Base implementation of PEP 696

Include/internal/pycore_global_strings.h

@@ -98,6 +98,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(__contains__)
         STRUCT_FOR_ID(__copy__)
         STRUCT_FOR_ID(__ctypes_from_outparam__)
+        STRUCT_FOR_ID(__default__)
         STRUCT_FOR_ID(__del__)
         STRUCT_FOR_ID(__delattr__)
         STRUCT_FOR_ID(__delete__)

Lib/test/test_types.py

@@ -828,6 +828,14 @@ def test_union_parameter_chaining(self):
         self.assertEqual((list[T] | list[S])[int, T], list[int] | list[T])
         self.assertEqual((list[T] | list[S])[int, int], list[int])
 
+    def test_union_parameter_default_ordering(self):
+        T = typing.TypeVar("T")
+        U = typing.TypeVar("U", default=int)
+
+        self.assertEqual((list[U] | list[T]).__parameters__, (U, T))
+        with self.assertRaises(TypeError):
+            list[U] | list[T]
+
     def test_union_parameter_substitution(self):
         def eq(actual, expected, typed=True):
             self.assertEqual(actual, expected)
@@ -996,6 +1004,18 @@ def __eq__(self, other):
                 with self.assertRaises(TypeError):
                     issubclass(int, type_)
 
+    def test_generic_alias_subclass_with_defaults(self):
+        T = typing.TypeVar("T")
+        U = typing.TypeVar("U", default=int)
+        class MyGeneric:
+            __class_getitem__ = classmethod(types.GenericAlias)
+
+        class Fine(MyGeneric[T, U]):
+            ...
+
+        with self.assertRaises(TypeError):
+            class NonDefaultFollows(MyGeneric[U, T]): ...
+
     def test_or_type_operator_with_bad_module(self):
         class BadMeta(type):
             __qualname__ = 'TypeVar'

Lib/test/test_typing.py

@@ -444,6 +444,22 @@ def test_bound_errors(self):
         with self.assertRaises(TypeError):
             TypeVar('X', str, float, bound=Employee)
 
+    def test_default_error(self):
+        with self.assertRaises(TypeError):
+            TypeVar('X', default=Union)
+
+    def test_default_ordering(self):
+        T = TypeVar("T")
+        U = TypeVar("U", default=int)
+        V = TypeVar("V", default=float)
+
+        class Foo(Generic[T, U]): ...
+        with self.assertRaises(TypeError):
+            class Bar(Generic[U, T]): ...
+
+        class Baz(Foo[V]): ...
+
+
     def test_missing__name__(self):
         # See bpo-39942
         code = ("import typing\n"
@@ -3971,22 +3987,24 @@ def test_immutability_by_copy_and_pickle(self):
         TPB = TypeVar('TPB', bound=int)
         TPV = TypeVar('TPV', bytes, str)
         PP = ParamSpec('PP')
-        for X in [TP, TPB, TPV, PP,
+        TD = TypeVar('TD', default=int)
+        for X in [TP, TPB, TPV, PP, TD,
                   List, typing.Mapping, ClassVar, typing.Iterable,
                   Union, Any, Tuple, Callable]:
             with self.subTest(thing=X):
                 self.assertIs(copy(X), X)
                 self.assertIs(deepcopy(X), X)
                 for proto in range(pickle.HIGHEST_PROTOCOL + 1):
                     self.assertIs(pickle.loads(pickle.dumps(X, proto)), X)
-        del TP, TPB, TPV, PP
+        del TP, TPB, TPV, PP, TD
 
         # Check that local type variables are copyable.
         TL = TypeVar('TL')
         TLB = TypeVar('TLB', bound=int)
         TLV = TypeVar('TLV', bytes, str)
         PL = ParamSpec('PL')
-        for X in [TL, TLB, TLV, PL]:
+        TDL = TypeVar('TDL', default=int)
+        for X in [TL, TLB, TLV, PL, TDL]:
             with self.subTest(thing=X):
                 self.assertIs(copy(X), X)
                 self.assertIs(deepcopy(X), X)

Lib/typing.py

@@ -260,6 +260,7 @@ def _collect_parameters(args):
         _collect_parameters((T, Callable[P, T])) == (T, P)
     """
     parameters = []
+    seen_default = False
     for t in args:
         if isinstance(t, type):
             # We don't want __parameters__ descriptor of a bare Python class.
@@ -273,10 +274,24 @@ def _collect_parameters(args):
                         parameters.append(collected)
         elif hasattr(t, '__typing_subst__'):
             if t not in parameters:
+                if hasattr(t, "__default__") and t.__default__ is not None:
+                    seen_default = True
+                elif seen_default:
+                    raise TypeError(
+                        f"non-default type parameter {t!r} follows default type parameter"
+                    )
+
                 parameters.append(t)
         else:
             for x in getattr(t, '__parameters__', ()):
                 if x not in parameters:
+                    if hasattr(x, "__default__") and x.__default__ is not None:
+                        seen_default = True
+                    elif seen_default:
+                        raise TypeError(
+                            f"non-default type parameter {x!r} follows default type parameter"
+                        )
+
                     parameters.append(x)
     return tuple(parameters)
 

Objects/genericaliasobject.c

@@ -214,6 +214,7 @@ _Py_make_parameters(PyObject *args)
     if (parameters == NULL)
         return NULL;
     Py_ssize_t iparam = 0;
+    bool seen_default = false;
     for (Py_ssize_t iarg = 0; iarg < nargs; iarg++) {
         PyObject *t = PyTuple_GET_ITEM(args, iarg);
         PyObject *subst;
@@ -226,6 +227,25 @@ _Py_make_parameters(PyObject *args)
             return NULL;
         }
         if (subst) {
+            PyObject *default_;
+            bool does_not_contain = tuple_index(parameters, nargs, t) == -1;
+            if (does_not_contain) {
+                if (_PyObject_LookupAttr(t, &_Py_ID(__default__), &default_) < 0) {
+                    Py_DECREF(subst);
+                    return NULL;
+                }
+                if (!Py_IsNone(default_)) {
+                    seen_default = true;
+                } else if (seen_default) {
+                    return PyErr_Format(
+                        PyExc_TypeError,
+                        "non-default type parameter %R follows default type parameter",
+                        t
+                    );
+                }
+                Py_DECREF(default_);
+            }
+
             iparam += tuple_add(parameters, iparam, t);
             Py_DECREF(subst);
         }
@@ -248,7 +268,25 @@ _Py_make_parameters(PyObject *args)
                     }
                 }
                 for (Py_ssize_t j = 0; j < len2; j++) {
+                    PyObject *default_;
                     PyObject *t2 = PyTuple_GET_ITEM(subparams, j);
+                    bool does_not_contain = tuple_index(parameters, nargs, t2) == -1;
+                    if (does_not_contain) {
+                        if (_PyObject_LookupAttr(t2, &_Py_ID(__default__), &default_) < 0) {
+                            Py_DECREF(subst);
+                            return NULL;
+                        }
+                        if (default_ && !Py_IsNone(default_)) {
+                            seen_default = true;
+                        } else if (seen_default) {
+                            return PyErr_Format(
+                                PyExc_TypeError,
+                                "non-default type parameter %R follows default type parameter",
+                                t2
+                            );
+                        }
+                        Py_DECREF(default_);
+                    }
                     iparam += tuple_add(parameters, iparam, t2);
                 }
             }