gh-85943: Fix BytesWarning in the struct format cache under -bb (GH-1…

…53627) 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 190d2ff) Co-authored-by: Serhiy Storchaka <storchaka@gmail.com> Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
serhiy-storchaka · Jul 17, 2026 · Jul 17, 2026 · Jul 17, 2026 · e3616a92854cb7cd70faf0962366199539d510c3
commit e3616a92854cb7cd70faf0962366199539d510c3
diff --git a/Lib/test/test_struct.py b/Lib/test/test_struct.py
@@ -181,6 +181,16 @@ def test_calcsize(self):
         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
@@ -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
@@ -2504,32 +2504,50 @@ 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) {
         Py_SETREF(*ptr, NULL);
         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_CAST(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;
 }