gh-157880: Write visible headers of BabylMessage in mailbox.Babyl

Lib/mailbox.py

@@ -1495,6 +1495,7 @@ def _install_message(self, message):
                 vis_buffer = io.BytesIO()
                 vis_generator = email.generator.BytesGenerator(vis_buffer, False, 0)
                 vis_generator.flatten(message.get_visible())
+                vis_buffer.seek(0)
                 while True:
                     line = vis_buffer.readline()
                     self._file.write(line.replace(b'\n', linesep))

Lib/test/test_mailbox.py

@@ -1521,6 +1521,19 @@ def test_labels(self):
         self._box.remove(key1)
         self.assertEqual(set(self._box.get_labels()), set(['blah']))
 
+    def test_visible(self):
+        # gh-157880: visible headers are written to the file and read back
+        msg = mailbox.BabylMessage(self._template % 0)
+        msg.update_visible()
+        key = self._box.add(msg)
+        for retrieved in (self._box.get_message(key), self._box[key]):
+            self.assertEqual(retrieved['from'], 'foo')
+            self.assertEqual(retrieved.get_payload(), '0\n')
+            self.assertEqual(retrieved.get_visible().keys(), ['From'])
+            self.assertEqual(retrieved.get_visible()['from'], 'foo')
+        self.assertEqual(self._box.get_bytes(key),
+                         (self._template % 0).encode('ascii'))
+
 
 class FakeFileLikeObject:
 

Misc/NEWS.d/next/Library/2026-09-20-21-24-05.gh-issue-157880.aE7wIu.rst

@@ -0,0 +1,2 @@
+Fix :class:`mailbox.Babyl` to write the visible headers of a
+:class:`mailbox.BabylMessage` to the mailbox file.