[Python-checkins] bpo-38979: fix ContextVar "__class_getitem__" method (GH-17497)

[Miss Islington (bot)]

https://github.com/python/cpython/commit/960fca1a5887a277fd6031cf4c4b6fb31b08ebf5
commit: 960fca1a5887a277fd6031cf4c4b6fb31b08ebf5
branch: 3.8
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: GitHub <noreply at github.com>
date: 2019-12-08T04:49:07-08:00
summary:

bpo-38979: fix ContextVar "__class_getitem__" method (GH-17497)


now contextvars.ContextVar "__class_getitem__" method returns ContextVar class, not None.

https://bugs.python.org/issue38979

Automerge-Triggered-By: @asvetlov
(cherry picked from commit 28c91631c24e53713ad0e8a2bbae716373f5e53d)

Co-authored-by: AMIR <31338382+amiremohamadi at users.noreply.github.com>

files:
A Misc/NEWS.d/next/Library/2019-12-07-16-32-42.bpo-38979.q0sIHy.rst
M Lib/test/test_context.py
M Python/context.c

diff --git a/Lib/test/test_context.py b/Lib/test/test_context.py
index efd7319a23ae0..b9e991a400092 100644
--- a/Lib/test/test_context.py
+++ b/Lib/test/test_context.py
@@ -38,9 +38,6 @@ def test_context_var_new_1(self):
 
         self.assertNotEqual(hash(c), hash('aaa'))
 
-    def test_context_var_new_2(self):
-        self.assertIsNone(contextvars.ContextVar[int])
-
     @isolated_context
     def test_context_var_repr_1(self):
         c = contextvars.ContextVar('a')
@@ -361,6 +358,10 @@ def sub(num):
             tp.shutdown()
         self.assertEqual(results, list(range(10)))
 
+    def test_contextvar_getitem(self):
+        clss = contextvars.ContextVar
+        self.assertEqual(clss[str], clss)
+
 
 # HAMT Tests
 
diff --git a/Misc/NEWS.d/next/Library/2019-12-07-16-32-42.bpo-38979.q0sIHy.rst b/Misc/NEWS.d/next/Library/2019-12-07-16-32-42.bpo-38979.q0sIHy.rst
new file mode 100644
index 0000000000000..6a91a12e4930a
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-12-07-16-32-42.bpo-38979.q0sIHy.rst
@@ -0,0 +1 @@
+Return class from ``ContextVar.__class_getitem__`` to simplify subclassing.
diff --git a/Python/context.c b/Python/context.c
index f48c376b4ffaa..5c30e47f35dd7 100644
--- a/Python/context.c
+++ b/Python/context.c
@@ -1010,9 +1010,10 @@ _contextvars_ContextVar_reset(PyContextVar *self, PyObject *token)
 
 
 static PyObject *
-contextvar_cls_getitem(PyObject *self, PyObject *args)
+contextvar_cls_getitem(PyObject *self, PyObject *arg)
 {
-    Py_RETURN_NONE;
+    Py_INCREF(self);
+    return self;
 }
 
 static PyMemberDef PyContextVar_members[] = {
@@ -1025,7 +1026,7 @@ static PyMethodDef PyContextVar_methods[] = {
     _CONTEXTVARS_CONTEXTVAR_SET_METHODDEF
     _CONTEXTVARS_CONTEXTVAR_RESET_METHODDEF
     {"__class_getitem__", contextvar_cls_getitem,
-        METH_VARARGS | METH_STATIC, NULL},
+        METH_O | METH_CLASS, NULL},
     {NULL, NULL}
 };