/_sqlite/row.c
+++ b/Modules/_sqlite/row.c
@@ -237,7 +237,7 @@ static PyType_Slot row_slots[] = {
 static PyType_Spec row_spec = {
     .name = MODULE_NAME ".Row",
     .basicsize = sizeof(pysqlite_Row),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE,
     .slots = row_slots,
 };
 
diff --git a/Modules/_sqlite/statement.c b/Modules/_sqlite/statement.c
index 57026270e1..4a171b9d62 100644
--- a/Modules/_sqlite/statement.c
+++ b/Modules/_sqlite/statement.c
@@ -475,7 +475,7 @@ static PyType_Slot stmt_slots[] = {
 static PyType_Spec stmt_spec = {
     .name = MODULE_NAME ".Statement",
     .basicsize = sizeof(pysqlite_Statement),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = stmt_slots,
 };
 PyTypeObject *pysqlite_StatementType = NULL;
diff --git a/Modules/_sre.c b/Modules/_sre.c
index d4bfff6e84..b2aba56c9e 100644
--- a/Modules/_sre.c
+++ b/Modules/_sre.c
@@ -2690,7 +2690,7 @@ static PyType_Spec pattern_spec = {
     .name = "re.Pattern",
     .basicsize = sizeof(PatternObject),
     .itemsize = sizeof(SRE_CODE),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = pattern_slots,
 };
 
@@ -2755,7 +2755,7 @@ static PyType_Spec match_spec = {
     .name = "re.Match",
     .basicsize = sizeof(MatchObject),
     .itemsize = sizeof(Py_ssize_t),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = match_slots,
 };
 
@@ -2781,7 +2781,7 @@ static PyType_Slot scanner_slots[] = {
 static PyType_Spec scanner_spec = {
     .name = "_" SRE_MODULE ".SRE_Scanner",
     .basicsize = sizeof(ScannerObject),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = scanner_slots,
 };
 
diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index e28c128678..14abf045f5 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -425,7 +425,7 @@ static PyType_Spec sslerror_type_spec = {
     "ssl.SSLError",
     sizeof(PyOSErrorObject),
     0,
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE,
     sslerror_type_slots
 };
 
@@ -2825,7 +2825,7 @@ static PyType_Spec PySSLSocket_spec = {
     "_ssl._SSLSocket",
     sizeof(PySSLSocket),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     PySSLSocket_slots,
 };
 
@@ -4533,7 +4533,7 @@ static PyType_Spec PySSLContext_spec = {
     "_ssl._SSLContext",
     sizeof(PySSLContext),
     0,
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
     PySSLContext_slots,
 };
 
@@ -4739,7 +4739,7 @@ static PyType_Spec PySSLMemoryBIO_spec = {
     "_ssl.MemoryBIO",
     sizeof(PySSLMemoryBIO),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     PySSLMemoryBIO_slots,
 };
 
@@ -4914,7 +4914,7 @@ static PyType_Spec PySSLSession_spec = {
     "_ssl.SSLSession",
     sizeof(PySSLSession),
     0,
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC,
     PySSLSession_slots,
 };
 
diff --git a/Modules/_struct.c b/Modules/_struct.c
index 30ad9f2b79..07c6192b84 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -1724,7 +1724,7 @@ static PyType_Spec unpackiter_type_spec = {
     "_struct.unpack_iterator",
     sizeof(unpackiterobject),
     0,
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC,
     unpackiter_type_slots
 };
 
@@ -2093,7 +2093,7 @@ static PyType_Spec PyStructType_spec = {
     "_struct.Struct",
     sizeof(PyStructObject),
     0,
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE,
     PyStructType_slots
 };
 
diff --git a/Modules/_threadmodule.c b/Modules/_threadmodule.c
index 7feb0b8a1f..22f13dc9d9 100644
--- a/Modules/_threadmodule.c
+++ b/Modules/_threadmodule.c
@@ -312,7 +312,7 @@ static PyType_Slot lock_type_slots[] = {
 static PyType_Spec lock_type_spec = {
     .name = "_thread.lock",
     .basicsize = sizeof(lockobject),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC,
     .slots = lock_type_slots,
 };
 
@@ -584,7 +584,7 @@ static PyType_Slot rlock_type_slots[] = {
 static PyType_Spec rlock_type_spec = {
     .name = "_thread.RLock",
     .basicsize = sizeof(rlockobject),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE,
     .slots = rlock_type_slots,
 };
 
@@ -683,7 +683,7 @@ static PyType_Slot local_dummy_type_slots[] = {
 static PyType_Spec local_dummy_type_spec = {
     .name = "_thread._localdummy",
     .basicsize = sizeof(localdummyobject),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = local_dummy_type_slots,
 };
 
@@ -967,7 +967,7 @@ static PyType_Slot local_type_slots[] = {
 static PyType_Spec local_type_spec = {
     .name = "_thread._local",
     .basicsize = sizeof(localobject),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
     .slots = local_type_slots,
 };
 
diff --git a/Modules/_tkinter.c b/Modules/_tkinter.c
index 46d6a6e095..078d507a4e 100644
--- a/Modules/_tkinter.c
+++ b/Modules/_tkinter.c
@@ -1002,7 +1002,7 @@ static PyType_Spec PyTclObject_Type_spec = {
     "_tkinter.Tcl_Obj",
     sizeof(PyTclObject),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     PyTclObject_Type_slots,
 };
 
@@ -3294,7 +3294,7 @@ static PyType_Spec Tktt_Type_spec = {
     "_tkinter.tktimertoken",
     sizeof(TkttObject),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     Tktt_Type_slots,
 };
 
@@ -3349,7 +3349,7 @@ static PyType_Spec Tkapp_Type_spec = {
     "_tkinter.tkapp",
     sizeof(TkappObject),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     Tkapp_Type_slots,
 };
 
diff --git a/Modules/_winapi.c b/Modules/_winapi.c
index 9d5a45adac..02d806799a 100644
--- a/Modules/_winapi.c
+++ b/Modules/_winapi.c
@@ -331,7 +331,7 @@ static PyType_Slot winapi_overlapped_type_slots[] = {
 static PyType_Spec winapi_overlapped_type_spec = {
     .name = "_winapi.Overlapped",
     .basicsize = sizeof(OverlappedObject),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = winapi_overlapped_type_slots,
 };
 
diff --git a/Modules/arraymodule.c b/Modules/arraymodule.c
index f532678952..3c098f4a88 100644
--- a/Modules/arraymodule.c
+++ b/Modules/arraymodule.c
@@ -2985,7 +2985,7 @@ static PyType_Slot arrayiter_slots[] = {
 static PyType_Spec arrayiter_spec = {
     .name = "array.arrayiterator",
     .basicsize = sizeof(arrayiterobject),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC,
     .slots = arrayiter_slots,
 };
 
diff --git a/Modules/cjkcodecs/multibytecodec.c b/Modules/cjkcodecs/multibytecodec.c
index 5070c983d4..8d62269803 100644
--- a/Modules/cjkcodecs/multibytecodec.c
+++ b/Modules/cjkcodecs/multibytecodec.c
@@ -748,7 +748,7 @@ static PyType_Slot multibytecodec_slots[] = {
 static PyType_Spec multibytecodec_spec = {
     .name = MODULE_NAME ".MultibyteCodec",
     .basicsize = sizeof(MultibyteCodecObject),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC,
     .slots = multibytecodec_slots,
 };
 
@@ -1110,7 +1110,7 @@ static PyType_Slot encoder_slots[] = {
 static PyType_Spec encoder_spec = {
     .name = MODULE_NAME ".MultibyteIncrementalEncoder",
     .basicsize = sizeof(MultibyteIncrementalEncoderObject),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE,
     .slots = encoder_slots,
 };
 
@@ -1383,7 +1383,7 @@ static PyType_Slot decoder_slots[] = {
 static PyType_Spec decoder_spec = {
     .name = MODULE_NAME ".MultibyteIncrementalDecoder",
     .basicsize = sizeof(MultibyteIncrementalDecoderObject),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE,
     .slots = decoder_slots,
 };
 
@@ -1704,7 +1704,7 @@ static PyType_Slot reader_slots[] = {
 static PyType_Spec reader_spec = {
     .name = MODULE_NAME ".MultibyteStreamReader",
     .basicsize = sizeof(MultibyteStreamReaderObject),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE,
     .slots = reader_slots,
 };
 
@@ -1924,7 +1924,7 @@ static PyType_Slot writer_slots[] = {
 static PyType_Spec writer_spec = {
     .name = MODULE_NAME ".MultibyteStreamWriter",
     .basicsize = sizeof(MultibyteStreamWriterObject),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE,
     .slots = writer_slots,
 };
 
diff --git a/Modules/md5module.c b/Modules/md5module.c
index b2e65a0a5f..86b131610e 100644
--- a/Modules/md5module.c
+++ b/Modules/md5module.c
@@ -484,7 +484,7 @@ static PyType_Slot md5_type_slots[] = {
 static PyType_Spec md5_type_spec = {
     .name = "_md5.md5",
     .basicsize =  sizeof(MD5object),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = md5_type_slots
 };
 
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index 1e66962d37..9f4091c762 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -1114,7 +1114,7 @@ static PyType_Slot mmap_object_slots[] = {
 static PyType_Spec mmap_object_spec = {
     .name = "mmap.mmap",
     .basicsize = sizeof(mmap_object),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_BASETYPE,
     .slots = mmap_object_slots,
 };
 
diff --git a/Modules/overlapped.c b/Modules/overlapped.c
index 38dd98f084..81fb05896a 100644
--- a/Modules/overlapped.c
+++ b/Modules/overlapped.c
@@ -1876,7 +1876,7 @@ static PyType_Slot overlapped_type_slots[] = {
 static PyType_Spec overlapped_type_spec = {
     .name = "_overlapped.Overlapped",
     .basicsize = sizeof(OverlappedObject),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = overlapped_type_slots
 };
 
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 85e1e6976d..44c499eb21 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -13776,7 +13776,7 @@ static PyType_Spec DirEntryType_spec = {
     MODNAME ".DirEntry",
     sizeof(DirEntry),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     DirEntryType_slots
 };
 
@@ -14211,7 +14211,7 @@ static PyType_Spec ScandirIteratorType_spec = {
     0,
     // bpo-40549: Py_TPFLAGS_BASETYPE should not be used, since
     // PyType_GetModule(Py_TYPE(self)) doesn't work on a subclass instance.
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_FINALIZE,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_FINALIZE,
     ScandirIteratorType_slots
 };
 
diff --git a/Modules/pyexpat.c b/Modules/pyexpat.c
index a13d340a3e..4047d627a5 100644
--- a/Modules/pyexpat.c
+++ b/Modules/pyexpat.c
@@ -1506,7 +1506,7 @@ static PyType_Slot _xml_parse_type_spec_slots[] = {
 static PyType_Spec _xml_parse_type_spec = {
     .name = "pyexpat.xmlparser",
     .basicsize = sizeof(xmlparseobject),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_HAVE_GC,
     .slots = _xml_parse_type_spec_slots,
 };
 
diff --git a/Modules/selectmodule.c b/Modules/selectmodule.c
index f80da58954..3a975e0636 100644
--- a/Modules/selectmodule.c
+++ b/Modules/selectmodule.c
@@ -1146,7 +1146,7 @@ static PyType_Spec devpoll_Type_spec = {
     "select.devpoll",
     sizeof(devpollObject),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     devpoll_Type_slots
 };
 
@@ -1920,7 +1920,7 @@ static PyType_Spec kqueue_event_Type_spec = {
     "select.kevent",
     sizeof(kqueue_event_Object),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     kqueue_event_Type_slots
 };
 
@@ -2283,7 +2283,7 @@ static PyType_Spec poll_Type_spec = {
     "select.poll",
     sizeof(pollObject),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     poll_Type_slots
 };
 
@@ -2332,7 +2332,7 @@ static PyType_Spec pyEpoll_Type_spec = {
     "select.epoll",
     sizeof(pyEpoll_Object),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     pyEpoll_Type_slots
 };
 
@@ -2361,7 +2361,7 @@ static PyType_Spec kqueue_queue_Type_spec = {
     "select.kqueue",
     sizeof(kqueue_queue_Object),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     kqueue_queue_Type_slots
 };
 
diff --git a/Modules/sha1module.c b/Modules/sha1module.c
index 7126db93b1..4e5beac28e 100644
--- a/Modules/sha1module.c
+++ b/Modules/sha1module.c
@@ -462,7 +462,7 @@ static PyType_Slot sha1_type_slots[] = {
 static PyType_Spec sha1_type_spec = {
     .name = "_sha1.sha1",
     .basicsize =  sizeof(SHA1object),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = sha1_type_slots
 };
 
diff --git a/Modules/sha256module.c b/Modules/sha256module.c
index b90e5df782..9121f7365c 100644
--- a/Modules/sha256module.c
+++ b/Modules/sha256module.c
@@ -544,14 +544,14 @@ static PyType_Slot sha256_types_slots[] = {
 static PyType_Spec sha224_type_spec = {
     .name = "_sha256.sha224",
     .basicsize = sizeof(SHAobject),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = sha256_types_slots
 };
 
 static PyType_Spec sha256_type_spec = {
     .name = "_sha256.sha256",
     .basicsize = sizeof(SHAobject),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = sha256_types_slots
 };
 
diff --git a/Modules/sha512module.c b/Modules/sha512module.c
index 0d8f51e5ae..b82e6938bf 100644
--- a/Modules/sha512module.c
+++ b/Modules/sha512module.c
@@ -602,7 +602,7 @@ static PyType_Slot sha512_sha384_type_slots[] = {
 static PyType_Spec sha512_sha384_type_spec = {
     .name = "_sha512.sha384",
     .basicsize =  sizeof(SHAobject),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = sha512_sha384_type_slots
 };
 
@@ -619,7 +619,7 @@ static PyType_Slot sha512_sha512_type_slots[] = {
 static PyType_Spec sha512_sha512_type_spec = {
     .name = "_sha512.sha512",
     .basicsize =  sizeof(SHAobject),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = sha512_sha512_type_slots
 };
 
diff --git a/Modules/unicodedata.c b/Modules/unicodedata.c
index aebae7da57..e1786bad3a 100644
--- a/Modules/unicodedata.c
+++ b/Modules/unicodedata.c
@@ -1454,7 +1454,7 @@ static PyType_Slot ucd_type_slots[] = {
 static PyType_Spec ucd_type_spec = {
     .name = "unicodedata.UCD",
     .basicsize = sizeof(PreviousDBVersion),
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = ucd_type_slots
 };
 
diff --git a/Modules/zlibmodule.c b/Modules/zlibmodule.c
index a537087d19..d2e586bcf0 100644
--- a/Modules/zlibmodule.c
+++ b/Modules/zlibmodule.c
@@ -1337,7 +1337,7 @@ static PyType_Spec Comptype_spec = {
     "zlib.Compress",
     sizeof(compobject),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     Comptype_slots
 };
 
@@ -1352,7 +1352,7 @@ static PyType_Spec Decomptype_spec = {
     "zlib.Decompress",
     sizeof(compobject),
     0,
-    Py_TPFLAGS_DEFAULT,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_IMMUTABLETYPE,
     Decomptype_slots
 };
 
diff --git a/Doc/c-api/typeobj.rst b/Doc/c-api/typeobj.rst
index 9efe3aac2e1c9..10b8b2584aef7 100644
--- a/Doc/c-api/typeobj.rst
+++ b/Doc/c-api/typeobj.rst
@@ -1177,6 +1177,17 @@ and :c:type:`PyType_Type` effectively act as defaults.)
 
       .. versionadded:: 3.10
 
+   .. data:: Py_TPFLAGS_IMMUTABLETYPE
+
+      This bit is set for type objects that are immutable. Static types are
+      immutable by default; setting this flag has no effect for static types.
+
+      **Inheritance:**
+
+      This flag is never inherited.
+
+      .. versionadded:: 3.10
+
 
 .. c:member:: const char* PyTypeObject.tp_doc
 
diff --git a/Include/object.h b/Include/object.h
index 695f01564282c..a239063e3965c 100644
--- a/Include/object.h
+++ b/Include/object.h
@@ -320,6 +320,9 @@ Code can use PyType_HasFeature(type_ob, flag_value) to test whether the
 given type object has a specified feature.
 */
 
+/* Set if the type object is immutable */
+#define Py_TPFLAGS_IMMUTABLETYPE (1UL << 8)
+
 /* Set if the type object is dynamically allocated */
 #define Py_TPFLAGS_HEAPTYPE (1UL << 9)
 
diff --git a/Lib/test/test_array.py b/Lib/test/test_array.py
index bdcd1254b304f..d419d59ad2d50 100644
--- a/Lib/test/test_array.py
+++ b/Lib/test/test_array.py
@@ -50,6 +50,10 @@ def test_empty(self):
         a += a
         self.assertEqual(len(a), 0)
 
+    def test_immutable(self):
+        with self.assertRaises(TypeError):
+            array.array.foo = 1
+
 
 # Machine format codes.
 #
diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-04-22-10-46-40.bpo-43908.Co3YhZ.rst b/Misc/NEWS.d/next/Core and Builtins/2021-04-22-10-46-40.bpo-43908.Co3YhZ.rst
new file mode 100644
index 0000000000000..1558498329efd
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2021-04-22-10-46-40.bpo-43908.Co3YhZ.rst	
@@ -0,0 +1,3 @@
+Introduce :const:`Py_TPFLAGS_IMMUTABLETYPE` for immutable type objects and
+apply it to types that lately have been converted to heap types. Patch by
+Erlend E. Aasland.
diff --git a/Modules/arraymodule.c b/Modules/arraymodule.c
index fb9ebbe9f4870..b98358fa5d8a4 100644
--- a/Modules/arraymodule.c
+++ b/Modules/arraymodule.c
@@ -2846,7 +2846,7 @@ static PyType_Slot array_slots[] = {
 static PyType_Spec array_spec = {
     .name = "array.array",
     .basicsize = sizeof(arrayobject),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_IMMUTABLETYPE,
     .slots = array_slots,
 };
 
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index a957c832b90bd..cf6cf0fc25f45 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -3859,7 +3859,7 @@ static int
 type_setattro(PyTypeObject *type, PyObject *name, PyObject *value)
 {
     int res;
-    if (!(type->tp_flags & Py_TPFLAGS_HEAPTYPE)) {
+    if (type->tp_flags & Py_TPFLAGS_IMMUTABLETYPE) {
         PyErr_Format(
             PyExc_TypeError,
             "can't set attributes of built-in/extension type '%s'",
@@ -6213,6 +6213,10 @@ PyType_Ready(PyTypeObject *type)
 
     type->tp_flags |= Py_TPFLAGS_READYING;
 
+    if (!(type->tp_flags & Py_TPFLAGS_HEAPTYPE)) {
+        type->tp_flags |= Py_TPFLAGS_IMMUTABLETYPE;
+    }
+
     if (type_ready(type) < 0) {
         type->tp_flags &= ~Py_TPFLAGS_READYING;
         return -1;