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

diff --git a/Lib/test/test_context.py b/Lib/test/test_context.py
index efd7319a23ae0268b2d037aa76174adc343474bc..b9e991a400092915695ef23e971bf0a5c187f4f6 100644 (file)
--- a/Lib/test/test_context.py
+++ b/Lib/test/test_context.py
@@ -38,9 +38,6 @@ class ContextTest(unittest.TestCase):
 
         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 @@ 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
 

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 (file)
index 0000000..6a91a12
--- /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 26f22994eecb7a9728836dd00319bf2b3e308627..e0338c97e1873a31a7b0e15c508529d42ebf0970 100644 (file)
--- a/Python/context.c
+++ b/Python/context.c
@@ -1024,9 +1024,10 @@ _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[] = {
@@ -1039,7 +1040,7 @@ 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}
 };