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 28c9163)

Co-authored-by: AMIR <[email protected]>

Author: miss-islington

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
 

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.

Python/context.c

@@ -974,9 +974,10 @@ _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[] = {
@@ -989,7 +990,7 @@ 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}
 };