gh-96531: tempfile.TemporaryFile should honour write-only mode

Lib/tempfile.py

@@ -57,6 +57,12 @@
 if hasattr(_os, 'O_BINARY'):
     _bin_openflags |= _os.O_BINARY
 
+def _get_bin_openflags(mode):
+    if "r" in mode or "+" in mode:
+        return _bin_openflags
+    else:
+        return (_bin_openflags & ~_os.O_RDWR) | _os.O_WRONLY
+
 if hasattr(_os, 'TMP_MAX'):
     TMP_MAX = _os.TMP_MAX
 else:
@@ -583,7 +589,7 @@ def NamedTemporaryFile(mode='w+b', buffering=-1, encoding=None,
 
     prefix, suffix, dir, output_type = _sanitize_params(prefix, suffix, dir)
 
-    flags = _bin_openflags
+    flags = _get_bin_openflags(mode)
 
     # Setting O_TEMPORARY in the flags causes the OS to delete
     # the file when it is closed.  This is only supported by Windows.
@@ -650,7 +656,7 @@ def TemporaryFile(mode='w+b', buffering=-1, encoding=None,
 
         prefix, suffix, dir, output_type = _sanitize_params(prefix, suffix, dir)
 
-        flags = _bin_openflags
+        flags = _get_bin_openflags(mode)
         if _O_TMPFILE_WORKS:
             fd = None
             def opener(*args):

Lib/test/test_tempfile.py

@@ -940,14 +940,30 @@ def test_many(self):
 # We test _TemporaryFileWrapper by testing NamedTemporaryFile.
 
 
-class TestNamedTemporaryFile(BaseTestCase):
+class BaseTemporaryFileTestCases:
+    def test_wronly_mode(self):
+        with mock.patch('os.open', wraps=os.open) as spy:
+            with self.do_create(mode='wb') as fileobj:
+                fileobj.write(b'abc')
+                fileobj.seek(0)
+                self.assertRaises(io.UnsupportedOperation, fileobj.read)
+                with self.assertRaises(OSError):
+                    os.read(fileobj.fileno(), 1)
+
+        flags = spy.call_args[0][1]
+        self.assertEqual(flags & os.O_RDONLY, 0x0)
+        self.assertNotEqual(flags & os.O_RDWR, os.O_RDWR)
+        self.assertEqual(flags & os.O_WRONLY, os.O_WRONLY)
+        self.assertEqual(flags & os.O_EXCL, os.O_EXCL)
+
+class TestNamedTemporaryFile(BaseTestCase, BaseTemporaryFileTestCases):
     """Test NamedTemporaryFile()."""
 
-    def do_create(self, dir=None, pre="", suf="", delete=True):
+    def do_create(self, dir=None, pre="", suf="", delete=True, mode='w+b'):
         if dir is None:
             dir = tempfile.gettempdir()
         file = tempfile.NamedTemporaryFile(dir=dir, prefix=pre, suffix=suf,
-                                           delete=delete)
+                                           delete=delete, mode=mode)
 
         self.nameCheck(file.name, dir, pre, suf)
         return file
@@ -1519,9 +1535,12 @@ def test_class_getitem(self):
 
 if tempfile.NamedTemporaryFile is not tempfile.TemporaryFile:
 
-    class TestTemporaryFile(BaseTestCase):
+    class TestTemporaryFile(BaseTestCase, BaseTemporaryFileTestCases):
         """Test TemporaryFile()."""
 
+        def do_create(self, *args, **kwargs):
+            return tempfile.TemporaryFile(*args, **kwargs)
+
         def test_basic(self):
             # TemporaryFile can create files
             # No point in testing the name params - the file has no name.

Misc/NEWS.d/next/Library/2025-05-01-16-10-47.gh-issue-96531.4WHMzW.rst

@@ -0,0 +1,3 @@
+Use the ``O_WRONLY`` flag instead of ``O_RDWR`` when opening file descriptors
+for :class:`tempfile.TemporaryFile` and :class:`tempfile.NamedTemporaryFile`
+objects created with a write-only mode).