mailbox.Babyl returns labels as bytes after reading a mailbox file #160598

Bug report

When a Babyl mailbox is read back from its file, the message labels are bytes instead of str:

import mailbox

box = mailbox.Babyl('box.babyl')
msg = mailbox.BabylMessage('Subject: one\n\nbody\n')
msg.set_labels(['unseen', 'work'])
box.add(msg)
box.add(mailbox.BabylMessage('Subject: two\n\nbody\n'))
box.close()

box = mailbox.Babyl('box.babyl')
msg = box[0]
print(sorted(msg.get_labels()))  # [b'unseen', b'work']
print(sorted(box.get_labels()))  # [b'unseen', b'work']
print(mailbox.MaildirMessage(msg).get_flags(), mailbox.mboxMessage(msg).get_flags())  # S RO
box.remove(1)
box.flush()  # AttributeError: 'bytes' object has no attribute 'encode'

As a result:

• 'unseen' in msg.get_labels() is false, so converting the message to MaildirMessage or mboxMessage marks an unseen message as read (S, R).
• Babyl.get_labels() does not leave out the special labels such as unseen.
• If any message has labels, a change that makes flush() rewrite the file, such as remove(), fails with AttributeError, and so does adding such a message to another Babyl mailbox.

Babyl._generate_toc() reads the labels line in binary mode and keeps the labels as bytes, while _install_message() and _pre_mailbox_hook() write them with label.encode(). Decoding them in _generate_toc() fixes all of the above. Using errors='surrogateescape' for both the decoding and the encoding keeps files with labels that are not valid UTF-8 readable (today they can be read, only rewriting them fails). The existing test_labels never reopens the mailbox, so it doesn't catch this.

In the releases I tried (3.13.17, 3.14.9, 3.15.0) the example fails even earlier, with an AssertionError in get_message(), because of gh-157880, which is fixed on main, 3.15 and 3.14 after those releases. With the gh-157880 fix applied, 3.13 shows the same label problem.

CPython versions tested on:

3.13, 3.14, CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-160598: Preserve Babyl labels when reopening mailboxes #160604
• gh-160598: Decode labels read from a Babyl mailbox file #160613