Add typing.NoDefault as the default for TypeVar's default

python · JelleZijlstra · May 3, 2024 · Feb 29, 2024 · Feb 29, 2024 · Feb 29, 2024
commit e0ccfeb8a3dfd4813e1b7823d14e78b0cdcb183b
@@ -1616,7 +1616,7 @@ without the dedicated syntax, as documented below.
 
 .. _typevar:
 
-.. class:: TypeVar(name, *constraints, bound=None, covariant=False, contravariant=False, infer_variance=False, default=<unrepresentable>)
+.. class:: TypeVar(name, *constraints, bound=None, covariant=False, contravariant=False, infer_variance=False, default=typing.NoDefault)
 
    Type variable.
 
@@ -1774,7 +1774,7 @@ without the dedicated syntax, as documented below.
 
 .. _typevartuple:
 
-.. class:: TypeVarTuple(name, default=<unrepresentable>)
+.. class:: TypeVarTuple(name, default=typing.NoDefault)
 
    Type variable tuple. A specialized form of :ref:`type variable <typevar>`
    that enables *variadic* generics.
@@ -1903,7 +1903,7 @@ without the dedicated syntax, as documented below.
 
       Support for default values was added.
 
-.. class:: ParamSpec(name, *, bound=None, covariant=False, contravariant=False, default=<unrepresentable>)
+.. class:: ParamSpec(name, *, bound=None, covariant=False, contravariant=False, default=typing.NoDefault)
 
    Parameter specification variable.  A specialized version of
    :ref:`type variables <typevar>`.
@@ -3209,6 +3209,13 @@ Introspection helpers
 
    .. versionadded:: 3.7.4
 
+.. data:: NoDefault
+
+   A sentinel object used to represent the default value of certain parameters
+   used in this module.
+
+   .. versionadded:: 3.13
+
 Constant
 --------
 

diff --git a/Doc/whatsnew/3.13.rst b/Doc/whatsnew/3.13.rst
@@ -831,6 +831,10 @@ typing
   an item of a :class:`typing.TypedDict` as read-only for type checkers.
   See :pep:`705` for more details.
 
+* Add :data:`typing.NoDefault`, a sentinel object used to represent the defaults
+  of some parameters in the :mod:`typing` module. (Contributed by Jelle Zijlstra in
+  :gh:`116126`.)
+
 unicodedata
 -----------
 

diff --git a/Include/internal/pycore_typevarobject.h b/Include/internal/pycore_typevarobject.h
@@ -18,6 +18,7 @@ extern int _Py_initialize_generic(PyInterpreterState *);
 extern void _Py_clear_generic_types(PyInterpreterState *);
 
 extern PyTypeObject _PyTypeAlias_Type;
+extern PyObject _Py_NoDefaultStruct;
 
 #ifdef __cplusplus
 }

@@ -38,7 +38,7 @@
 from typing import Self, LiteralString
 from typing import TypeAlias
 from typing import ParamSpec, Concatenate, ParamSpecArgs, ParamSpecKwargs
-from typing import TypeGuard, TypeIs
+from typing import TypeGuard, TypeIs, NoDefault
 import abc
 import textwrap
 import typing
@@ -592,8 +592,12 @@ class A(Generic[T]): ...
     def test_typevar_none(self):
         U = TypeVar('U')
         U_None = TypeVar('U_None', default=None)
-        self.assertEqual(U.__default__, None)
-        self.assertEqual(U_None.__default__, type(None))
+        self.assertIs(U.__default__, NoDefault)
+        self.assertIs(U_None.__default__, None)
+
+        class X[T]: ...
+        T, = X.__type_params__
+        self.assertIs(T.__default__, NoDefault)
 
     def test_paramspec(self):
         P = ParamSpec('P', default=(str, int))
@@ -609,8 +613,12 @@ class A(Generic[P]): ...
     def test_paramspec_none(self):
         U = ParamSpec('U')
         U_None = ParamSpec('U_None', default=None)
-        self.assertEqual(U.__default__, None)
-        self.assertEqual(U_None.__default__, type(None))
+        self.assertIs(U.__default__, NoDefault)
+        self.assertIs(U_None.__default__, None)
+
+        class X[**P]: ...
+        P, = X.__type_params__
+        self.assertIs(P.__default__, NoDefault)
 
     def test_typevartuple(self):
         Ts = TypeVarTuple('Ts', default=Unpack[Tuple[str, int]])
@@ -682,8 +690,12 @@ class A(Generic[T, P, U]): ...
     def test_typevartuple_none(self):
         U = TypeVarTuple('U')
         U_None = TypeVarTuple('U_None', default=None)
-        self.assertEqual(U.__default__, None)
-        self.assertEqual(U_None.__default__, type(None))
+        self.assertIs(U.__default__, NoDefault)
+        self.assertIs(U_None.__default__, None)
+
+        class X[**Ts]: ...
+        Ts, = X.__type_params__
+        self.assertIs(Ts.__default__, NoDefault)
 
     def test_no_default_after_non_default(self):
         DefaultStrT = TypeVar('DefaultStrT', default=str)
@@ -10145,6 +10157,26 @@ class CustomerModel(ModelBase, init=False):
         self.assertIsInstance(CustomerModel, Decorated)
 
 
+class NoDefaultTests(BaseTestCase):
+    def test_pickling(self):
+        for proto in range(pickle.HIGHEST_PROTOCOL + 1):
+            s = pickle.dumps(NoDefault, proto)
+            loaded = pickle.loads(s)
+            self.assertIs(NoDefault, loaded)
+
+    def test_constructor(self):
+        self.assertIs(NoDefault, type(NoDefault)())
+        with self.assertRaises(TypeError):
+            NoDefault(1)
+
+    def test_repr(self):
+        self.assertEqual(repr(NoDefault), 'typing.NoDefault')
+
+    def test_no_call(self):
+        with self.assertRaises(TypeError):
+            NoDefault()
+
+
 class AllTests(BaseTestCase):
     """Tests for __all__."""
 

@@ -38,6 +38,7 @@
     ParamSpecKwargs,
     TypeAliasType,
     Generic,
+    NoDefault,
 )
 
 # Please keep __all__ alphabetized within each category.
@@ -138,6 +139,7 @@
     'NewType',
     'no_type_check',
     'no_type_check_decorator',
+    'NoDefault',
     'NoReturn',
     'NotRequired',
     'overload',
@@ -284,11 +286,11 @@ def _collect_parameters(args):
                         parameters.append(collected)
         elif hasattr(t, '__typing_subst__'):
             if t not in parameters:
-                if type_var_tuple_encountered and t.__default__ is not None:
+                if type_var_tuple_encountered and t.__default__ is not NoDefault:
                     raise TypeError('Type parameter with a default'
                                     ' follows TypeVarTuple')
 
-                if t.__default__ is not None:
+                if t.__default__ is not NoDefault:
                     default_encountered = True
                 elif default_encountered:
                     raise TypeError(f'Type parameter {t!r} without a default'
@@ -323,10 +325,10 @@ def _check_generic_specialization(cls, arguments):
             # the number of arguments being passed not matching the number
             # of parameters: all parameters that aren't explicitly
             # specialized in this call are parameters with default values.
-            if cls.__parameters__[actual_len].__default__ is not None:
+            if cls.__parameters__[actual_len].__default__ is not NoDefault:
                 return
 
-            expected_len -= sum(p.__default__ is not None for p in cls.__parameters__)
+            expected_len -= sum(p.__default__ is not NoDefault for p in cls.__parameters__)
             expect_val = f"at least {expected_len}"
         else:
             expect_val = expected_len
@@ -1124,7 +1126,7 @@ def _typevartuple_prepare_subst(self, alias, args):
     elif left + right > alen:
         raise TypeError(f"Too few arguments for {alias};"
                         f" actual {alen}, expected at least {plen-1}")
-    if left == alen - right and self.__default__ is not None:
+    if left == alen - right and self.__default__ is not NoDefault:
         replacement = _unpack_args(self.__default__)
     else:
         replacement = args[left: alen - right]
@@ -1150,7 +1152,7 @@ def _paramspec_subst(self, arg):
 def _paramspec_prepare_subst(self, alias, args):
     params = alias.__parameters__
     i = params.index(self)
-    if i == len(args) and self.__default__ is not None:
+    if i == len(args) and self.__default__ is not NoDefault:
         args = [*args, self.__default__]
     if i >= len(args):
         raise TypeError(f"Too few arguments for {alias}")

@@ -63,6 +63,9 @@ _typing_exec(PyObject *m)
     if (PyModule_AddObjectRef(m, "TypeAliasType", (PyObject *)&_PyTypeAlias_Type) < 0) {
         return -1;
     }
+    if (PyModule_AddObjectRef(m, "NoDefault", (PyObject *)&_Py_NoDefaultStruct) < 0) {
+        return -1;
+    }
     return 0;
 }
 

diff --git a/Objects/clinic/typevarobject.c.h b/Objects/clinic/typevarobject.c.h