[3.13] gh-85943: Fix BytesWarning in the struct format cache under -bb (GH-153627...

Normalize bytes format strings to str before using them as the cache key,
so that equal str and bytes formats no longer collide and get compared.

(cherry picked from commit 190d2ffbb969ad571fd12d4988b37519091194d5)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

diff --git a/Lib/test/test_struct.py b/Lib/test/test_struct.py
index 460e9e94aacf81bbd6a6ac2fe452b3dd6ea95c66..a5a1edcb0d193c8a48b931758c52985ce23da28f 100644 (file)
--- a/Lib/test/test_struct.py
+++ b/Lib/test/test_struct.py
@@ -176,6 +176,16 @@ class StructTest(unittest.TestCase):
         self.assertGreaterEqual(struct.calcsize('n'), struct.calcsize('i'))
         self.assertGreaterEqual(struct.calcsize('n'), struct.calcsize('P'))
 
+    def test_cache_bytes_vs_str_bb(self):
+        # Mixing str and bytes formats must not raise BytesWarning under -bb.
+        code = (
+            'import struct\n'
+            'struct.calcsize(b"!d"); struct.calcsize("!d")\n'
+            'struct.calcsize(">d"); struct.calcsize(b">d")\n'
+            'struct.Struct(b"i"); struct.Struct("i")\n'
+        )
+        assert_python_ok('-bb', '-c', code)
+
     def test_integers(self):
         # Integer tests (bBhHiIlLqQnN).
         import binascii

diff --git a/Misc/NEWS.d/next/Library/2026-07-12-00-00-00.gh-issue-85943.8f906e.rst b/Misc/NEWS.d/next/Library/2026-07-12-00-00-00.gh-issue-85943.8f906e.rst
new file mode 100644 (file)
index 0000000..0a93bf2
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-07-12-00-00-00.gh-issue-85943.8f906e.rst
@@ -0,0 +1,4 @@
+Fix :mod:`struct` functions raising :exc:`BytesWarning` under the ``-bb``
+command line option when a :class:`str` format is used after an equal
+:class:`bytes` format (or vice versa).  The internal format cache no longer
+mixes :class:`str` and :class:`bytes` keys.

diff --git a/Modules/_struct.c b/Modules/_struct.c
index 1195f6f0d7434ffa7638961c8e7761a80f5fd5c7..36d5ece390e8a21366e7bc3b148ba7810ab26b28 100644 (file)
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -2289,7 +2289,8 @@ static PyType_Spec PyStructType_spec = {
 static int
 cache_struct_converter(PyObject *module, PyObject *fmt, PyStructObject **ptr)
 {
-    PyObject * s_object;
+    PyObject *s_object;
+    PyObject *key;
     _structmodulestate *state = get_struct_state(module);
 
     if (fmt == NULL) {
@@ -2297,24 +2298,41 @@ cache_struct_converter(PyObject *module, PyObject *fmt, PyStructObject **ptr)
         return 1;
     }
 
-    if (PyDict_GetItemRef(state->cache, fmt, &s_object) < 0) {
+    /* Use a str cache key: an equal str and bytes would collide and be
+       compared, raising BytesWarning under -bb. */
+    if (PyBytes_Check(fmt)) {
+        key = PyUnicode_DecodeASCII(PyBytes_AS_STRING(fmt),
+                                    PyBytes_GET_SIZE(fmt), "surrogateescape");
+        if (key == NULL) {
+            return 0;
+        }
+    }
+    else {
+        key = Py_NewRef(fmt);
+    }
+
+    if (PyDict_GetItemRef(state->cache, key, &s_object) < 0) {
+        Py_DECREF(key);
         return 0;
     }
     if (s_object != NULL) {
+        Py_DECREF(key);
         *ptr = (PyStructObject *)s_object;
         return Py_CLEANUP_SUPPORTED;
     }
 
-    s_object = PyObject_CallOneArg(state->PyStructType, fmt);
+    s_object = PyObject_CallOneArg(state->PyStructType, key);
     if (s_object != NULL) {
         if (PyDict_GET_SIZE(state->cache) >= MAXCACHE)
             PyDict_Clear(state->cache);
         /* Attempt to cache the result */
-        if (PyDict_SetItem(state->cache, fmt, s_object) == -1)
+        if (PyDict_SetItem(state->cache, key, s_object) == -1)
             PyErr_Clear();
+        Py_DECREF(key);
         *ptr = (PyStructObject *)s_object;
         return Py_CLEANUP_SUPPORTED;
     }
+    Py_DECREF(key);
     return 0;
 }