gh-143904: Raise OverflowError instead of IndexError for too large offset in struct.pack_into()

Lib/test/test_struct.py

@@ -478,9 +478,9 @@ def _test_pack_into(self, pack_into):
             pack_into(writable_buf, None, test_string)
         with self.assertRaises(TypeError):
             pack_into(writable_buf, 0.0, test_string)
-        with self.assertRaises((IndexError, OverflowError)):
+        with self.assertRaises(OverflowError):
             pack_into(writable_buf, 2**1000, test_string)
-        with self.assertRaises((IndexError, OverflowError)):
+        with self.assertRaises(OverflowError):
             pack_into(writable_buf, -2**1000, test_string)
 
     def test_pack_into(self):

Misc/NEWS.d/next/Library/2026-01-16-14-02-39.gh-issue-143904.rErHHA.rst

@@ -0,0 +1,2 @@
+:func:`struct.pack_into` now raises OverflowError instead of IndexError for
+too large *offset* argument.

Modules/_struct.c

@@ -2275,7 +2275,7 @@ Struct_pack_impl(PyStructObject *self, PyObject * const *values,
 Struct.pack_into
 
     buffer: Py_buffer(accept={rwbuffer})
-    offset as offset_obj: object
+    offset: Py_ssize_t
     /
     *values: array
 
@@ -2289,11 +2289,10 @@ help(struct) for more on format strings.
 
 static PyObject *
 Struct_pack_into_impl(PyStructObject *self, Py_buffer *buffer,
-                      PyObject *offset_obj, PyObject * const *values,
+                      Py_ssize_t offset, PyObject * const *values,
                       Py_ssize_t values_length)
-/*[clinic end generated code: output=b0c2ef496135dad3 input=d0de9b9f138c782d]*/
+/*[clinic end generated code: output=aa9d9a93f5f8f77b input=9d842a368ee14245]*/
 {
-    Py_ssize_t offset;
     _structmodulestate *state = get_struct_state_structinst(self);
 
     ENSURE_STRUCT_IS_READY(self);
@@ -2304,12 +2303,6 @@ Struct_pack_into_impl(PyStructObject *self, Py_buffer *buffer,
         return NULL;
     }
 
-    /* Extract the offset from the first argument */
-    offset = PyNumber_AsSsize_t(offset_obj, PyExc_IndexError);
-    if (offset == -1 && PyErr_Occurred()) {
-        return NULL;
-    }
-
     /* Support negative offsets. */
     if (offset < 0) {
          /* Check that negative offset is low enough to fit data */
@@ -2546,7 +2539,7 @@ pack_into
 
     format as s_object: cache_struct
     buffer: Py_buffer(accept={rwbuffer})
-    offset as offset_obj: object
+    offset: Py_ssize_t
     /
     *values: array
 
@@ -2560,11 +2553,11 @@ strings.
 
 static PyObject *
 pack_into_impl(PyObject *module, PyStructObject *s_object, Py_buffer *buffer,
-               PyObject *offset_obj, PyObject * const *values,
+               Py_ssize_t offset, PyObject * const *values,
                Py_ssize_t values_length)
-/*[clinic end generated code: output=148ef659a490eec3 input=3c5fe5bd3b6fd396]*/
+/*[clinic end generated code: output=e8bf7d422b2088ef input=086867c0f5d8a8e4]*/
 {
-    return Struct_pack_into_impl(s_object, buffer, offset_obj,
+    return Struct_pack_into_impl(s_object, buffer, offset,
                                  values, values_length);
 }