test_mbox is an mbox mailbox with a few messages in it.

>>> import mailbox
>>> inbox = mailbox.mbox('test_mbox')
>>> inbox.lock()
>>> inbox.popitem()
(0, <mailbox.mboxMessage instance at 0x7f78016bc680>)
>>> inbox.flush()
>>> inbox.unlock()
>>> inbox.lock()
>>> inbox.popitem()
(1, <mailbox.mboxMessage instance at 0x7f7801653320>)
>>> inbox.flush()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/mailbox.py", line 633, in flush
    (self._file_length, cur_len))
mailbox.ExternalClashError: Size of mailbox file changed (expected 141289, found 141147)