gh-69528: Distinguish between file modes wb+ and rb+

Lib/_pyio.py

@@ -1498,6 +1498,7 @@ class FileIO(RawIOBase):
     _writable = False
     _appending = False
     _seekable = None
+    _truncate = False
     _closefd = True
 
     def __init__(self, file, mode='r', closefd=True, opener=None):
@@ -1553,6 +1554,7 @@ def __init__(self, file, mode='r', closefd=True, opener=None):
             flags = 0
         elif 'w' in mode:
             self._writable = True
+            self._truncate = True
             flags = os.O_CREAT | os.O_TRUNC
         elif 'a' in mode:
             self._writable = True
@@ -1877,7 +1879,10 @@ def mode(self):
                 return 'ab'
         elif self._readable:
             if self._writable:
-                return 'rb+'
+                if self._truncate:
+                    return 'wb+'
+                else:
+                    return 'rb+'
             else:
                 return 'rb'
         else:

Lib/test/test_fileio.py

@@ -567,8 +567,8 @@ def testModeStrings(self):
         # test that the mode attribute is correct for various mode strings
         # given as init args
         try:
-            for modes in [('w', 'wb'), ('wb', 'wb'), ('wb+', 'rb+'),
-                          ('w+b', 'rb+'), ('a', 'ab'), ('ab', 'ab'),
+            for modes in [('w', 'wb'), ('wb', 'wb'), ('wb+', 'wb+'),
+                          ('w+b', 'wb+'), ('a', 'ab'), ('ab', 'ab'),
                           ('ab+', 'ab+'), ('a+b', 'ab+'), ('r', 'rb'),
                           ('rb', 'rb'), ('rb+', 'rb+'), ('r+b', 'rb+')]:
                 # read modes are last so that TESTFN will exist first

Lib/test/test_gzip.py

@@ -326,6 +326,8 @@ def test_mode(self):
         os_helper.unlink(self.filename)
         with gzip.GzipFile(self.filename, 'x') as f:
             self.assertEqual(f.myfileobj.mode, 'xb')
+        with gzip.GzipFile(self.filename, 'wb+') as f:
+            self.assertEqual(f.myfileobj.mode, 'wb+')
 
     def test_1647484(self):
         for mode in ('wb', 'rb'):

Lib/test/test_io.py

@@ -4337,8 +4337,8 @@ def test_attributes(self):
 
         f = self.open(os_helper.TESTFN, "w+", encoding="utf-8")
         self.assertEqual(f.mode,            "w+")
-        self.assertEqual(f.buffer.mode,     "rb+") # Does it really matter?
-        self.assertEqual(f.buffer.raw.mode, "rb+")
+        self.assertEqual(f.buffer.mode,     "wb+")
+        self.assertEqual(f.buffer.raw.mode, "wb+")
 
         g = self.open(f.fileno(), "wb", closefd=False)
         self.assertEqual(g.mode,     "wb")

Lib/test/test_tempfile.py

@@ -1386,7 +1386,7 @@ def test_properties(self):
 
         f.write(b'x')
         self.assertTrue(f._rolled)
-        self.assertEqual(f.mode, 'rb+')
+        self.assertEqual(f.mode, 'wb+')
         self.assertIsNotNone(f.name)
         with self.assertRaises(AttributeError):
             f.newlines

Misc/NEWS.d/next/Core_and_Builtins/2025-08-15-20-35-30.gh-issue-69528.qc-Eh_.rst

@@ -0,0 +1 @@
+:mod:`io`: Fix a bug where the ``'wb+'`` mode was treated as ``'rb+'``.

Modules/_io/fileio.c

@@ -70,6 +70,7 @@ typedef struct {
     unsigned int writable : 1;
     unsigned int appending : 1;
     signed int seekable : 2; /* -1 means unknown */
+    unsigned int truncate : 1;
     unsigned int closefd : 1;
     char finalizing;
     /* Stat result which was grabbed at file open, useful for optimizing common
@@ -209,6 +210,7 @@ fileio_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     self->writable = 0;
     self->appending = 0;
     self->seekable = -1;
+    self->truncate = 0;
     self->stat_atopen = NULL;
     self->closefd = 1;
     self->weakreflist = NULL;
@@ -341,6 +343,7 @@ _io_FileIO___init___impl(fileio *self, PyObject *nameobj, const char *mode,
                 goto bad_mode;
             rwa = 1;
             self->writable = 1;
+            self->truncate = 1;
             flags |= O_CREAT | O_TRUNC;
             break;
         case 'a':
@@ -1156,10 +1159,17 @@ mode_string(fileio *self)
             return "ab";
     }
     else if (self->readable) {
-        if (self->writable)
-            return "rb+";
-        else
+        if (self->writable) {
+            if (self->truncate) {
+                return "wb+";
+            }
+            else {
+                return "rb+";
+            }
+        }
+        else {
             return "rb";
+        }
     }
     else
         return "wb";