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Date: Thu, 16 Oct 2025 22:26:24 +0200
Subject: [PATCH] =?utf8?q?[3.13]=20gh-138859:=20Account=20for=20`ParamSpec?=
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gh-138859: Account for `ParamSpec` defaults that are not lists … (GH-138868)
(cherry picked from commit 379fd020a0116754f22b04fa2f7f27a8f7b372b0)

Co-authored-by: bzoracler <50305397+bzoracler@users.noreply.github.com>
---
 Lib/test/test_typing.py                                | 10 ++++++++++
 Lib/typing.py                                          |  2 +-
 .../2025-09-13-12-19-17.gh-issue-138859.PxjIoN.rst     |  1 +
 3 files changed, 12 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2025-09-13-12-19-17.gh-issue-138859.PxjIoN.rst

diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
index fb4cf26982d3..abe78c38439e 100644
--- a/Lib/test/test_typing.py
+++ b/Lib/test/test_typing.py
@@ -752,6 +752,16 @@ class TypeParameterDefaultsTests(BaseTestCase):
         self.assertEqual(A[float, [range]].__args__, (float, (range,), float))
         self.assertEqual(A[float, [range], int].__args__, (float, (range,), int))
 
+    def test_paramspec_and_typevar_specialization_2(self):
+        T = TypeVar("T")
+        P = ParamSpec('P', default=...)
+        U = TypeVar("U", default=float)
+        self.assertEqual(P.__default__, ...)
+        class A(Generic[T, P, U]): ...
+        self.assertEqual(A[float].__args__, (float, ..., float))
+        self.assertEqual(A[float, [range]].__args__, (float, (range,), float))
+        self.assertEqual(A[float, [range], int].__args__, (float, (range,), int))
+
     def test_typevartuple_none(self):
         U = TypeVarTuple('U')
         U_None = TypeVarTuple('U_None', default=None)
diff --git a/Lib/typing.py b/Lib/typing.py
index 95e469c4fff7..35ffaabc719b 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1191,7 +1191,7 @@ def _paramspec_prepare_subst(self, alias, args):
     params = alias.__parameters__
     i = params.index(self)
     if i == len(args) and self.has_default():
-        args = [*args, self.__default__]
+        args = (*args, self.__default__)
     if i >= len(args):
         raise TypeError(f"Too few arguments for {alias}")
     # Special case where Z[[int, str, bool]] == Z[int, str, bool] in PEP 612.
diff --git a/Misc/NEWS.d/next/Library/2025-09-13-12-19-17.gh-issue-138859.PxjIoN.rst b/Misc/NEWS.d/next/Library/2025-09-13-12-19-17.gh-issue-138859.PxjIoN.rst
new file mode 100644
index 000000000000..a5d4dd042fcd
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-09-13-12-19-17.gh-issue-138859.PxjIoN.rst
@@ -0,0 +1 @@
+Fix generic type parameterization raising a :exc:`TypeError` when omitting a :class:`ParamSpec` that has a default which is not a list of types.
-- 
2.47.3