From bb7713bbe24506a4642cc1cecae24f5abfe64a8a Mon Sep 17 00:00:00 2001
From: Yurii Karabas <1998uriyyo@gmail.com>
Date: Thu, 1 Jul 2021 12:02:10 +0300
Subject: [PATCH 01/13] bpo-44490: Add __parameters__ to types.Union

---
 Include/genericaliasobject.h                  |  2 +
 Lib/test/test_types.py                        |  5 ++
 .../2021-07-01-11-59-34.bpo-44490.xY80VR.rst  |  2 +
 Objects/genericaliasobject.c                  | 50 +++++++++++-------
 Objects/unionobject.c                         | 52 +++++++++++++++++++
 5 files changed, 92 insertions(+), 19 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2021-07-01-11-59-34.bpo-44490.xY80VR.rst

diff --git a/Include/genericaliasobject.h b/Include/genericaliasobject.h
index cf002976b27cd7..6df56fbf41df79 100644
--- a/Include/genericaliasobject.h
+++ b/Include/genericaliasobject.h
@@ -5,6 +5,8 @@
 extern "C" {
 #endif
 
+PyAPI_FUNC(PyObject *) _Py_apply_parameters(PyObject *, PyObject *, PyObject *, PyObject *);
+PyAPI_FUNC(PyObject *) _Py_make_parameters(PyObject *);
 PyAPI_FUNC(PyObject *) Py_GenericAlias(PyObject *, PyObject *);
 PyAPI_DATA(PyTypeObject) Py_GenericAliasType;
 
diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
index ae7b17bd590e61..3f938c3a81f98f 100644
--- a/Lib/test/test_types.py
+++ b/Lib/test/test_types.py
@@ -666,6 +666,11 @@ def test_or_type_operator_with_TypeVar(self):
         assert TV | str == typing.Union[TV, str]
         assert str | TV == typing.Union[str, TV]
 
+    def test_union_parameter_chaining(self):
+        T = typing.TypeVar("T")
+
+        assert (float | list[T])[int] == float | list[int]
+
     def test_or_type_operator_with_forward(self):
         T = typing.TypeVar('T')
         ForwardAfter = T | 'Forward'
diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-07-01-11-59-34.bpo-44490.xY80VR.rst b/Misc/NEWS.d/next/Core and Builtins/2021-07-01-11-59-34.bpo-44490.xY80VR.rst
new file mode 100644
index 00000000000000..1f8f3e90505d14
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2021-07-01-11-59-34.bpo-44490.xY80VR.rst	
@@ -0,0 +1,2 @@
+Add ``__parameters__`` attribute to ``types.Union``. Add ``__getitem__``
+operator to ``types.Union``. Patch provided by Yurii Karabas.
diff --git a/Objects/genericaliasobject.c b/Objects/genericaliasobject.c
index 756a7ce474aee9..d64c323a51edb2 100644
--- a/Objects/genericaliasobject.c
+++ b/Objects/genericaliasobject.c
@@ -198,8 +198,8 @@ tuple_add(PyObject *self, Py_ssize_t len, PyObject *item)
     return 0;
 }
 
-static PyObject *
-make_parameters(PyObject *args)
+PyObject *
+_Py_make_parameters(PyObject *args)
 {
     Py_ssize_t nargs = PyTuple_GET_SIZE(args);
     Py_ssize_t len = nargs;
@@ -294,18 +294,10 @@ subs_tvars(PyObject *obj, PyObject *params, PyObject **argitems)
     return obj;
 }
 
-static PyObject *
-ga_getitem(PyObject *self, PyObject *item)
+PyObject *
+_Py_apply_parameters(PyObject *self, PyObject *args, PyObject *parameters, PyObject *item)
 {
-    gaobject *alias = (gaobject *)self;
-    // do a lookup for __parameters__ so it gets populated (if not already)
-    if (alias->parameters == NULL) {
-        alias->parameters = make_parameters(alias->args);
-        if (alias->parameters == NULL) {
-            return NULL;
-        }
-    }
-    Py_ssize_t nparams = PyTuple_GET_SIZE(alias->parameters);
+    Py_ssize_t nparams = PyTuple_GET_SIZE(parameters);
     if (nparams == 0) {
         return PyErr_Format(PyExc_TypeError,
                             "There are no type variables left in %R",
@@ -320,32 +312,32 @@ ga_getitem(PyObject *self, PyObject *item)
                             nitems > nparams ? "many" : "few",
                             self);
     }
-    /* Replace all type variables (specified by alias->parameters)
+    /* Replace all type variables (specified by parameters)
        with corresponding values specified by argitems.
         t = list[T];          t[int]      -> newargs = [int]
         t = dict[str, T];     t[int]      -> newargs = [str, int]
         t = dict[T, list[S]]; t[str, int] -> newargs = [str, list[int]]
      */
-    Py_ssize_t nargs = PyTuple_GET_SIZE(alias->args);
+    Py_ssize_t nargs = PyTuple_GET_SIZE(args);
     PyObject *newargs = PyTuple_New(nargs);
     if (newargs == NULL) {
         return NULL;
     }
     for (Py_ssize_t iarg = 0; iarg < nargs; iarg++) {
-        PyObject *arg = PyTuple_GET_ITEM(alias->args, iarg);
+        PyObject *arg = PyTuple_GET_ITEM(args, iarg);
         int typevar = is_typevar(arg);
         if (typevar < 0) {
             Py_DECREF(newargs);
             return NULL;
         }
         if (typevar) {
-            Py_ssize_t iparam = tuple_index(alias->parameters, nparams, arg);
+            Py_ssize_t iparam = tuple_index(parameters, nparams, arg);
             assert(iparam >= 0);
             arg = argitems[iparam];
             Py_INCREF(arg);
         }
         else {
-            arg = subs_tvars(arg, alias->parameters, argitems);
+            arg = subs_tvars(arg, parameters, argitems);
             if (arg == NULL) {
                 Py_DECREF(newargs);
                 return NULL;
@@ -354,6 +346,26 @@ ga_getitem(PyObject *self, PyObject *item)
         PyTuple_SET_ITEM(newargs, iarg, arg);
     }
 
+    return newargs;
+}
+
+static PyObject *
+ga_getitem(PyObject *self, PyObject *item)
+{
+    gaobject *alias = (gaobject *)self;
+    // do a lookup for __parameters__ so it gets populated (if not already)
+    if (alias->parameters == NULL) {
+        alias->parameters = _Py_make_parameters(alias->args);
+        if (alias->parameters == NULL) {
+            return NULL;
+        }
+    }
+
+    PyObject *newargs = _Py_apply_parameters(self, alias->args, alias->parameters, item);
+    if (newargs == NULL) {
+        return NULL;
+    }
+
     PyObject *res = Py_GenericAlias(alias->origin, newargs);
 
     Py_DECREF(newargs);
@@ -550,7 +562,7 @@ ga_parameters(PyObject *self, void *unused)
 {
     gaobject *alias = (gaobject *)self;
     if (alias->parameters == NULL) {
-        alias->parameters = make_parameters(alias->args);
+        alias->parameters = _Py_make_parameters(alias->args);
         if (alias->parameters == NULL) {
             return NULL;
         }
diff --git a/Objects/unionobject.c b/Objects/unionobject.c
index cc7181d2475caf..81c613c150e0c4 100644
--- a/Objects/unionobject.c
+++ b/Objects/unionobject.c
@@ -8,6 +8,7 @@
 typedef struct {
     PyObject_HEAD
     PyObject *args;
+    PyObject *parameters;
 } unionobject;
 
 static void
@@ -18,6 +19,7 @@ unionobject_dealloc(PyObject *self)
     _PyObject_GC_UNTRACK(self);
 
     Py_XDECREF(alias->args);
+    Py_XDECREF(alias->parameters);
     Py_TYPE(self)->tp_free(self);
 }
 
@@ -435,6 +437,53 @@ static PyMethodDef union_methods[] = {
         {"__subclasscheck__", union_subclasscheck, METH_O},
         {0}};
 
+
+static PyObject *
+union_getitem(PyObject *self, PyObject *item)
+{
+    unionobject *alias = (unionobject *)self;
+    // do a lookup for __parameters__ so it gets populated (if not already)
+    if (alias->parameters == NULL) {
+        alias->parameters = _Py_make_parameters(alias->args);
+        if (alias->parameters == NULL) {
+            return NULL;
+        }
+    }
+
+    PyObject *newargs = _Py_apply_parameters(self, alias->args, alias->parameters, item);
+    if (newargs == NULL) {
+        return NULL;
+    }
+
+    PyObject *res = _Py_Union(newargs);
+
+    Py_DECREF(newargs);
+    return res;
+}
+
+static PyMappingMethods union_as_mapping = {
+    .mp_subscript = union_getitem,
+};
+
+static PyObject *
+union_parameters(PyObject *self, void *unused)
+{
+    unionobject *alias = (unionobject *)self;
+    if (alias->parameters == NULL) {
+        alias->parameters = _Py_make_parameters(alias->args);
+        if (alias->parameters == NULL) {
+            return NULL;
+        }
+    }
+    Py_INCREF(alias->parameters);
+    return alias->parameters;
+}
+
+static PyGetSetDef union_properties[] = {
+    {"__parameters__", union_parameters, (setter)NULL, "Type variables in the types.Union.", NULL},
+    {0}
+};
+
 static PyNumberMethods union_as_number = {
         .nb_or = _Py_union_type_or, // Add __or__ function
 };
@@ -456,8 +505,10 @@ PyTypeObject _Py_UnionType = {
     .tp_members = union_members,
     .tp_methods = union_methods,
     .tp_richcompare = union_richcompare,
+    .tp_as_mapping = &union_as_mapping,
     .tp_as_number = &union_as_number,
     .tp_repr = union_repr,
+    .tp_getset = union_properties,
 };
 
 PyObject *
@@ -489,6 +540,7 @@ _Py_Union(PyObject *args)
         return NULL;
     }
 
+    result->parameters = NULL;
     result->args = dedup_and_flatten_args(args);
     if (result->args == NULL) {
         PyObject_GC_Del(result);

From 4d1a60b3f37cbb3a89774a5804a3f02fffafb459 Mon Sep 17 00:00:00 2001
From: Yurii Karabas <1998uriyyo@gmail.com>
Date: Thu, 1 Jul 2021 12:14:46 +0300
Subject: [PATCH 02/13] Add Py_LIMITED_API to genericaliasobject

---
 Include/genericaliasobject.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Include/genericaliasobject.h b/Include/genericaliasobject.h
index 6df56fbf41df79..7d52e8a8e57b2f 100644
--- a/Include/genericaliasobject.h
+++ b/Include/genericaliasobject.h
@@ -5,8 +5,11 @@
 extern "C" {
 #endif
 
+#ifndef Py_LIMITED_API
 PyAPI_FUNC(PyObject *) _Py_apply_parameters(PyObject *, PyObject *, PyObject *, PyObject *);
 PyAPI_FUNC(PyObject *) _Py_make_parameters(PyObject *);
+#endif
+
 PyAPI_FUNC(PyObject *) Py_GenericAlias(PyObject *, PyObject *);
 PyAPI_DATA(PyTypeObject) Py_GenericAliasType;
 

From 71b306ef6cabc59a4ad2e72a5a29b0872423e4b3 Mon Sep 17 00:00:00 2001
From: Yurii Karabas <1998uriyyo@gmail.com>
Date: Thu, 1 Jul 2021 18:21:57 +0300
Subject: [PATCH 03/13] Update Lib/test/test_types.py

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
---
 Lib/test/test_types.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
index 3f938c3a81f98f..edc4d1b0203b5f 100644
--- a/Lib/test/test_types.py
+++ b/Lib/test/test_types.py
@@ -669,7 +669,7 @@ def test_or_type_operator_with_TypeVar(self):
     def test_union_parameter_chaining(self):
         T = typing.TypeVar("T")
 
-        assert (float | list[T])[int] == float | list[int]
+        self.assertEqual((float | list[T])[int], float | list[int])
 
     def test_or_type_operator_with_forward(self):
         T = typing.TypeVar('T')

From 09d441cf0e6187bba2d1e712671b768e19ca98e5 Mon Sep 17 00:00:00 2001
From: Yurii Karabas <1998uriyyo@gmail.com>
Date: Thu, 1 Jul 2021 18:22:08 +0300
Subject: [PATCH 04/13] Update Misc/NEWS.d/next/Core and
 Builtins/2021-07-01-11-59-34.bpo-44490.xY80VR.rst

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
---
 .../Core and Builtins/2021-07-01-11-59-34.bpo-44490.xY80VR.rst  | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-07-01-11-59-34.bpo-44490.xY80VR.rst b/Misc/NEWS.d/next/Core and Builtins/2021-07-01-11-59-34.bpo-44490.xY80VR.rst
index 1f8f3e90505d14..4912bca837bb10 100644
--- a/Misc/NEWS.d/next/Core and Builtins/2021-07-01-11-59-34.bpo-44490.xY80VR.rst	
+++ b/Misc/NEWS.d/next/Core and Builtins/2021-07-01-11-59-34.bpo-44490.xY80VR.rst	
@@ -1,2 +1,2 @@
-Add ``__parameters__`` attribute to ``types.Union``. Add ``__getitem__``
+Add ``__parameters__`` attribute and ``__getitem__``
 operator to ``types.Union``. Patch provided by Yurii Karabas.

From 8033e53ea44a77b2fe9c8ff52f82c6ef7cce754c Mon Sep 17 00:00:00 2001
From: Yurii Karabas <1998uriyyo@gmail.com>
Date: Thu, 1 Jul 2021 18:22:25 +0300
Subject: [PATCH 05/13] Update Objects/unionobject.c

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
---
 Objects/unionobject.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Objects/unionobject.c b/Objects/unionobject.c
index 81c613c150e0c4..7c3d436a4c2eee 100644
--- a/Objects/unionobject.c
+++ b/Objects/unionobject.c
@@ -466,7 +466,7 @@ static PyMappingMethods union_as_mapping = {
 };
 
 static PyObject *
-union_parameters(PyObject *self, void *unused)
+union_parameters(PyObject *self, void *Py_UNUSED(unused))
 {
     unionobject *alias = (unionobject *)self;
     if (alias->parameters == NULL) {

From 59f54afa5ede83df2e00b679ffd406b507b6ef3f Mon Sep 17 00:00:00 2001
From: Yurii Karabas <1998uriyyo@gmail.com>
Date: Thu, 1 Jul 2021 18:32:58 +0300
Subject: [PATCH 06/13] Update Include/genericaliasobject.h

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
---
 Include/genericaliasobject.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Include/genericaliasobject.h b/Include/genericaliasobject.h
index 7d52e8a8e57b2f..4ce9244bb4ce79 100644
--- a/Include/genericaliasobject.h
+++ b/Include/genericaliasobject.h
@@ -6,7 +6,7 @@ extern "C" {
 #endif
 
 #ifndef Py_LIMITED_API
-PyAPI_FUNC(PyObject *) _Py_apply_parameters(PyObject *, PyObject *, PyObject *, PyObject *);
+PyAPI_FUNC(PyObject *) _Py_subs_parameters(PyObject *, PyObject *, PyObject *, PyObject *);
 PyAPI_FUNC(PyObject *) _Py_make_parameters(PyObject *);
 #endif
 

From 2072f7e73a22ff29718a3238b228a1b2988ed314 Mon Sep 17 00:00:00 2001
From: Yurii Karabas <1998uriyyo@gmail.com>
Date: Thu, 1 Jul 2021 18:34:34 +0300
Subject: [PATCH 07/13] Update genericaliasobject.c

---
 Objects/genericaliasobject.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Objects/genericaliasobject.c b/Objects/genericaliasobject.c
index d64c323a51edb2..30e11ab7b4da87 100644
--- a/Objects/genericaliasobject.c
+++ b/Objects/genericaliasobject.c
@@ -295,7 +295,7 @@ subs_tvars(PyObject *obj, PyObject *params, PyObject **argitems)
 }
 
 PyObject *
-_Py_apply_parameters(PyObject *self, PyObject *args, PyObject *parameters, PyObject *item)
+_Py_subs_parameters(PyObject *self, PyObject *args, PyObject *parameters, PyObject *item)
 {
     Py_ssize_t nparams = PyTuple_GET_SIZE(parameters);
     if (nparams == 0) {
@@ -361,7 +361,7 @@ ga_getitem(PyObject *self, PyObject *item)
         }
     }
 
-    PyObject *newargs = _Py_apply_parameters(self, alias->args, alias->parameters, item);
+    PyObject *newargs = _Py_subs_parameters(self, alias->args, alias->parameters, item);
     if (newargs == NULL) {
         return NULL;
     }

From cea63d4ac51938e2a250e622a6ce07a68a86af3c Mon Sep 17 00:00:00 2001
From: Yurii Karabas <1998uriyyo@gmail.com>
Date: Thu, 1 Jul 2021 18:35:21 +0300
Subject: [PATCH 08/13] Update unionobject.c

---
 Objects/unionobject.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Objects/unionobject.c b/Objects/unionobject.c
index 7c3d436a4c2eee..2c2221c7f9b84f 100644
--- a/Objects/unionobject.c
+++ b/Objects/unionobject.c
@@ -450,7 +450,7 @@ union_getitem(PyObject *self, PyObject *item)
         }
     }
 
-    PyObject *newargs = _Py_apply_parameters(self, alias->args, alias->parameters, item);
+    PyObject *newargs = _Py_subs_parameters(self, alias->args, alias->parameters, item);
     if (newargs == NULL) {
         return NULL;
     }

From 27980f1c896b39955da20f327f68af083371004c Mon Sep 17 00:00:00 2001
From: Yurii Karabas <1998uriyyo@gmail.com>
Date: Thu, 1 Jul 2021 19:09:22 +0300
Subject: [PATCH 09/13] Update Lib/test/test_types.py

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
---
 Lib/test/test_types.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
index edc4d1b0203b5f..070692fa0c6eda 100644
--- a/Lib/test/test_types.py
+++ b/Lib/test/test_types.py
@@ -670,7 +670,8 @@ def test_union_parameter_chaining(self):
         T = typing.TypeVar("T")
 
         self.assertEqual((float | list[T])[int], float | list[int])
-
+self.assertEqual(list[int | list[T]].__parameters__, (T,))
+self.assertEqual(list[int | list[T]][str], list[int | list[str]])
     def test_or_type_operator_with_forward(self):
         T = typing.TypeVar('T')
         ForwardAfter = T | 'Forward'

From afbb03868e6ecc8ab963ff736a7d47d82c8ad6da Mon Sep 17 00:00:00 2001
From: Yurii Karabas <1998uriyyo@gmail.com>
Date: Thu, 1 Jul 2021 19:12:04 +0300
Subject: [PATCH 10/13] Update test_types.py

---
 Lib/test/test_types.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
index 070692fa0c6eda..22ae20d8c4efc3 100644
--- a/Lib/test/test_types.py
+++ b/Lib/test/test_types.py
@@ -670,8 +670,9 @@ def test_union_parameter_chaining(self):
         T = typing.TypeVar("T")
 
         self.assertEqual((float | list[T])[int], float | list[int])
-self.assertEqual(list[int | list[T]].__parameters__, (T,))
-self.assertEqual(list[int | list[T]][str], list[int | list[str]])
+        self.assertEqual(list[int | list[T]].__parameters__, (T,))
+        self.assertEqual(list[int | list[T]][str], list[int | list[str]])
+
     def test_or_type_operator_with_forward(self):
         T = typing.TypeVar('T')
         ForwardAfter = T | 'Forward'

From 8cfa980308abec90a29f54cc17218335d079f882 Mon Sep 17 00:00:00 2001
From: Yurii Karabas <1998uriyyo@gmail.com>
Date: Sat, 3 Jul 2021 17:27:58 +0300
Subject: [PATCH 11/13] Add parameters to union_traverse

---
 Objects/unionobject.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Objects/unionobject.c b/Objects/unionobject.c
index 2c2221c7f9b84f..9fc0620a28f380 100644
--- a/Objects/unionobject.c
+++ b/Objects/unionobject.c
@@ -28,6 +28,7 @@ union_traverse(PyObject *self, visitproc visit, void *arg)
 {
     unionobject *alias = (unionobject *)self;
     Py_VISIT(alias->args);
+    Py_VISIT(alias->parameters);
     return 0;
 }
 

From 1a1ac841b4f3ad0b5b1ca09afd6d753e327cf5ad Mon Sep 17 00:00:00 2001
From: Yurii Karabas <1998uriyyo@gmail.com>
Date: Mon, 5 Jul 2021 20:00:19 +0300
Subject: [PATCH 12/13] Update Objects/genericaliasobject.c

Co-authored-by: Guido van Rossum <gvanrossum@gmail.com>
---
 Objects/genericaliasobject.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Objects/genericaliasobject.c b/Objects/genericaliasobject.c
index 30e11ab7b4da87..87b04851e08da1 100644
--- a/Objects/genericaliasobject.c
+++ b/Objects/genericaliasobject.c
@@ -353,7 +353,7 @@ static PyObject *
 ga_getitem(PyObject *self, PyObject *item)
 {
     gaobject *alias = (gaobject *)self;
-    // do a lookup for __parameters__ so it gets populated (if not already)
+    // Populate __parameters__ if needed.
     if (alias->parameters == NULL) {
         alias->parameters = _Py_make_parameters(alias->args);
         if (alias->parameters == NULL) {

From 7799a159b616e9d90754bfa013f6ac27a284e4f6 Mon Sep 17 00:00:00 2001
From: Yurii Karabas <1998uriyyo@gmail.com>
Date: Mon, 5 Jul 2021 20:09:43 +0300
Subject: [PATCH 13/13] Add more tests to cover types.Union

---
 Lib/test/test_types.py | 3 +++
 Objects/unionobject.c  | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
index 22ae20d8c4efc3..7f7ce86ff08ef3 100644
--- a/Lib/test/test_types.py
+++ b/Lib/test/test_types.py
@@ -668,10 +668,13 @@ def test_or_type_operator_with_TypeVar(self):
 
     def test_union_parameter_chaining(self):
         T = typing.TypeVar("T")
+        S = typing.TypeVar("S")
 
         self.assertEqual((float | list[T])[int], float | list[int])
         self.assertEqual(list[int | list[T]].__parameters__, (T,))
         self.assertEqual(list[int | list[T]][str], list[int | list[str]])
+        self.assertEqual((list[T] | list[S]).__parameters__, (T, S))
+        self.assertEqual((list[T] | list[S])[int, T], list[int] | list[T])
 
     def test_or_type_operator_with_forward(self):
         T = typing.TypeVar('T')
diff --git a/Objects/unionobject.c b/Objects/unionobject.c
index 9fc0620a28f380..9d714fb073711a 100644
--- a/Objects/unionobject.c
+++ b/Objects/unionobject.c
@@ -443,7 +443,7 @@ static PyObject *
 union_getitem(PyObject *self, PyObject *item)
 {
     unionobject *alias = (unionobject *)self;
-    // do a lookup for __parameters__ so it gets populated (if not already)
+    // Populate __parameters__ if needed.
     if (alias->parameters == NULL) {
         alias->parameters = _Py_make_parameters(alias->args);
         if (alias->parameters == NULL) {