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