If a mailbox.MH() object is created by pointing at a path that exists but doesn't contain a ".mh_sequences" file, it raises an exception upon iteration over .{iter,}items() rather than gracefully assuming that the file is empty.  I encountered this by pointing it at a Claws Mail IMAP-cache folder (which claims to store its messages in MH format¹ but it doesn't place a .mh_sequences file in those folders) only to have it raise an exception.

To replicate:
$ mkdir empty
$ python
>>> import mailbox
>>> for msg in mailbox.MH('empty').values(): pass

I suspect this could simply wrap the "f = open(os.path.join(self._path, '.mh_sequences'), 'r')" and following lines in a check to ignore the file if it doesn't exist (returning the empty "results").

¹ http://www.claws-mail.org/faq/index.php/General_Information#How_does_Claws_Mail_store_mails.3F

I had to tweak the example reproduction code as it seemed to succeed (i.e., fail to demonstrate the problem) in some instances.  The same exception occurs, but here's the full original traceback:


$ cd /home/tim/.claws-mail/imapcache/mail.example.com/tim@example.com/INBOX/

$ python3
Python 3.2.3 (default, Feb 20 2013, 14:44:27) 
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import mailbox
>>> m = mailbox.MH('.')
>>> for msg in m:
...     print(msg)
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.2/mailbox.py", line 114, in itervalues
    value = self[key]
  File "/usr/lib/python3.2/mailbox.py", line 78, in __getitem__
    return self.get_message(key)
  File "/usr/lib/python3.2/mailbox.py", line 1019, in get_message
    for name, key_list in self.get_sequences().items():
  File "/usr/lib/python3.2/mailbox.py", line 1128, in get_sequences
    f = open(os.path.join(self._path, '.mh_sequences'), 'r')
IOError: [Errno 2] No such file or directory: '/home/tim/.claws-mail/imapcache/mail.example.com/tim@example.com/INBOX/.mh_sequences'

I consider this issue as a new feature.

PR 804 makes get_sequences() and set_sequences() working when the ".mh_sequences" file does not exist.

The open question is what to do with lock(). Currently it fails if the ".mh_sequences" file does not exist. Is it correct to create the ".mh_sequences" file in lock() or this invalidates the lock?

Is it safe to change the file open mode in set_sequences() from "r+" to "w" (the file is truncated later)?

Ping.

Honestly, given the open questions my inclination would be to reject this.

Do you mean rejecting the support of Claws Mail IMAP-cache folder or just the patch?

The support.  ClawsMail is broken, IMO.

If there is a way to resolve the lock issue we can consider it.  But (without looking at the code again) I suspect the locking is too deeply embedded in the mbox logic for this to be a safe change.