From c0a18a62b39b00a414a461eadf380d0d989ebc4b Mon Sep 17 00:00:00 2001
From: AZero13 <gfunni234@gmail.com>
Date: Tue, 9 Dec 2025 09:22:43 -0500
Subject: [PATCH] gh-142462: Ensure kw_defaults is always populated in AST
 arguments

Normalize AST arguments construction to always allocate kwonlyargs/kw_defaults (with length parity) and assert the invariant.
---
 ...025-12-09-09-22-38.gh-issue-142462.WlnNMW.rst |  2 ++
 Parser/asdl_c.py                                 | 16 ++++++++++++++++
 Python/Python-ast.c                              | 13 +++++++++++++
 3 files changed, 31 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2025-12-09-09-22-38.gh-issue-142462.WlnNMW.rst

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-12-09-09-22-38.gh-issue-142462.WlnNMW.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-12-09-09-22-38.gh-issue-142462.WlnNMW.rst
new file mode 100644
index 00000000000000..faa957068ab2d2
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-12-09-09-22-38.gh-issue-142462.WlnNMW.rst
@@ -0,0 +1,2 @@
+Normalize AST arguments construction to always allocate
+kwonlyargs/kw_defaults (with length parity) and assert the invariant.
diff --git a/Parser/asdl_c.py b/Parser/asdl_c.py
index 3e252cbc4883d1..6a2d8de0270c13 100755
--- a/Parser/asdl_c.py
+++ b/Parser/asdl_c.py
@@ -437,6 +437,22 @@ def emit(s, depth=0, reflow=True):
         emit("p = (%s)_PyArena_Malloc(arena, sizeof(*p));" % ctype, 1);
         emit("if (!p)", 1)
         emit("return NULL;", 2)
+        # Special-case normalization for the arguments product to guarantee
+        # non-NULL kwonlyargs/kw_defaults with matched lengths.
+        if name == "arguments":
+            emit("if (kwonlyargs == NULL) {", 1)
+            emit("kwonlyargs = _Py_asdl_arg_seq_new(0, arena);", 2)
+            emit("if (!kwonlyargs) {", 2)
+            emit("return NULL;", 3)
+            emit("}", 2)
+            emit("}", 1)
+            emit("if (kw_defaults == NULL) {", 1)
+            emit("kw_defaults = _Py_asdl_expr_seq_new(asdl_seq_LEN(kwonlyargs), arena);", 2)
+            emit("if (!kw_defaults) {", 2)
+            emit("return NULL;", 3)
+            emit("}", 2)
+            emit("}", 1)
+            emit("assert(asdl_seq_LEN(kw_defaults) == asdl_seq_LEN(kwonlyargs));", 1)
         if union:
             self.emit_body_union(name, args, attrs)
         else:
diff --git a/Python/Python-ast.c b/Python/Python-ast.c
index aac24ed7d3c0c5..5a3f78b6462c26 100644
--- a/Python/Python-ast.c
+++ b/Python/Python-ast.c
@@ -8520,6 +8520,19 @@ _PyAST_arguments(asdl_arg_seq * posonlyargs, asdl_arg_seq * args, arg_ty
     p = (arguments_ty)_PyArena_Malloc(arena, sizeof(*p));
     if (!p)
         return NULL;
+    if (kwonlyargs == NULL) {
+        kwonlyargs = _Py_asdl_arg_seq_new(0, arena);
+        if (!kwonlyargs) {
+            return NULL;
+        }
+    }
+    if (kw_defaults == NULL) {
+        kw_defaults = _Py_asdl_expr_seq_new(asdl_seq_LEN(kwonlyargs), arena);
+        if (!kw_defaults) {
+            return NULL;
+        }
+    }
+    assert(asdl_seq_LEN(kw_defaults) == asdl_seq_LEN(kwonlyargs));
     p->posonlyargs = posonlyargs;
     p->args = args;
     p->vararg = vararg;