Probably a parsing bug in email.utils.parseaddr.

How to recreate:

>>> import email.utils
>>> test = 'Subject: I am a bug [Random]\r\nFrom: someone <some@email.address>\r\n\r\n'
>>> email.utils.parseaddr(test)
('', 'I')
>>> email.utils.parseaddr(test.replace('[', '').replace(']',''))
('someone', 'some@email.address')

Expected behaviour: no need to remove the []'s

parseaddr is for parsing the contents of an address header, not for parsing any additional text.  So the correct way to call it is parseaddress('someone <some@email.address>').

In any case, please look in to the new email policies, which provide a much more convenient API:

    >>> from email import message_from_bytes
    >>> from email.policy import default
    >>> m = message_from_bytes(b'Subject: I am a bug [Random]\r\nFrom: someone <some@email.address>\r\n\r\n', policy=default)
    >>> m['from']
    'someone <some@email.address>'
    >>> m['from'].addresses
    (Address(display_name='someone', username='some', domain='email.address'),)
    >>> m['from'].addresses[0].display_name
    'someone'
    >>> m['from'].addresses[0].username
    'some'
    >>> m['from'].addresses[0].addr_spec
    'some@email.address'

I do get this data from an IMAP fetch statement, see my code below:

    rv, data = imap.fetch(num, "(BODY[HEADER.FIELDS (FROM SUBJECT)])")
    if rv != 'OK':
        logging.error("Error getting message sender and subject (" + num.decode("ascii") + ")")
        return
    logging.info("Got message " + num.decode("ascii"))

    sender_subject = data[0][1].decode("utf-8")
    sender = email.utils.parseaddr(sender_subject.replace('[', '').replace(']',''))[1].replace("\r\n", "")

Thank you for providing this new API though, I will make sure to switch to that.

Unfortunately the imap module in the stdlib doesn't provide a whole lot in the way of tools for parsing the imap data, just for sending it back and forth to the server.