Lib/mailbox.py

#! /usr/bin/env python

"""Read/write support for Maildir, mbox, MH, Babyl, and MMDF mailboxes."""

import sys
import os
import time
import calendar
import socket
import errno
import copy
import email
import email.Message
import email.Generator
import rfc822
import StringIO
try:
    if sys.platform == 'os2emx':
        # OS/2 EMX fcntl() not adequate
        raise ImportError
    import fcntl
except ImportError:
    fcntl = None

__all__ = [ 'Mailbox', 'Maildir', 'mbox', 'MH', 'Babyl', 'MMDF',
            'Message', 'MaildirMessage', 'mboxMessage', 'MHMessage',
            'BabylMessage', 'MMDFMessage', 'UnixMailbox',
            'PortableUnixMailbox', 'MmdfMailbox', 'MHMailbox', 'BabylMailbox' ]

class Mailbox:
    """A group of messages in a particular place."""

    def __init__(self, path, factory=None, create=True):
        """Initialize a Mailbox instance."""
        self._path = os.path.abspath(os.path.expanduser(path))
        self._factory = factory

    def add(self, message):
        """Add message and return assigned key."""
        raise NotImplementedError('Method must be implemented by subclass')

    def remove(self, key):
        """Remove the keyed message; raise KeyError if it doesn't exist."""
        raise NotImplementedError('Method must be implemented by subclass')

    def __delitem__(self, key):
        self.remove(key)

    def discard(self, key):
        """If the keyed message exists, remove it."""
        try:
            self.remove(key)
        except KeyError:
            pass

    def __setitem__(self, key, message):
        """Replace the keyed message; raise KeyError if it doesn't exist."""
        raise NotImplementedError('Method must be implemented by subclass')

    def get(self, key, default=None):
        """Return the keyed message, or default if it doesn't exist."""
        try:
            return self.__getitem__(key)
        except KeyError:
            return default

    def __getitem__(self, key):
        """Return the keyed message; raise KeyError if it doesn't exist."""
        if not self._factory:
            return self.get_message(key)
        else:
            return self._factory(self.get_file(key))

    def get_message(self, key):
        """Return a Message representation or raise a KeyError."""
        raise NotImplementedError('Method must be implemented by subclass')

    def get_string(self, key):
        """Return a string representation or raise a KeyError."""
        raise NotImplementedError('Method must be implemented by subclass')

    def get_file(self, key):
        """Return a file-like representation or raise a KeyError."""
        raise NotImplementedError('Method must be implemented by subclass')

    def iterkeys(self):
        """Return an iterator over keys."""
        raise NotImplementedError('Method must be implemented by subclass')

    def keys(self):
        """Return a list of keys."""
        return list(self.iterkeys())

    def itervalues(self):
        """Return an iterator over all messages."""
        for key in self.iterkeys():
            try:
                value = self[key]
            except KeyError:
                continue
            yield value

    def __iter__(self):
        return self.itervalues()

    def values(self):
        """Return a list of messages. Memory intensive."""
        return list(self.itervalues())

    def iteritems(self):
        """Return an iterator over (key, message) tuples."""
        for key in self.iterkeys():
            try:
                value = self[key]
            except KeyError:
                continue
            yield (key, value)

    def items(self):
        """Return a list of (key, message) tuples. Memory intensive."""
        return list(self.iteritems())

    def __contains__(self, key):
        """Return True if the keyed message exists, False otherwise."""
        raise NotImplementedError('Method must be implemented by subclass')

    def __len__(self):
        """Return a count of messages in the mailbox."""
        raise NotImplementedError('Method must be implemented by subclass')

    def clear(self):
        """Delete all messages."""
        for key in self.iterkeys():
            self.discard(key)

    def pop(self, key, default=None):
        """Delete the keyed message and return it, or default."""
        try:
            result = self[key]
        except KeyError:
            return default
        self.discard(key)
        return result

    def popitem(self):
        """Delete an arbitrary (key, message) pair and return it."""
        for key in self.iterkeys():
            return (key, self.pop(key))     # This is only run once.
        else:
            raise KeyError('No messages in mailbox')

    def update(self, arg=None):
        """Change the messages that correspond to certain keys."""
        if hasattr(arg, 'iteritems'):
            source = arg.iteritems()
        elif hasattr(arg, 'items'):
            source = arg.items()
        else:
            source = arg
        bad_key = False
        for key, message in source:
            try:
                self[key] = message
            except KeyError:
                bad_key = True
        if bad_key:
            raise KeyError('No message with key(s)')

    def flush(self):
        """Write any pending changes to the disk."""
        raise NotImplementedError('Method must be implemented by subclass')

    def lock(self):
        """Lock the mailbox."""
        raise NotImplementedError('Method must be implemented by subclass')

    def unlock(self):
        """Unlock the mailbox if it is locked."""
        raise NotImplementedError('Method must be implemented by subclass')

    def close(self):
        """Flush and close the mailbox."""
        raise NotImplementedError('Method must be implemented by subclass')

    def _dump_message(self, message, target, mangle_from_=False):
        # Most files are opened in binary mode to allow predictable seeking.
        # To get native line endings on disk, the user-friendly \n line endings
        # used in strings and by email.Message are translated here.
        """Dump message contents to target file."""
        if isinstance(message, email.Message.Message):
            buffer = StringIO.StringIO()
            gen = email.Generator.Generator(buffer, mangle_from_, 0)
            gen.flatten(message)
            buffer.seek(0)
            target.write(buffer.read().replace('\n', os.linesep))
        elif isinstance(message, str):
            if mangle_from_:
                message = message.replace('\nFrom ', '\n>From ')
            message = message.replace('\n', os.linesep)
            target.write(message)
        elif hasattr(message, 'read'):
            while True:
                line = message.readline()
                if line == '':
                    break
                if mangle_from_ and line.startswith('From '):
                    line = '>From ' + line[5:]
                line = line.replace('\n', os.linesep)
                target.write(line)
        else:
            raise TypeError('Invalid message type: %s' % type(message))


class Maildir(Mailbox):
    """A qmail-style Maildir mailbox."""

    colon = ':'

    def __init__(self, dirname, factory=rfc822.Message, create=True):
        """Initialize a Maildir instance."""
        Mailbox.__init__(self, dirname, factory, create)
        if not os.path.exists(self._path):
            if create:
                os.mkdir(self._path, 0700)
                os.mkdir(os.path.join(self._path, 'tmp'), 0700)
                os.mkdir(os.path.join(self._path, 'new'), 0700)
                os.mkdir(os.path.join(self._path, 'cur'), 0700)
            else:
                raise NoSuchMailboxError(self._path)
        self._toc = {}

    def add(self, message):
        """Add message and return assigned key."""
        tmp_file = self._create_tmp()
        try:
            self._dump_message(message, tmp_file)
        finally:
            tmp_file.close()
        if isinstance(message, MaildirMessage):
            subdir = message.get_subdir()
            suffix = self.colon + message.get_info()
            if suffix == self.colon:
                suffix = ''
        else:
            subdir = 'new'
            suffix = ''
        uniq = os.path.basename(tmp_file.name).split(self.colon)[0]
        dest = os.path.join(self._path, subdir, uniq + suffix)
        os.rename(tmp_file.name, dest)
        if isinstance(message, MaildirMessage):
            os.utime(dest, (os.path.getatime(dest), message.get_date()))
        return uniq

    def remove(self, key):
        """Remove the keyed message; raise KeyError if it doesn't exist."""
        os.remove(os.path.join(self._path, self._lookup(key)))

    def discard(self, key):
        """If the keyed message exists, remove it."""
        # This overrides an inapplicable implementation in the superclass.
        try:
            self.remove(key)
        except KeyError:
            pass
        except OSError, e:
            if e.errno != errno.ENOENT:
                raise

    def __setitem__(self, key, message):
        """Replace the keyed message; raise KeyError if it doesn't exist."""
        old_subpath = self._lookup(key)
        temp_key = self.add(message)
        temp_subpath = self._lookup(temp_key)
        if isinstance(message, MaildirMessage):
            # temp's subdir and suffix were specified by message.
            dominant_subpath = temp_subpath
        else:
            # temp's subdir and suffix were defaults from add().
            dominant_subpath = old_subpath
        subdir = os.path.dirname(dominant_subpath)
        if self.colon in dominant_subpath:
            suffix = self.colon + dominant_subpath.split(self.colon)[-1]
        else:
            suffix = ''
        self.discard(key)
        new_path = os.path.join(self._path, subdir, key + suffix)
        os.rename(os.path.join(self._path, temp_subpath), new_path)
        if isinstance(message, MaildirMessage):
            os.utime(new_path, (os.path.getatime(new_path),
                                message.get_date()))

    def get_message(self, key):
        """Return a Message representation or raise a KeyError."""
        subpath = self._lookup(key)
        f = open(os.path.join(self._path, subpath), 'r')
        try:
            msg = MaildirMessage(f)
        finally:
            f.close()
        subdir, name = os.path.split(subpath)
        msg.set_subdir(subdir)
        if self.colon in name:
            msg.set_info(name.split(self.colon)[-1])
        msg.set_date(os.path.getmtime(os.path.join(self._path, subpath)))
        return msg

    def get_string(self, key):
        """Return a string representation or raise a KeyError."""
        f = open(os.path.join(self._path, self._lookup(key)), 'r')
        try:
            return f.read()
        finally:
            f.close()

    def get_file(self, key):
        """Return a file-like representation or raise a KeyError."""
        f = open(os.path.join(self._path, self._lookup(key)), 'rb')
        return _ProxyFile(f)

    def iterkeys(self):
        """Return an iterator over keys."""
        self._refresh()
        for key in self._toc:
            try:
                self._lookup(key)
            except KeyError:
                continue
            yield key

    def __contains__(self, key):
        """Return True if the keyed message exists, False otherwise."""
        self._refresh()
        return key in self._toc

    def __len__(self):
        """Return a count of messages in the mailbox."""
        self._refresh()
        return len(self._toc)

    def flush(self):
        """Write any pending changes to disk."""
        return  # Maildir changes are always written immediately.

    def lock(self):
        """Lock the mailbox."""
        return

    def unlock(self):
        """Unlock the mailbox if it is locked."""
        return

    def close(self):
        """Flush and close the mailbox."""
        return

    def list_folders(self):
        """Return a list of folder names."""
        result = []
        for entry in os.listdir(self._path):
            if len(entry) > 1 and entry[0] == '.' and \
               os.path.isdir(os.path.join(self._path, entry)):
                result.append(entry[1:])
        return result

    def get_folder(self, folder):
        """Return a Maildir instance for the named folder."""
        return Maildir(os.path.join(self._path, '.' + folder), create=False)

    def add_folder(self, folder):
        """Create a folder and return a Maildir instance representing it."""
        path = os.path.join(self._path, '.' + folder)
        result = Maildir(path)
        maildirfolder_path = os.path.join(path, 'maildirfolder')
        if not os.path.exists(maildirfolder_path):
            os.close(os.open(maildirfolder_path, os.O_CREAT | os.O_WRONLY))
        return result

    def remove_folder(self, folder):
        """Delete the named folder, which must be empty."""
        path = os.path.join(self._path, '.' + folder)
        for entry in os.listdir(os.path.join(path, 'new')) + \
                     os.listdir(os.path.join(path, 'cur')):
            if len(entry) < 1 or entry[0] != '.':
                raise NotEmptyError('Folder contains message(s): %s' % folder)
        for entry in os.listdir(path):
            if entry != 'new' and entry != 'cur' and entry != 'tmp' and \
               os.path.isdir(os.path.join(path, entry)):
                raise NotEmptyError("Folder contains subdirectory '%s': %s" %
                                    (folder, entry))
        for root, dirs, files in os.walk(path, topdown=False):
            for entry in files:
                os.remove(os.path.join(root, entry))
            for entry in dirs:
                os.rmdir(os.path.join(root, entry))
        os.rmdir(path)

    def clean(self):
        """Delete old files in "tmp"."""
        now = time.time()
        for entry in os.listdir(os.path.join(self._path, 'tmp')):
            path = os.path.join(self._path, 'tmp', entry)
            if now - os.path.getatime(path) > 129600:   # 60 * 60 * 36
                os.remove(path)

    _count = 1  # This is used to generate unique file names.

    def _create_tmp(self):
        """Create a file in the tmp subdirectory and open and return it."""
        now = time.time()
        hostname = socket.gethostname()
        if '/' in hostname:
            hostname = hostname.replace('/', r'\057')
        if ':' in hostname:
            hostname = hostname.replace(':', r'\072')
        uniq = "%s.M%sP%sQ%s.%s" % (int(now), int(now % 1 * 1e6), os.getpid(),
                                    Maildir._count, hostname)
        path = os.path.join(self._path, 'tmp', uniq)
        try:
            os.stat(path)
        except OSError, e:
            if e.errno == errno.ENOENT:
                Maildir._count += 1
                return open(path, 'wb+')
            else:
                raise
        else:
            raise ExternalClashError('Name clash prevented file creation: %s' %
                                     path)

    def _refresh(self):
        """Update table of contents mapping."""
        self._toc = {}
        for subdir in ('new', 'cur'):
            for entry in os.listdir(os.path.join(self._path, subdir)):
                uniq = entry.split(self.colon)[0]
                self._toc[uniq] = os.path.join(subdir, entry)

    def _lookup(self, key):
        """Use TOC to return subpath for given key, or raise a KeyError."""
        try:
            if os.path.exists(os.path.join(self._path, self._toc[key])):
                return self._toc[key]
        except KeyError:
            pass
        self._refresh()
        try:
            return self._toc[key]
        except KeyError:
            raise KeyError('No message with key: %s' % key)

    # This method is for backward compatibility only.
    def next(self):
        """Return the next message in a one-time iteration."""
        if not hasattr(self, '_onetime_keys'):
            self._onetime_keys = self.iterkeys()
        while True:
            try:
                return self[self._onetime_keys.next()]
            except StopIteration:
                return None
            except KeyError:
                continue


class _singlefileMailbox(Mailbox):
    """A single-file mailbox."""

    def __init__(self, path, factory=None, create=True):
        """Initialize a single-file mailbox."""
        Mailbox.__init__(self, path, factory, create)
        try:
            f = open(self._path, 'rb+')
        except IOError, e:
            if e.errno == errno.ENOENT:
                if create:
                    f = open(self._path, 'wb+')
                else:
                    raise NoSuchMailboxError(self._path)
            elif e.errno == errno.EACCES:
                f = open(self._path, 'rb')
            else:
                raise
        self._file = f
        self._toc = None
        self._next_key = 0
        self._pending = False   # No changes require rewriting the file.
        self._locked = False

    def add(self, message):
        """Add message and return assigned key."""
        self._lookup()
        self._toc[self._next_key] = self._append_message(message)
        self._next_key += 1
        self._pending = True
        return self._next_key - 1

    def remove(self, key):
        """Remove the keyed message; raise KeyError if it doesn't exist."""
        self._lookup(key)
        del self._toc[key]
        self._pending = True

    def __setitem__(self, key, message):
        """Replace the keyed message; raise KeyError if it doesn't exist."""
        self._lookup(key)
        self._toc[key] = self._append_message(message)
        self._pending = True

    def iterkeys(self):
        """Return an iterator over keys."""
        self._lookup()
        for key in self._toc.keys():
            yield key

    def __contains__(self, key):
        """Return True if the keyed message exists, False otherwise."""
        self._lookup()
        return key in self._toc

    def __len__(self):
        """Return a count of messages in the mailbox."""
        self._lookup()
        return len(self._toc)

    def lock(self):
        """Lock the mailbox."""
        if not self._locked:
            _lock_file(self._file)
            self._locked = True

    def unlock(self):
        """Unlock the mailbox if it is locked."""
        if self._locked:
            _unlock_file(self._file)
            self._locked = False

    def flush(self):
        """Write any pending changes to disk."""
        if not self._pending:
            return
        self._lookup()
        new_file = _create_temporary(self._path)
        try:
            new_toc = {}
            self._pre_mailbox_hook(new_file)
            for key in sorted(self._toc.keys()):
                start, stop = self._toc[key]
                self._file.seek(start)
                self._pre_message_hook(new_file)
                new_start = new_file.tell()
                while True:
                    buffer = self._file.read(min(4096,
                                                 stop - self._file.tell()))
                    if buffer == '':
                        break
                    new_file.write(buffer)
                new_toc[key] = (new_start, new_file.tell())
                self._post_message_hook(new_file)
        except:
            new_file.close()
            os.remove(new_file.name)
            raise
        new_file.close()
        self._file.close()
        try:
            os.rename(new_file.name, self._path)
        except OSError, e:
            if e.errno == errno.EEXIST or \
              (os.name == 'os2' and e.errno == errno.EACCES):
                os.remove(self._path)
                os.rename(new_file.name, self._path)
            else:
                raise
        self._file = open(self._path, 'rb+')
        self._toc = new_toc
        self._pending = False
        if self._locked:
            _lock_file(new_file, dotlock=False)

    def _pre_mailbox_hook(self, f):
        """Called before writing the mailbox to file f."""
        return

    def _pre_message_hook(self, f):
        """Called before writing each message to file f."""
        return

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

    def close(self):
        """Flush and close the mailbox."""
        self.flush()
        if self._locked:
            self.unlock()
        self._file.close()

    def _lookup(self, key=None):
        """Return (start, stop) or raise KeyError."""
        if self._toc is None:
            self._generate_toc()
        if key is not None:
            try:
                return self._toc[key]
            except KeyError:
                raise KeyError('No message with key: %s' % key)

    def _append_message(self, message):
        """Append message to mailbox and return (start, stop) offsets."""
        self._file.seek(0, 2)
        self._pre_message_hook(self._file)
        offsets = self._install_message(message)
        self._post_message_hook(self._file)
        self._file.flush()
        return offsets



class _mboxMMDF(_singlefileMailbox):
    """An mbox or MMDF mailbox."""

    _mangle_from_ = True

    def get_message(self, key):
        """Return a Message representation or raise a KeyError."""
        start, stop = self._lookup(key)
        self._file.seek(start)
        from_line = self._file.readline().replace(os.linesep, '')
        string = self._file.read(stop - self._file.tell())
        msg = self._message_factory(string.replace(os.linesep, '\n'))
        msg.set_from(from_line[5:])
        return msg

    def get_string(self, key, from_=False):
        """Return a string representation or raise a KeyError."""
        start, stop = self._lookup(key)
        self._file.seek(start)
        if not from_:
            self._file.readline()
        string = self._file.read(stop - self._file.tell())
        return string.replace(os.linesep, '\n')

    def get_file(self, key, from_=False):
        """Return a file-like representation or raise a KeyError."""
        start, stop = self._lookup(key)
        self._file.seek(start)
        if not from_:
            self._file.readline()
        return _PartialFile(self._file, self._file.tell(), stop)

    def _install_message(self, message):
        """Format a message and blindly write to self._file."""
        from_line = None
        if isinstance(message, str) and message.startswith('From '):
            newline = message.find('\n')
            if newline != -1:
                from_line = message[:newline]
                message = message[newline + 1:]
            else:
                from_line = message
                message = ''
        elif isinstance(message, _mboxMMDFMessage):
            from_line = 'From ' + message.get_from()
        elif isinstance(message, email.Message.Message):
            from_line = message.get_unixfrom()  # May be None.
        if from_line is None:
            from_line = 'From MAILER-DAEMON %s' % time.asctime(time.gmtime())
        start = self._file.tell()
        self._file.write(from_line + os.linesep)
        self._dump_message(message, self._file, self._mangle_from_)
        stop = self._file.tell()
        return (start, stop)


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

    _mangle_from_ = True

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

    def _pre_message_hook(self, f):
        """Called before writing each message to file f."""
        if f.tell() != 0:
            f.write(os.linesep)

    def _generate_toc(self):
        """Generate key-to-(start, stop) table of contents."""
        starts, stops = [], []
        self._file.seek(0)
        while True:
            line_pos = self._file.tell()
            line = self._file.readline()
            if line.startswith('From '):
                if len(stops) < len(starts):
                    stops.append(line_pos - len(os.linesep))
                starts.append(line_pos)
            elif line == '':
                stops.append(line_pos)
                break
        self._toc = dict(enumerate(zip(starts, stops)))
        self._next_key = len(self._toc)


class MMDF(_mboxMMDF):
    """An MMDF mailbox."""

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

    def _pre_message_hook(self, f):
        """Called before writing each message to file f."""
        f.write('\001\001\001\001' + os.linesep)

    def _post_message_hook(self, f):
        """Called after writing each message to file f."""
        f.write(os.linesep + '\001\001\001\001' + os.linesep)

    def _generate_toc(self):
        """Generate key-to-(start, stop) table of contents."""
        starts, stops = [], []
        self._file.seek(0)
        next_pos = 0
        while True:
            line_pos = next_pos
            line = self._file.readline()
            next_pos = self._file.tell()
            if line.startswith('\001\001\001\001' + os.linesep):
                starts.append(next_pos)
                while True:
                    line_pos = next_pos
                    line = self._file.readline()
                    next_pos = self._file.tell()
                    if line == '\001\001\001\001' + os.linesep:
                        stops.append(line_pos - len(os.linesep))
                        break
                    elif line == '':
                        stops.append(line_pos)
                        break
            elif line == '':
                break
        self._toc = dict(enumerate(zip(starts, stops)))
        self._next_key = len(self._toc)


class MH(Mailbox):
    """An MH mailbox."""

    def __init__(self, path, factory=None, create=True):
        """Initialize an MH instance."""
        Mailbox.__init__(self, path, factory, create)
        if not os.path.exists(self._path):
            if create:
                os.mkdir(self._path, 0700)
                os.close(os.open(os.path.join(self._path, '.mh_sequences'),
                                 os.O_CREAT | os.O_EXCL | os.O_WRONLY, 0600))
            else:
                raise NoSuchMailboxError(self._path)
        self._locked = False

    def add(self, message):
        """Add message and return assigned key."""
        keys = self.keys()
        if len(keys) == 0:
            new_key = 1
        else:
            new_key = max(keys) + 1
        new_path = os.path.join(self._path, str(new_key))
        f = _create_carefully(new_path)
        try:
            if self._locked:
                _lock_file(f)
            try:
                self._dump_message(message, f)
                if isinstance(message, MHMessage):
                    self._dump_sequences(message, new_key)
            finally:
                if self._locked:
                    _unlock_file(f)
        finally:
            f.close()
        return new_key

    def remove(self, key):
        """Remove the keyed message; raise KeyError if it doesn't exist."""
        path = os.path.join(self._path, str(key))
        try:
            f = open(path, 'rb+')
        except IOError, e:
            if e.errno == errno.ENOENT:
                raise KeyError('No message with key: %s' % key)
            else:
                raise
        try:
            if self._locked:
                _lock_file(f)
            try:
                f.close()
                os.remove(os.path.join(self._path, str(key)))
            finally:
                if self._locked:
                    _unlock_file(f)
        finally:
            f.close()

    def __setitem__(self, key, message):
        """Replace the keyed message; raise KeyError if it doesn't exist."""
        path = os.path.join(self._path, str(key))
        try:
            f = open(path, 'rb+')
        except IOError, e:
            if e.errno == errno.ENOENT:
                raise KeyError('No message with key: %s' % key)
            else:
                raise
        try:
            if self._locked:
                _lock_file(f)
            try:
                os.close(os.open(path, os.O_WRONLY | os.O_TRUNC))
                self._dump_message(message, f)
                if isinstance(message, MHMessage):
                    self._dump_sequences(message, key)
            finally:
                if self._locked:
                    _unlock_file(f)
        finally:
            f.close()

    def get_message(self, key):
        """Return a Message representation or raise a KeyError."""
        try:
            if self._locked:
                f = open(os.path.join(self._path, str(key)), 'r+')
            else:
                f = open(os.path.join(self._path, str(key)), 'r')
        except IOError, e:
            if e.errno == errno.ENOENT:
                raise KeyError('No message with key: %s' % key)
            else:
                raise
        try:
            if self._locked:
                _lock_file(f)
            try:
                msg = MHMessage(f)
            finally:
                if self._locked:
                    _unlock_file(f)
        finally:
            f.close()
        for name, key_list in self.get_sequences():
            if key in key_list:
                msg.add_sequence(name)
        return msg

    def get_string(self, key):
        """Return a string representation or raise a KeyError."""
        try:
            if self._locked:
                f = open(os.path.join(self._path, str(key)), 'r+')
            else:
                f = open(os.path.join(self._path, str(key)), 'r')
        except IOError, e:
            if e.errno == errno.ENOENT:
                raise KeyError('No message with key: %s' % key)
            else:
                raise
        try:
            if self._locked:
                _lock_file(f)
            try:
                return f.read()
            finally:
                if self._locked:
                    _unlock_file(f)
        finally:
            f.close()

    def get_file(self, key):
        """Return a file-like representation or raise a KeyError."""
        try:
            f = open(os.path.join(self._path, str(key)), 'rb')
        except IOError, e:
            if e.errno == errno.ENOENT:
                raise KeyError('No message with key: %s' % key)
            else:
                raise
        return _ProxyFile(f)

    def iterkeys(self):
        """Return an iterator over keys."""
        return iter(sorted(int(entry) for entry in os.listdir(self._path)
                                      if entry.isdigit()))

    def __contains__(self, key):
        """Return True if the keyed message exists, False otherwise."""
        return os.path.exists(os.path.join(self._path, str(key)))

    def __len__(self):
        """Return a count of messages in the mailbox."""
        return len(list(self.iterkeys()))

    def lock(self):
        """Lock the mailbox."""
        if not self._locked:
            self._file = open(os.path.join(self._path, '.mh_sequences'), 'rb+')
            _lock_file(self._file)
            self._locked = True

    def unlock(self):
        """Unlock the mailbox if it is locked."""
        if self._locked:
            _unlock_file(self._file)
            self._file.close()
            del self._file
            self._locked = False

    def flush(self):
        """Write any pending changes to the disk."""
        return

    def close(self):
        """Flush and close the mailbox."""
        if self._locked:
            self.unlock()

    def list_folders(self):
        """Return a list of folder names."""
        result = []
        for entry in os.listdir(self._path):
            if os.path.isdir(os.path.join(self._path, entry)):
                result.append(entry)
        return result

    def get_folder(self, folder):
        """Return an MH instance for the named folder."""
        return MH(os.path.join(self._path, folder), create=False)

    def add_folder(self, folder):
        """Create a folder and return an MH instance representing it."""
        return MH(os.path.join(self._path, folder))

    def remove_folder(self, folder):
        """Delete the named folder, which must be empty."""
        path = os.path.join(self._path, folder)
        entries = os.listdir(path)
        if entries == ['.mh_sequences']:
            os.remove(os.path.join(path, '.mh_sequences'))
        elif entries == []:
            pass
        else:
            raise NotEmptyError('Folder not empty: %s' % self._path)
        os.rmdir(path)

    def get_sequences(self):
        """Return a name-to-key-list dictionary to define each sequence."""
        results = {}
        f = open(os.path.join(self._path, '.mh_sequences'), 'r')
        try:
            all_keys = set(self.keys())
            for line in f:
                try:
                    name, contents = line.split(':')
                    keys = set()
                    for spec in contents.split():
                        if spec.isdigit():
                            keys.add(int(spec))
                        else:
                            start, stop = (int(x) for x in spec.split('-'))
                            keys.update(range(start, stop + 1))
                    results[name] = [key for key in sorted(keys) \
                                         if key in all_keys]
                    if len(results[name]) == 0:
                        del results[name]
                except ValueError:
                    raise FormatError('Invalid sequence specification: %s' %
                                      line.rstrip())
        finally:
            f.close()
        return results

    def set_sequences(self, sequences):
        """Set sequences using the given name-to-key-list dictionary."""
        f = open(os.path.join(self._path, '.mh_sequences'), 'r+')
        try:
            os.close(os.open(f.name, os.O_WRONLY | os.O_TRUNC))
            for name, keys in sequences.iteritems():
                if len(keys) == 0:
                    continue
                f.write('%s:' % name)
                prev = None
                completing = False
                for key in sorted(set(keys)):
                    if key - 1 == prev:
                        if not completing:
                            completing = True
                            f.write('-')
                    elif completing:
                        completing = False
                        f.write('%s %s' % (prev, key))
                    else:
                        f.write(' %s' % key)
                    prev = key
                if completing:
                    f.write(str(prev) + '\n')
                else:
                    f.write('\n')
        finally:
            f.close()

    def pack(self):
        """Re-name messages to eliminate numbering gaps. Invalidates keys."""
        sequences = self.get_sequences()
        prev = 0
        changes = []
        for key in self.iterkeys():
            if key - 1 != prev:
                changes.append((key, prev + 1))
                f = open(os.path.join(self._path, str(key)), 'r+')
                try:
                    if self._locked:
                        _lock_file(f)
                    try:
                        if hasattr(os, 'link'):
                            os.link(os.path.join(self._path, str(key)),
                                    os.path.join(self._path, str(prev + 1)))
                            if sys.platform == 'os2emx':
                                # cannot unlink an open file on OS/2
                                f.close()
                            os.unlink(os.path.join(self._path, str(key)))
                        else:
                            f.close()
                            os.rename(os.path.join(self._path, str(key)),
                                      os.path.join(self._path, str(prev + 1)))
                    finally:
                        if self._locked:
                            _unlock_file(f)
                finally:
                    f.close()
            prev += 1
        self._next_key = prev + 1
        if len(changes) == 0:
            return
        for name, key_list in sequences.items():
            for old, new in changes:
                if old in key_list:
                    key_list[key_list.index(old)] = new
        self.set_sequences(sequences)

    def _dump_sequences(self, message, key):
        """Inspect a new MHMessage and update sequences appropriately."""
        pending_sequences = message.get_sequences()
        all_sequences = self.get_sequences()
        for name, key_list in all_sequences.iteritems():
            if name in pending_sequences:
                key_list.append(key)
            elif key in key_list:
                del key_list[key_list.index(key)]
        for sequence in pending_sequences:
            if sequence not in all_sequences:
                all_sequences[sequence] = [key]
        self.set_sequences(all_sequences)


class Babyl(_singlefileMailbox):
    """An Rmail-style Babyl mailbox."""

    _special_labels = frozenset(('unseen', 'deleted', 'filed', 'answered',
                                 'forwarded', 'edited', 'resent'))

    def __init__(self, path, factory=None, create=True):
        """Initialize a Babyl mailbox."""
        _singlefileMailbox.__init__(self, path, factory, create)
        self._labels = {}

    def add(self, message):
        """Add message and return assigned key."""
        key = _singlefileMailbox.add(self, message)
        if isinstance(message, BabylMessage):
            self._labels[key] = message.get_labels()
        return key

    def remove(self, key):
        """Remove the keyed message; raise KeyError if it doesn't exist."""
        _singlefileMailbox.remove(self, key)
        if key in self._labels:
            del self._labels[key]

    def __setitem__(self, key, message):
        """Replace the keyed message; raise KeyError if it doesn't exist."""
        _singlefileMailbox.__setitem__(self, key, message)
        if isinstance(message, BabylMessage):
            self._labels[key] = message.get_labels()

    def get_message(self, key):
        """Return a Message representation or raise a KeyError."""
        start, stop = self._lookup(key)
        self._file.seek(start)
        self._file.readline()   # Skip '1,' line specifying labels.
        original_headers = StringIO.StringIO()
        while True:
            line = self._file.readline()
            if line == '*** EOOH ***' + os.linesep or line == '':
                break
            original_headers.write(line.replace(os.linesep, '\n'))
        visible_headers = StringIO.StringIO()
        while True:
            line = self._file.readline()
            if line == os.linesep or line == '':
                break
            visible_headers.write(line.replace(os.linesep, '\n'))
        body = self._file.read(stop - self._file.tell()).replace(os.linesep,
                                                                 '\n')
        msg = BabylMessage(original_headers.getvalue() + body)
        msg.set_visible(visible_headers.getvalue())
        if key in self._labels:
            msg.set_labels(self._labels[key])
        return msg

    def get_string(self, key):
        """Return a string representation or raise a KeyError."""
        start, stop = self._lookup(key)
        self._file.seek(start)
        self._file.readline()   # Skip '1,' line specifying labels.
        original_headers = StringIO.StringIO()
        while True:
            line = self._file.readline()
            if line == '*** EOOH ***' + os.linesep or line == '':
                break
            original_headers.write(line.replace(os.linesep, '\n'))
        while True:
            line = self._file.readline()
            if line == os.linesep or line == '':
                break
        return original_headers.getvalue() + \
               self._file.read(stop - self._file.tell()).replace(os.linesep,
                                                                 '\n')

    def get_file(self, key):
        """Return a file-like representation or raise a KeyError."""
        return StringIO.StringIO(self.get_string(key).replace('\n',
                                                              os.linesep))

    def get_labels(self):
        """Return a list of user-defined labels in the mailbox."""
        self._lookup()
        labels = set()
        for label_list in self._labels.values():
            labels.update(label_list)
        labels.difference_update(self._special_labels)
        return list(labels)

    def _generate_toc(self):
        """Generate key-to-(start, stop) table of contents."""
        starts, stops = [], []
        self._file.seek(0)
        next_pos = 0
        label_lists = []
        while True:
            line_pos = next_pos
            line = self._file.readline()
            next_pos = self._file.tell()
            if line == '\037\014' + os.linesep:
                if len(stops) < len(starts):
                    stops.append(line_pos - len(os.linesep))
                starts.append(next_pos)
                labels = [label.strip() for label
                                        in self._file.readline()[1:].split(',')
                                        if label.strip() != '']
                label_lists.append(labels)
            elif line == '\037' or line == '\037' + os.linesep:
                if len(stops) < len(starts):
                    stops.append(line_pos - len(os.linesep))
            elif line == '':
                stops.append(line_pos - len(os.linesep))
                break
        self._toc = dict(enumerate(zip(starts, stops)))
        self._labels = dict(enumerate(label_lists))
        self._next_key = len(self._toc)

    def _pre_mailbox_hook(self, f):
        """Called before writing the mailbox to file f."""
        f.write('BABYL OPTIONS:%sVersion: 5%sLabels:%s%s\037' %
                (os.linesep, os.linesep, ','.join(self.get_labels()),
                 os.linesep))

    def _pre_message_hook(self, f):
        """Called before writing each message to file f."""
        f.write('\014' + os.linesep)

    def _post_message_hook(self, f):
        """Called after writing each message to file f."""
        f.write(os.linesep + '\037')

    def _install_message(self, message):
        """Write message contents and return (start, stop)."""
        start = self._file.tell()
        if isinstance(message, BabylMessage):
            special_labels = []
            labels = []
            for label in message.get_labels():
                if label in self._special_labels:
                    special_labels.append(label)
                else:
                    labels.append(label)
            self._file.write('1')
            for label in special_labels:
                self._file.write(', ' + label)
            self._file.write(',,')
            for label in labels:
                self._file.write(' ' + label + ',')
            self._file.write(os.linesep)
        else:
            self._file.write('1,,' + os.linesep)
        if isinstance(message, email.Message.Message):
            orig_buffer = StringIO.StringIO()
            orig_generator = email.Generator.Generator(orig_buffer, False, 0)
            orig_generator.flatten(message)
            orig_buffer.seek(0)
            while True:
                line = orig_buffer.readline()
                self._file.write(line.replace('\n', os.linesep))
                if line == '\n' or line == '':
                    break
            self._file.write('*** EOOH ***' + os.linesep)
            if isinstance(message, BabylMessage):
                vis_buffer = StringIO.StringIO()
                vis_generator = email.Generator.Generator(vis_buffer, False, 0)
                vis_generator.flatten(message.get_visible())
                while True:
                    line = vis_buffer.readline()
                    self._file.write(line.replace('\n', os.linesep))
                    if line == '\n' or line == '':
                        break
            else:
                orig_buffer.seek(0)
                while True:
                    line = orig_buffer.readline()
                    self._file.write(line.replace('\n', os.linesep))
                    if line == '\n' or line == '':
                        break
            while True:
                buffer = orig_buffer.read(4096) # Buffer size is arbitrary.
                if buffer == '':
                    break
                self._file.write(buffer.replace('\n', os.linesep))
        elif isinstance(message, str):
            body_start = message.find('\n\n') + 2
            if body_start - 2 != -1:
                self._file.write(message[:body_start].replace('\n',
                                                              os.linesep))
                self._file.write('*** EOOH ***' + os.linesep)
                self._file.write(message[:body_start].replace('\n',
                                                              os.linesep))
                self._file.write(message[body_start:].replace('\n',
                                                              os.linesep))
            else:
                self._file.write('*** EOOH ***' + os.linesep + os.linesep)
                self._file.write(message.replace('\n', os.linesep))
        elif hasattr(message, 'readline'):
            original_pos = message.tell()
            first_pass = True
            while True:
                line = message.readline()
                self._file.write(line.replace('\n', os.linesep))
                if line == '\n' or line == '':
                    self._file.write('*** EOOH ***' + os.linesep)
                    if first_pass:
                        first_pass = False
                        message.seek(original_pos)
                    else:
                        break
            while True:
                buffer = message.read(4096)     # Buffer size is arbitrary.
                if buffer == '':
                    break
                self._file.write(buffer.replace('\n', os.linesep))
        else:
            raise TypeError('Invalid message type: %s' % type(message))
        stop = self._file.tell()
        return (start, stop)


class Message(email.Message.Message):
    """Message with mailbox-format-specific properties."""

    def __init__(self, message=None):
        """Initialize a Message instance."""
        if isinstance(message, email.Message.Message):
            self._become_message(copy.deepcopy(message))
            if isinstance(message, Message):
                message._explain_to(self)
        elif isinstance(message, str):
            self._become_message(email.message_from_string(message))
        elif hasattr(message, "read"):
            self._become_message(email.message_from_file(message))
        elif message is None:
            email.Message.Message.__init__(self)
        else:
            raise TypeError('Invalid message type: %s' % type(message))

    def _become_message(self, message):
        """Assume the non-format-specific state of message."""
        for name in ('_headers', '_unixfrom', '_payload', '_charset',
                     'preamble', 'epilogue', 'defects', '_default_type'):
            self.__dict__[name] = message.__dict__[name]

    def _explain_to(self, message):
        """Copy format-specific state to message insofar as possible."""
        if isinstance(message, Message):
            return  # There's nothing format-specific to explain.
        else:
            raise TypeError('Cannot convert to specified type')


class MaildirMessage(Message):
    """Message with Maildir-specific properties."""

    def __init__(self, message=None):
        """Initialize a MaildirMessage instance."""
        self._subdir = 'new'
        self._info = ''
        self._date = time.time()
        Message.__init__(self, message)

    def get_subdir(self):
        """Return 'new' or 'cur'."""
        return self._subdir

    def set_subdir(self, subdir):
        """Set subdir to 'new' or 'cur'."""
        if subdir == 'new' or subdir == 'cur':
            self._subdir = subdir
        else:
            raise ValueError("subdir must be 'new' or 'cur': %s" % subdir)

    def get_flags(self):
        """Return as a string the flags that are set."""
        if self._info.startswith('2,'):
            return self._info[2:]
        else:
            return ''

    def set_flags(self, flags):
        """Set the given flags and unset all others."""
        self._info = '2,' + ''.join(sorted(flags))

    def add_flag(self, flag):
        """Set the given flag(s) without changing others."""
        self.set_flags(''.join(set(self.get_flags()) | set(flag)))

    def remove_flag(self, flag):
        """Unset the given string flag(s) without changing others."""
        if self.get_flags() != '':
            self.set_flags(''.join(set(self.get_flags()) - set(flag)))

    def get_date(self):
        """Return delivery date of message, in seconds since the epoch."""
        return self._date

    def set_date(self, date):
        """Set delivery date of message, in seconds since the epoch."""
        try:
            self._date = float(date)
        except ValueError:
            raise TypeError("can't convert to float: %s" % date)

    def get_info(self):
        """Get the message's "info" as a string."""
        return self._info

    def set_info(self, info):
        """Set the message's "info" string."""
        if isinstance(info, str):
            self._info = info
        else:
            raise TypeError('info must be a string: %s' % type(info))

    def _explain_to(self, message):
        """Copy Maildir-specific state to message insofar as possible."""
        if isinstance(message, MaildirMessage):
            message.set_flags(self.get_flags())
            message.set_subdir(self.get_subdir())
            message.set_date(self.get_date())
        elif isinstance(message, _mboxMMDFMessage):
            flags = set(self.get_flags())
            if 'S' in flags:
                message.add_flag('R')
            if self.get_subdir() == 'cur':
                message.add_flag('O')
            if 'T' in flags:
                message.add_flag('D')
            if 'F' in flags:
                message.add_flag('F')
            if 'R' in flags:
                message.add_flag('A')
            message.set_from('MAILER-DAEMON', time.gmtime(self.get_date()))
        elif isinstance(message, MHMessage):
            flags = set(self.get_flags())
            if 'S' not in flags:
                message.add_sequence('unseen')
            if 'R' in flags:
                message.add_sequence('replied')
            if 'F' in flags:
                message.add_sequence('flagged')
        elif isinstance(message, BabylMessage):
            flags = set(self.get_flags())
            if 'S' not in flags:
                message.add_label('unseen')
            if 'T' in flags:
                message.add_label('deleted')
            if 'R' in flags:
                message.add_label('answered')
            if 'P' in flags:
                message.add_label('forwarded')
        elif isinstance(message, Message):
            pass
        else:
            raise TypeError('Cannot convert to specified type: %s' %
                            type(message))


class _mboxMMDFMessage(Message):
    """Message with mbox- or MMDF-specific properties."""

    def __init__(self, message=None):
        """Initialize an mboxMMDFMessage instance."""
        self.set_from('MAILER-DAEMON', True)
        if isinstance(message, email.Message.Message):
            unixfrom = message.get_unixfrom()
            if unixfrom is not None and unixfrom.startswith('From '):
                self.set_from(unixfrom[5:])
        Message.__init__(self, message)

    def get_from(self):
        """Return contents of "From " line."""
        return self._from

    def set_from(self, from_, time_=None):
        """Set "From " line, formatting and appending time_ if specified."""
        if time_ is not None:
            if time_ is True:
                time_ = time.gmtime()
            from_ += ' ' + time.asctime(time_)
        self._from = from_

    def get_flags(self):
        """Return as a string the flags that are set."""
        return self.get('Status', '') + self.get('X-Status', '')

    def set_flags(self, flags):
        """Set the given flags and unset all others."""
        flags = set(flags)
        status_flags, xstatus_flags = '', ''
        for flag in ('R', 'O'):
            if flag in flags:
                status_flags += flag
                flags.remove(flag)
        for flag in ('D', 'F', 'A'):
            if flag in flags:
                xstatus_flags += flag
                flags.remove(flag)
        xstatus_flags += ''.join(sorted(flags))
        try:
            self.replace_header('Status', status_flags)
        except KeyError:
            self.add_header('Status', status_flags)
        try:
            self.replace_header('X-Status', xstatus_flags)
        except KeyError:
            self.add_header('X-Status', xstatus_flags)

    def add_flag(self, flag):
        """Set the given flag(s) without changing others."""
        self.set_flags(''.join(set(self.get_flags()) | set(flag)))

    def remove_flag(self, flag):
        """Unset the given string flag(s) without changing others."""
        if 'Status' in self or 'X-Status' in self:
            self.set_flags(''.join(set(self.get_flags()) - set(flag)))

    def _explain_to(self, message):
        """Copy mbox- or MMDF-specific state to message insofar as possible."""
        if isinstance(message, MaildirMessage):
            flags = set(self.get_flags())
            if 'O' in flags:
                message.set_subdir('cur')
            if 'F' in flags:
                message.add_flag('F')
            if 'A' in flags:
                message.add_flag('R')
            if 'R' in flags:
                message.add_flag('S')
            if 'D' in flags:
                message.add_flag('T')
            del message['status']
            del message['x-status']
            maybe_date = ' '.join(self.get_from().split()[-5:])
            try:
                message.set_date(calendar.timegm(time.strptime(maybe_date,
                                                      '%a %b %d %H:%M:%S %Y')))
            except (ValueError, OverflowError):
                pass
        elif isinstance(message, _mboxMMDFMessage):
            message.set_flags(self.get_flags())
            message.set_from(self.get_from())
        elif isinstance(message, MHMessage):
            flags = set(self.get_flags())
            if 'R' not in flags:
                message.add_sequence('unseen')
            if 'A' in flags:
                message.add_sequence('replied')
            if 'F' in flags:
                message.add_sequence('flagged')
            del message['status']
            del message['x-status']
        elif isinstance(message, BabylMessage):
            flags = set(self.get_flags())
            if 'R' not in flags:
                message.add_label('unseen')
            if 'D' in flags:
                message.add_label('deleted')
            if 'A' in flags:
                message.add_label('answered')
            del message['status']
            del message['x-status']
        elif isinstance(message, Message):
            pass
        else:
            raise TypeError('Cannot convert to specified type: %s' %
                            type(message))


class mboxMessage(_mboxMMDFMessage):
    """Message with mbox-specific properties."""


class MHMessage(Message):
    """Message with MH-specific properties."""

    def __init__(self, message=None):
        """Initialize an MHMessage instance."""
        self._sequences = []
        Message.__init__(self, message)

    def get_sequences(self):
        """Return a list of sequences that include the message."""
        return self._sequences[:]

    def set_sequences(self, sequences):
        """Set the list of sequences that include the message."""
        self._sequences = list(sequences)

    def add_sequence(self, sequence):
        """Add sequence to list of sequences including the message."""
        if isinstance(sequence, str):
            if not sequence in self._sequences:
                self._sequences.append(sequence)
        else:
            raise TypeError('sequence must be a string: %s' % type(sequence))

    def remove_sequence(self, sequence):
        """Remove sequence from the list of sequences including the message."""
        try:
            self._sequences.remove(sequence)
        except ValueError:
            pass

    def _explain_to(self, message):
        """Copy MH-specific state to message insofar as possible."""
        if isinstance(message, MaildirMessage):
            sequences = set(self.get_sequences())
            if 'unseen' in sequences:
                message.set_subdir('cur')
            else:
                message.set_subdir('cur')
                message.add_flag('S')
            if 'flagged' in sequences:
                message.add_flag('F')
            if 'replied' in sequences:
                message.add_flag('R')
        elif isinstance(message, _mboxMMDFMessage):
            sequences = set(self.get_sequences())
            if 'unseen' not in sequences:
                message.add_flag('RO')
            else:
                message.add_flag('O')
            if 'flagged' in sequences:
                message.add_flag('F')
            if 'replied' in sequences:
                message.add_flag('A')
        elif isinstance(message, MHMessage):
            for sequence in self.get_sequences():
                message.add_sequence(sequence)
        elif isinstance(message, BabylMessage):
            sequences = set(self.get_sequences())
            if 'unseen' in sequences:
                message.add_label('unseen')
            if 'replied' in sequences:
                message.add_label('answered')
        elif isinstance(message, Message):
            pass
        else:
            raise TypeError('Cannot convert to specified type: %s' %
                            type(message))


class BabylMessage(Message):
    """Message with Babyl-specific properties."""

    def __init__(self, message=None):
        """Initialize an BabylMessage instance."""
        self._labels = []
        self._visible = Message()
        Message.__init__(self, message)

    def get_labels(self):
        """Return a list of labels on the message."""
        return self._labels[:]

    def set_labels(self, labels):
        """Set the list of labels on the message."""
        self._labels = list(labels)

    def add_label(self, label):
        """Add label to list of labels on the message."""
        if isinstance(label, str):
            if label not in self._labels:
                self._labels.append(label)
        else:
            raise TypeError('label must be a string: %s' % type(label))

    def remove_label(self, label):
        """Remove label from the list of labels on the message."""
        try:
            self._labels.remove(label)
        except ValueError:
            pass

    def get_visible(self):
        """Return a Message representation of visible headers."""
        return Message(self._visible)

    def set_visible(self, visible):
        """Set the Message representation of visible headers."""
        self._visible = Message(visible)

    def update_visible(self):
        """Update and/or sensibly generate a set of visible headers."""
        for header in self._visible.keys():
            if header in self:
                self._visible.replace_header(header, self[header])
            else:
                del self._visible[header]
        for header in ('Date', 'From', 'Reply-To', 'To', 'CC', 'Subject'):
            if header in self and header not in self._visible:
                self._visible[header] = self[header]

    def _explain_to(self, message):
        """Copy Babyl-specific state to message insofar as possible."""
        if isinstance(message, MaildirMessage):
            labels = set(self.get_labels())
            if 'unseen' in labels:
                message.set_subdir('cur')
            else:
                message.set_subdir('cur')
                message.add_flag('S')
            if 'forwarded' in labels or 'resent' in labels:
                message.add_flag('P')
            if 'answered' in labels:
                message.add_flag('R')
            if 'deleted' in labels:
                message.add_flag('T')
        elif isinstance(message, _mboxMMDFMessage):
            labels = set(self.get_labels())
            if 'unseen' not in labels:
                message.add_flag('RO')
            else:
                message.add_flag('O')
            if 'deleted' in labels:
                message.add_flag('D')
            if 'answered' in labels:
                message.add_flag('A')
        elif isinstance(message, MHMessage):
            labels = set(self.get_labels())
            if 'unseen' in labels:
                message.add_sequence('unseen')
            if 'answered' in labels:
                message.add_sequence('replied')
        elif isinstance(message, BabylMessage):
            message.set_visible(self.get_visible())
            for label in self.get_labels():
                message.add_label(label)
        elif isinstance(message, Message):
            pass
        else:
            raise TypeError('Cannot convert to specified type: %s' %
                            type(message))


class MMDFMessage(_mboxMMDFMessage):
    """Message with MMDF-specific properties."""


class _ProxyFile:
    """A read-only wrapper of a file."""

    def __init__(self, f, pos=None):
        """Initialize a _ProxyFile."""
        self._file = f
        if pos is None:
            self._pos = f.tell()
        else:
            self._pos = pos

    def read(self, size=None):
        """Read bytes."""
        return self._read(size, self._file.read)

    def readline(self, size=None):
        """Read a line."""
        return self._read(size, self._file.readline)

    def readlines(self, sizehint=None):
        """Read multiple lines."""
        result = []
        for line in self:
            result.append(line)
            if sizehint is not None:
                sizehint -= len(line)
                if sizehint <= 0:
                    break
        return result

    def __iter__(self):
        """Iterate over lines."""
        return iter(self.readline, "")

    def tell(self):
        """Return the position."""
        return self._pos

    def seek(self, offset, whence=0):
        """Change position."""
        if whence == 1:
            self._file.seek(self._pos)
        self._file.seek(offset, whence)
        self._pos = self._file.tell()

    def close(self):
        """Close the file."""
        del self._file

    def _read(self, size, read_method):
        """Read size bytes using read_method."""
        if size is None:
            size = -1
        self._file.seek(self._pos)
        result = read_method(size)
        self._pos = self._file.tell()
        return result


class _PartialFile(_ProxyFile):
    """A read-only wrapper of part of a file."""

    def __init__(self, f, start=None, stop=None):
        """Initialize a _PartialFile."""
        _ProxyFile.__init__(self, f, start)
        self._start = start
        self._stop = stop

    def tell(self):
        """Return the position with respect to start."""
        return _ProxyFile.tell(self) - self._start

    def seek(self, offset, whence=0):
        """Change position, possibly with respect to start or stop."""
        if whence == 0:
            self._pos = self._start
            whence = 1
        elif whence == 2:
            self._pos = self._stop
            whence = 1
        _ProxyFile.seek(self, offset, whence)

    def _read(self, size, read_method):
        """Read size bytes using read_method, honoring start and stop."""
        remaining = self._stop - self._pos
        if remaining <= 0:
            return ''
        if size is None or size < 0 or size > remaining:
            size = remaining
        return _ProxyFile._read(self, size, read_method)


def _lock_file(f, dotlock=True):
    """Lock file f using lockf and dot locking."""
    dotlock_done = False
    try:
        if fcntl:
            try:
                fcntl.lockf(f, fcntl.LOCK_EX | fcntl.LOCK_NB)
            except IOError, e:
                if e.errno in (errno.EAGAIN, errno.EACCES):
                    raise ExternalClashError('lockf: lock unavailable: %s' %
                                             f.name)
                else:
                    raise
        if dotlock:
            try:
                pre_lock = _create_temporary(f.name + '.lock')
                pre_lock.close()
            except IOError, e:
                if e.errno == errno.EACCES:
                    return  # Without write access, just skip dotlocking.
                else:
                    raise
            try:
                if hasattr(os, 'link'):
                    os.link(pre_lock.name, f.name + '.lock')
                    dotlock_done = True
                    os.unlink(pre_lock.name)
                else:
                    os.rename(pre_lock.name, f.name + '.lock')
                    dotlock_done = True
            except OSError, e:
                if e.errno == errno.EEXIST or \
                  (os.name == 'os2' and e.errno == errno.EACCES):
                    os.remove(pre_lock.name)
                    raise ExternalClashError('dot lock unavailable: %s' %
                                             f.name)
                else:
                    raise
    except:
        if fcntl:
            fcntl.lockf(f, fcntl.LOCK_UN)
        if dotlock_done:
            os.remove(f.name + '.lock')
        raise

def _unlock_file(f):
    """Unlock file f using lockf and dot locking."""
    if fcntl:
        fcntl.lockf(f, fcntl.LOCK_UN)
    if os.path.exists(f.name + '.lock'):
        os.remove(f.name + '.lock')

def _create_carefully(path):
    """Create a file if it doesn't exist and open for reading and writing."""
    fd = os.open(path, os.O_CREAT | os.O_EXCL | os.O_RDWR)
    try:
        return open(path, 'rb+')
    finally:
        os.close(fd)

def _create_temporary(path):
    """Create a temp file based on path and open for reading and writing."""
    return _create_carefully('%s.%s.%s.%s' % (path, int(time.time()),
                                              socket.gethostname(),
                                              os.getpid()))


## Start: classes from the original module (for backward compatibility).

# Note that the Maildir class, whose name is unchanged, itself offers a next()
# method for backward compatibility.

class _Mailbox:

    def __init__(self, fp, factory=rfc822.Message):
        self.fp = fp
        self.seekp = 0
        self.factory = factory

    def __iter__(self):
        return iter(self.next, None)

    def next(self):
        while 1:
            self.fp.seek(self.seekp)
            try:
                self._search_start()
            except EOFError:
                self.seekp = self.fp.tell()
                return None
            start = self.fp.tell()
            self._search_end()
            self.seekp = stop = self.fp.tell()
            if start != stop:
                break
        return self.factory(_PartialFile(self.fp, start, stop))

# Recommended to use PortableUnixMailbox instead!
class UnixMailbox(_Mailbox):

    def _search_start(self):
        while 1:
            pos = self.fp.tell()
            line = self.fp.readline()
            if not line:
                raise EOFError
            if line[:5] == 'From ' and self._isrealfromline(line):
                self.fp.seek(pos)
                return

    def _search_end(self):
        self.fp.readline()      # Throw away header line
        while 1:
            pos = self.fp.tell()
            line = self.fp.readline()
            if not line:
                return
            if line[:5] == 'From ' and self._isrealfromline(line):
                self.fp.seek(pos)
                return

    # An overridable mechanism to test for From-line-ness.  You can either
    # specify a different regular expression or define a whole new
    # _isrealfromline() method.  Note that this only gets called for lines
    # starting with the 5 characters "From ".
    #
    # BAW: According to
    #http://home.netscape.com/eng/mozilla/2.0/relnotes/demo/content-length.html
    # the only portable, reliable way to find message delimiters in a BSD (i.e
    # Unix mailbox) style folder is to search for "\n\nFrom .*\n", or at the
    # beginning of the file, "^From .*\n".  While _fromlinepattern below seems
    # like a good idea, in practice, there are too many variations for more
    # strict parsing of the line to be completely accurate.
    #
    # _strict_isrealfromline() is the old version which tries to do stricter
    # parsing of the From_ line.  _portable_isrealfromline() simply returns
    # true, since it's never called if the line doesn't already start with
    # "From ".
    #
    # This algorithm, and the way it interacts with _search_start() and
    # _search_end() may not be completely correct, because it doesn't check
    # that the two characters preceding "From " are \n\n or the beginning of
    # the file.  Fixing this would require a more extensive rewrite than is
    # necessary.  For convenience, we've added a PortableUnixMailbox class
    # which uses the more lenient _fromlinepattern regular expression.

    _fromlinepattern = r"From \s*[^\s]+\s+\w\w\w\s+\w\w\w\s+\d?\d\s+" \
                       r"\d?\d:\d\d(:\d\d)?(\s+[^\s]+)?\s+\d\d\d\d\s*$"
    _regexp = None

    def _strict_isrealfromline(self, line):
        if not self._regexp:
            import re
            self._regexp = re.compile(self._fromlinepattern)
        return self._regexp.match(line)

    def _portable_isrealfromline(self, line):
        return True

    _isrealfromline = _strict_isrealfromline


class PortableUnixMailbox(UnixMailbox):
    _isrealfromline = UnixMailbox._portable_isrealfromline


class MmdfMailbox(_Mailbox):

    def _search_start(self):
        while 1:
            line = self.fp.readline()
            if not line:
                raise EOFError
            if line[:5] == '\001\001\001\001\n':
                return

    def _search_end(self):
        while 1:
            pos = self.fp.tell()
            line = self.fp.readline()
            if not line:
                return
            if line == '\001\001\001\001\n':
                self.fp.seek(pos)
                return


class MHMailbox:

    def __init__(self, dirname, factory=rfc822.Message):
        import re
        pat = re.compile('^[1-9][0-9]*$')
        self.dirname = dirname
        # the three following lines could be combined into:
        # list = map(long, filter(pat.match, os.listdir(self.dirname)))
        list = os.listdir(self.dirname)
        list = filter(pat.match, list)
        list = map(long, list)
        list.sort()
        # This only works in Python 1.6 or later;
        # before that str() added 'L':
        self.boxes = map(str, list)
        self.boxes.reverse()
        self.factory = factory

    def __iter__(self):
        return iter(self.next, None)

    def next(self):
        if not self.boxes:
            return None
        fn = self.boxes.pop()
        fp = open(os.path.join(self.dirname, fn))
        msg = self.factory(fp)
        try:
            msg._mh_msgno = fn
        except (AttributeError, TypeError):
            pass
        return msg


class BabylMailbox(_Mailbox):

    def _search_start(self):
        while 1:
            line = self.fp.readline()
            if not line:
                raise EOFError
            if line == '*** EOOH ***\n':
                return

    def _search_end(self):
        while 1:
            pos = self.fp.tell()
            line = self.fp.readline()
            if not line:
                return
            if line == '\037\014\n' or line == '\037':
                self.fp.seek(pos)
                return

## End: classes from the original module (for backward compatibility).


class Error(Exception):
    """Raised for module-specific errors."""

class NoSuchMailboxError(Error):
    """The specified mailbox does not exist and won't be created."""

class NotEmptyError(Error):
    """The specified mailbox is not empty and deletion was requested."""

class ExternalClashError(Error):
    """Another process caused an action to fail."""

class FormatError(Error):
    """A file appears to have an invalid format."""