bpo-39481: PEP 585 for dataclasses, mailbox, contextvars (GH-19425)

Lib/dataclasses.py

@@ -7,6 +7,7 @@ import keyword
 import builtins
 import functools
 import _thread
+from types import GenericAlias
 
 
 __all__ = ['dataclass',
@@ -284,6 +285,8 @@ class Field:
             # it.
             func(self.default, owner, name)
 
+    __class_getitem__ = classmethod(GenericAlias)
+
 
 class _DataclassParams:
     __slots__ = ('init',

Lib/mailbox.py

@@ -18,6 +18,7 @@ import email.message
 import email.generator
 import io
 import contextlib
+from types import GenericAlias
 try:
     import fcntl
 except ImportError:
@@ -260,6 +261,8 @@ class Mailbox:
         else:
             raise TypeError('Invalid message type: %s' % type(message))
 
+    __class_getitem__ = classmethod(GenericAlias)
+
 
 class Maildir(Mailbox):
     """A qmail-style Maildir mailbox."""
@@ -2015,6 +2018,8 @@ class _ProxyFile:
             return False
         return self._file.closed
 
+    __class_getitem__ = classmethod(GenericAlias)
+
 
 class _PartialFile(_ProxyFile):
     """A read-only wrapper of part of a file."""

Lib/test/test_context.py

@@ -358,10 +358,6 @@ class ContextTest(unittest.TestCase):
             tp.shutdown()
         self.assertEqual(results, list(range(10)))
 
-    def test_contextvar_getitem(self):
-        clss = contextvars.ContextVar
-        self.assertEqual(clss[str], clss)
-
 
 # HAMT Tests
 

Lib/test/test_genericalias.py

@@ -9,7 +9,10 @@ from collections.abc import *
 from concurrent.futures import Future
 from concurrent.futures.thread import _WorkItem
 from contextlib import AbstractContextManager, AbstractAsyncContextManager
-from functools import partial, partialmethod, _lru_cache_wrapper, cached_property
+from contextvars import ContextVar, Token
+from dataclasses import Field
+from functools import partial, partialmethod, cached_property
+from mailbox import Mailbox, _PartialFile
 from ctypes import Array, LibraryLoader
 from difflib import SequenceMatcher
 from filecmp import dircmp
@@ -60,6 +63,9 @@ class BaseTest(unittest.TestCase):
                   Reversible,
                   Container, Collection,
                   Callable,
+                  Mailbox, _PartialFile,
+                  ContextVar, Token,
+                  Field,
                   Set, MutableSet,
                   Mapping, MutableMapping, MappingView,
                   KeysView, ItemsView, ValuesView,

Python/context.c

@@ -1024,13 +1024,6 @@ _contextvars_ContextVar_reset(PyContextVar *self, PyObject *token)
 }
 
 
-static PyObject *
-contextvar_cls_getitem(PyObject *self, PyObject *arg)
-{
-    Py_INCREF(self);
-    return self;
-}
-
 static PyMemberDef PyContextVar_members[] = {
     {"name", T_OBJECT, offsetof(PyContextVar, var_name), READONLY},
     {NULL}
@@ -1040,8 +1033,8 @@ static PyMethodDef PyContextVar_methods[] = {
     _CONTEXTVARS_CONTEXTVAR_GET_METHODDEF
     _CONTEXTVARS_CONTEXTVAR_SET_METHODDEF
     _CONTEXTVARS_CONTEXTVAR_RESET_METHODDEF
-    {"__class_getitem__", contextvar_cls_getitem,
-        METH_O | METH_CLASS, NULL},
+    {"__class_getitem__", (PyCFunction)Py_GenericAlias,
+    METH_O|METH_CLASS,       PyDoc_STR("See PEP 585")},
     {NULL, NULL}
 };
 
@@ -1180,10 +1173,17 @@ static PyGetSetDef PyContextTokenType_getsetlist[] = {
     {NULL}
 };
 
+static PyMethodDef PyContextTokenType_methods[] = {
+    {"__class_getitem__",    (PyCFunction)Py_GenericAlias,
+    METH_O|METH_CLASS,       PyDoc_STR("See PEP 585")},
+    {NULL}
+};
+
 PyTypeObject PyContextToken_Type = {
     PyVarObject_HEAD_INIT(&PyType_Type, 0)
     "Token",
     sizeof(PyContextToken),
+    .tp_methods = PyContextTokenType_methods,
     .tp_getset = PyContextTokenType_getsetlist,
     .tp_dealloc = (destructor)token_tp_dealloc,
     .tp_getattro = PyObject_GenericGetAttr,