From 7336aaad1a7458630089c4aecc8806b15ec64b23 Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Tue, 10 Mar 2026 14:26:47 +0200
Subject: [PATCH 1/2] gh-145743: Fix inconsistancy after calling
 Struct.__init__() with invalid format

Only set the format attribute after successful (re-)initialization.
---
 Lib/test/test_struct.py | 20 ++++++++++++++++++--
 Modules/_struct.c       | 30 ++++++++++++++++--------------
 2 files changed, 34 insertions(+), 16 deletions(-)

diff --git a/Lib/test/test_struct.py b/Lib/test/test_struct.py
index 4cbfd7ad8b1e48..6904572d095d31 100644
--- a/Lib/test/test_struct.py
+++ b/Lib/test/test_struct.py
@@ -584,8 +584,24 @@ def test_Struct_reinitialization(self):
         # Issue 9422: there was a memory leak when reinitializing a
         # Struct instance.  This test can be used to detect the leak
         # when running with regrtest -L.
-        s = struct.Struct('i')
-        s.__init__('ii')
+        s = struct.Struct('>h')
+        s.__init__('>hh')
+        self.assertEqual(s.format, '>hh')
+        packed = b'\x00\x01\x00\x02'
+        self.assertEqual(s.pack(1, 2), packed)
+        self.assertEqual(s.unpack(packed), (1, 2))
+
+        with self.assertRaises(UnicodeEncodeError):
+            s.__init__('\udc00')
+        self.assertEqual(s.format, '>hh')
+        self.assertEqual(s.pack(1, 2), packed)
+        self.assertEqual(s.unpack(packed), (1, 2))
+
+        with self.assertRaises(struct.error):
+            s.__init__('$')
+        self.assertEqual(s.format, '>hh')
+        self.assertEqual(s.pack(1, 2), packed)
+        self.assertEqual(s.unpack(packed), (1, 2))
 
     def check_sizeof(self, format_str, number_of_codes):
         # The size of 'PyStructObject'
diff --git a/Modules/_struct.c b/Modules/_struct.c
index dcc3c7ec63e9e0..c4b562df63e581 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -1620,11 +1620,11 @@ align(Py_ssize_t size, char c, const formatdef *e)
 /* calculate the size of a format string */
 
 static int
-prepare_s(PyStructObject *self)
+prepare_s(PyStructObject *self, PyObject *format)
 {
     const formatdef *f;
     const formatdef *e;
-    formatcode *codes;
+    formatcode *codes, *codes0;
 
     const char *s;
     const char *fmt;
@@ -1634,8 +1634,8 @@ prepare_s(PyStructObject *self)
 
     _structmodulestate *state = get_struct_state_structinst(self);
 
-    fmt = PyBytes_AS_STRING(self->s_format);
-    if (strlen(fmt) != (size_t)PyBytes_GET_SIZE(self->s_format)) {
+    fmt = PyBytes_AS_STRING(format);
+    if (strlen(fmt) != (size_t)PyBytes_GET_SIZE(format)) {
         PyErr_SetString(state->StructError,
                         "embedded null character");
         return -1;
@@ -1711,13 +1711,7 @@ prepare_s(PyStructObject *self)
         PyErr_NoMemory();
         return -1;
     }
-    /* Free any s_codes value left over from a previous initialization. */
-    if (self->s_codes != NULL)
-        PyMem_Free(self->s_codes);
-    self->s_codes = codes;
-    self->s_size = size;
-    self->s_len = len;
-
+    codes0 = codes;
     s = fmt;
     size = 0;
     while ((c = *s++) != '\0') {
@@ -1757,6 +1751,15 @@ prepare_s(PyStructObject *self)
     codes->size = 0;
     codes->repeat = 0;
 
+    /* Free any s_codes value left over from a previous initialization. */
+    if (self->s_codes != NULL)
+        PyMem_Free(self->s_codes);
+    self->s_codes = codes0;
+    self->s_size = size;
+    self->s_len = len;
+    Py_INCREF(format);
+    Py_XSETREF(self->s_format, format);
+
     return 0;
 
   overflow:
@@ -1820,9 +1823,8 @@ Struct___init___impl(PyStructObject *self, PyObject *format)
         return -1;
     }
 
-    Py_SETREF(self->s_format, format);
-
-    ret = prepare_s(self);
+    ret = prepare_s(self, format);
+    Py_DECREF(format);
     return ret;
 }
 

From a23f9bb13d3b70d4db72f4a64d6de2cc7b7c5b3f Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Tue, 10 Mar 2026 15:26:28 +0200
Subject: [PATCH 2/2] Update Modules/_struct.c

Co-authored-by: Victor Stinner <vstinner@python.org>
---
 Modules/_struct.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/Modules/_struct.c b/Modules/_struct.c
index c4b562df63e581..c2f7b1fe0e800a 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -1757,8 +1757,7 @@ prepare_s(PyStructObject *self, PyObject *format)
     self->s_codes = codes0;
     self->s_size = size;
     self->s_len = len;
-    Py_INCREF(format);
-    Py_XSETREF(self->s_format, format);
+    Py_XSETREF(self->s_format, Py_NewRef(format));
 
     return 0;