Python 3.9's mailbox.mbox class patched to properly handle body lines that start with "From " in multipart messages

mailbox-mbox.py

class mbox(_mboxMMDF):
    """A classic mbox mailbox."""

    _mangle_from_ = True

    # All messages must end in a newline character, and
    # _post_message_hooks outputs an empty line between messages.
    _append_newline = True

    def __init__(self, path, factory=None, create=True):
        """Initialize an mbox mailbox."""
        self._message_factory = mboxMessage
        _mboxMMDF.__init__(self, path, factory, create)

    def _post_message_hook(self, f):
        """Called after writing each message to file f."""
        f.write(linesep)

    def _generate_toc(self):
        """Generate key-to-(start, stop) table of contents."""
        starts, stops = [], []
        last_was_empty = False
        multipart_boundaries = [] # multipart segments can be nested so we have a stack of boundaries
        self._file.seek(0)
        while True:
            line_pos = self._file.tell()
            line = self._file.readline()
            if line.startswith(b'From ') and not multipart_boundaries:
                if len(stops) < len(starts):
                    if last_was_empty:
                        stops.append(line_pos - len(linesep))
                    else:
                        # The last line before the "From " line wasn't
                        # blank, but we consider it a start of a
                        # message anyway.
                        stops.append(line_pos)
                starts.append(line_pos)
                last_was_empty = False
            elif not line:
                if last_was_empty:
                    stops.append(line_pos - len(linesep))
                else:
                    stops.append(line_pos)
                break
            elif line == linesep:
                last_was_empty = True
            else:
                last_was_empty = False
                # Check for new boundaries and push them on the stack right. We assume that the actual boundary line
                # will show up and don't explicitly check for it.
                m = re.search(r'boundary="([^"]+)"', line.decode('utf-8', 'replace'))
                if m:
                    multipart_boundaries.append(m.group(1))
                elif multipart_boundaries and line == ('--%s--\n' % (multipart_boundaries[-1])).encode('utf-8'):
                    multipart_boundaries.pop()
        self._toc = dict(enumerate(zip(starts, stops)))
        self._next_key = len(self._toc)
        self._file_length = self._file.tell()