[notmuch] / bindings / python / notmuch / database.py

"""
This file is part of notmuch.

Notmuch is free software: you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation, either version 3 of the License, or (at your
option) any later version.

Notmuch is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.

You should have received a copy of the GNU General Public License
along with notmuch.  If not, see <http://www.gnu.org/licenses/>.

Copyright 2010 Sebastian Spaeth <Sebastian@SSpaeth.de>'
"""

import os
import codecs
from ctypes import c_char_p, c_void_p, c_uint, c_long, byref, POINTER
from notmuch.globals import (
    nmlib,
    STATUS,
    FileError,
    NotmuchError,
    NullPointerError,
    NotInitializedError,
    Enum,
    _str,
    NotmuchDatabaseP,
    NotmuchDirectoryP,
    NotmuchMessageP,
    NotmuchTagsP,
    NotmuchFilenamesP
)
from notmuch.message import Message
from notmuch.tag import Tags
from .query import Query

class Database(object):
    """The :class:`Database` is the highest-level object that notmuch
    provides. It references a notmuch database, and can be opened in
    read-only or read-write mode. A :class:`Query` can be derived from
    or be applied to a specific database to find messages. Also adding
    and removing messages to the database happens via this
    object. Modifications to the database are not atmic by default (see
    :meth:`begin_atomic`) and once a database has been modified, all
    other database objects pointing to the same data-base will throw an
    :exc:`XapianError` as the underlying database has been
    modified. Close and reopen the database to continue working with it.

    :class:`Database` objects implement the context manager protocol
    so you can use the :keyword:`with` statement to ensure that the
    database is properly closed.

    .. note::

        Any function in this class can and will throw an
        :exc:`NotInitializedError` if the database was not intitialized
        properly.

    .. note::

        Do remember that as soon as we tear down (e.g. via `del db`) this
        object, all underlying derived objects such as queries, threads,
        messages, tags etc will be freed by the underlying library as well.
        Accessing these objects will lead to segfaults and other unexpected
        behavior. See above for more details.
    """
    _std_db_path = None
    """Class attribute to cache user's default database"""

    MODE = Enum(['READ_ONLY', 'READ_WRITE'])
    """Constants: Mode in which to open the database"""

    """notmuch_database_get_directory"""
    _get_directory = nmlib.notmuch_database_get_directory
    _get_directory.argtypes = [NotmuchDatabaseP, c_char_p]
    _get_directory.restype = NotmuchDirectoryP

    """notmuch_database_get_path"""
    _get_path = nmlib.notmuch_database_get_path
    _get_path.argtypes = [NotmuchDatabaseP]
    _get_path.restype = c_char_p

    """notmuch_database_get_version"""
    _get_version = nmlib.notmuch_database_get_version
    _get_version.argtypes = [NotmuchDatabaseP]
    _get_version.restype = c_uint

    """notmuch_database_open"""
    _open = nmlib.notmuch_database_open
    _open.argtypes = [c_char_p, c_uint]
    _open.restype = NotmuchDatabaseP

    """notmuch_database_upgrade"""
    _upgrade = nmlib.notmuch_database_upgrade
    _upgrade.argtypes = [NotmuchDatabaseP, c_void_p, c_void_p]
    _upgrade.restype = c_uint

    """ notmuch_database_find_message"""
    _find_message = nmlib.notmuch_database_find_message
    _find_message.argtypes = [NotmuchDatabaseP, c_char_p,
                              POINTER(NotmuchMessageP)]
    _find_message.restype = c_uint

    """notmuch_database_find_message_by_filename"""
    _find_message_by_filename = nmlib.notmuch_database_find_message_by_filename
    _find_message_by_filename.argtypes = [NotmuchDatabaseP, c_char_p,
                                          POINTER(NotmuchMessageP)]
    _find_message_by_filename.restype = c_uint

    """notmuch_database_get_all_tags"""
    _get_all_tags = nmlib.notmuch_database_get_all_tags
    _get_all_tags.argtypes = [NotmuchDatabaseP]
    _get_all_tags.restype = NotmuchTagsP

    """notmuch_database_create"""
    _create = nmlib.notmuch_database_create
    _create.argtypes = [c_char_p]
    _create.restype = NotmuchDatabaseP

    def __init__(self, path = None, create = False,
                 mode = MODE.READ_ONLY):
        """If *path* is `None`, we will try to read a users notmuch
        configuration and use his configured database. The location of the
        configuration file can be specified through the environment variable
        *NOTMUCH_CONFIG*, falling back to the default `~/.notmuch-config`.

        If *create* is `True`, the database will always be created in
        :attr:`MODE`.READ_WRITE mode. Default mode for opening is READ_ONLY.

        :param path:   Directory to open/create the database in (see
                       above for behavior if `None`)
        :type path:    `str` or `None`
        :param create: Pass `False` to open an existing, `True` to create a new
                       database.
        :type create:  bool
        :param mode:   Mode to open a database in. Is always
                       :attr:`MODE`.READ_WRITE when creating a new one.
        :type mode:    :attr:`MODE`
        :raises: :exc:`NotmuchError` or derived exception in case of
            failure.
        """
        self._db = None
        if path is None:
            # no path specified. use a user's default database
            if Database._std_db_path is None:
                #the following line throws a NotmuchError if it fails
                Database._std_db_path = self._get_user_default_db()
            path = Database._std_db_path

        if create == False:
            self.open(path, mode)
        else:
            self.create(path)

    def __del__(self):
        self.close()

    def _assert_db_is_initialized(self):
        """Raises :exc:`NotInitializedError` if self._db is `None`"""
        if not self._db:
            raise NotInitializedError()

    def create(self, path):
        """Creates a new notmuch database

        This function is used by __init__() and usually does not need
        to be called directly. It wraps the underlying
        *notmuch_database_create* function and creates a new notmuch
        database at *path*. It will always return a database in :attr:`MODE`
        .READ_WRITE mode as creating an empty database for
        reading only does not make a great deal of sense.

        :param path: A directory in which we should create the database.
        :type path: str
        :raises: :exc:`NotmuchError` in case of any failure
                    (possibly after printing an error message on stderr).
        """
        if self._db is not None:
            raise NotmuchError(message="Cannot create db, this Database() "
                                       "already has an open one.")

        res = Database._create(_str(path), Database.MODE.READ_WRITE)

        if not res:
            raise NotmuchError(
                message="Could not create the specified database")
        self._db = res

    def open(self, path, mode=0):
        """Opens an existing database

        This function is used by __init__() and usually does not need
        to be called directly. It wraps the underlying
        *notmuch_database_open* function.

        :param status: Open the database in read-only or read-write mode
        :type status:  :attr:`MODE`
        :raises: Raises :exc:`NotmuchError` in case of any failure
                    (possibly after printing an error message on stderr).
        """
        res = Database._open(_str(path), mode)

        if not res:
            raise NotmuchError(message="Could not open the specified database")
        self._db = res

    _close = nmlib.notmuch_database_close
    _close.argtypes = [NotmuchDatabaseP]
    _close.restype = None

    def close(self):
        """Close and free the notmuch database if needed"""
        if self._db is not None:
            self._close(self._db)
            self._db = None

    def __enter__(self):
        '''
        Implements the context manager protocol.
        '''
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        '''
        Implements the context manager protocol.
        '''
        self.close()

    def get_path(self):
        """Returns the file path of an open database"""
        self._assert_db_is_initialized()
        return Database._get_path(self._db).decode('utf-8')

    def get_version(self):
        """Returns the database format version

        :returns: The database version as positive integer
        """
        self._assert_db_is_initialized()
        return Database._get_version(self._db)

    _needs_upgrade = nmlib.notmuch_database_needs_upgrade
    _needs_upgrade.argtypes = [NotmuchDatabaseP]
    _needs_upgrade.restype = bool

    def needs_upgrade(self):
        """Does this database need to be upgraded before writing to it?

        If this function returns `True` then no functions that modify the
        database (:meth:`add_message`,
        :meth:`Message.add_tag`, :meth:`Directory.set_mtime`,
        etc.) will work unless :meth:`upgrade` is called successfully first.

        :returns: `True` or `False`
        """
        self._assert_db_is_initialized()
        return self._needs_upgrade(self._db)

    def upgrade(self):
        """Upgrades the current database

        After opening a database in read-write mode, the client should
        check if an upgrade is needed (notmuch_database_needs_upgrade) and
        if so, upgrade with this function before making any modifications.

        NOT IMPLEMENTED: The optional progress_notify callback can be
        used by the caller to provide progress indication to the
        user. If non-NULL it will be called periodically with
        'progress' as a floating-point value in the range of [0.0..1.0]
        indicating the progress made so far in the upgrade process.

        :TODO: catch exceptions, document return values and etc...
        """
        self._assert_db_is_initialized()
        status = Database._upgrade(self._db, None, None)
        #TODO: catch exceptions, document return values and etc
        return status

    _begin_atomic = nmlib.notmuch_database_begin_atomic
    _begin_atomic.argtypes = [NotmuchDatabaseP]
    _begin_atomic.restype = c_uint

    def begin_atomic(self):
        """Begin an atomic database operation

        Any modifications performed between a successful
        :meth:`begin_atomic` and a :meth:`end_atomic` will be applied to
        the database atomically.  Note that, unlike a typical database
        transaction, this only ensures atomicity, not durability;
        neither begin nor end necessarily flush modifications to disk.

        :returns: :attr:`STATUS`.SUCCESS or raises
        :raises: :exc:`NotmuchError`: :attr:`STATUS`.XAPIAN_EXCEPTION
                    Xapian exception occurred; atomic section not entered.

        *Added in notmuch 0.9*"""
        self._assert_db_is_initialized()
        status = self._begin_atomic(self._db)
        if status != STATUS.SUCCESS:
            raise NotmuchError(status)
        return status

    _end_atomic = nmlib.notmuch_database_end_atomic
    _end_atomic.argtypes = [NotmuchDatabaseP]
    _end_atomic.restype = c_uint

    def end_atomic(self):
        """Indicate the end of an atomic database operation

        See :meth:`begin_atomic` for details.

        :returns: :attr:`STATUS`.SUCCESS or raises

        :raises:
            :exc:`NotmuchError`:
                :attr:`STATUS`.XAPIAN_EXCEPTION
                    A Xapian exception occurred; atomic section not
                    ended.
                :attr:`STATUS`.UNBALANCED_ATOMIC:
                    end_atomic has been called more times than begin_atomic.

        *Added in notmuch 0.9*"""
        self._assert_db_is_initialized()
        status = self._end_atomic(self._db)
        if status != STATUS.SUCCESS:
            raise NotmuchError(status)
        return status

    def get_directory(self, path):
        """Returns a :class:`Directory` of path,
        (creating it if it does not exist(?))

        .. warning::

            This call needs a writeable database in
            :attr:`Database.MODE`.READ_WRITE mode. The underlying library will
            exit the program if this method is used on a read-only database!

        :param path: An unicode string containing the path relative to the path
              of database (see :meth:`get_path`), or else should be an absolute
              path with initial components that match the path of 'database'.
        :returns: :class:`Directory` or raises an exception.
        :raises:
            :exc:`NotmuchError` with :attr:`STATUS`.FILE_ERROR
                    If path is not relative database or absolute with initial
                    components same as database.
        """
        self._assert_db_is_initialized()
        # sanity checking if path is valid, and make path absolute
        if path and path[0] == os.sep:
            # we got an absolute path
            if not path.startswith(self.get_path()):
                # but its initial components are not equal to the db path
                raise FileError('Database().get_directory() called '
                                'with a wrong absolute path')
            abs_dirpath = path
        else:
            #we got a relative path, make it absolute
            abs_dirpath = os.path.abspath(os.path.join(self.get_path(), path))

        dir_p = Database._get_directory(self._db, _str(path))

        # return the Directory, init it with the absolute path
        return Directory(abs_dirpath, dir_p, self)

    _add_message = nmlib.notmuch_database_add_message
    _add_message.argtypes = [NotmuchDatabaseP, c_char_p,
                             POINTER(NotmuchMessageP)]
    _add_message.restype = c_uint

    def add_message(self, filename, sync_maildir_flags=False):
        """Adds a new message to the database

        :param filename: should be a path relative to the path of the
            open database (see :meth:`get_path`), or else should be an
            absolute filename with initial components that match the
            path of the database.

            The file should be a single mail message (not a
            multi-message mbox) that is expected to remain at its
            current location, since the notmuch database will reference
            the filename, and will not copy the entire contents of the
            file.

        :param sync_maildir_flags: If the message contains Maildir
            flags, we will -depending on the notmuch configuration- sync
            those tags to initial notmuch tags, if set to `True`. It is
            `False` by default to remain consistent with the libnotmuch
            API. You might want to look into the underlying method
            :meth:`Message.maildir_flags_to_tags`.

        :returns: On success, we return

           1) a :class:`Message` object that can be used for things
              such as adding tags to the just-added message.
           2) one of the following :attr:`STATUS` values:

              :attr:`STATUS`.SUCCESS
                  Message successfully added to database.
              :attr:`STATUS`.DUPLICATE_MESSAGE_ID
                  Message has the same message ID as another message already
                  in the database. The new filename was successfully added
                  to the list of the filenames for the existing message.

        :rtype:   2-tuple(:class:`Message`, :attr:`STATUS`)

        :raises: Raises a :exc:`NotmuchError` with the following meaning.
              If such an exception occurs, nothing was added to the database.

              :attr:`STATUS`.FILE_ERROR
                      An error occurred trying to open the file, (such as
                      permission denied, or file not found, etc.).
              :attr:`STATUS`.FILE_NOT_EMAIL
                      The contents of filename don't look like an email
                      message.
              :attr:`STATUS`.READ_ONLY_DATABASE
                      Database was opened in read-only mode so no message can
                      be added.
        """
        self._assert_db_is_initialized()
        msg_p = NotmuchMessageP()
        status = self._add_message(self._db, _str(filename), byref(msg_p))

        if not status in [STATUS.SUCCESS, STATUS.DUPLICATE_MESSAGE_ID]:
            raise NotmuchError(status)

        #construct Message() and return
        msg = Message(msg_p, self)
        #automatic sync initial tags from Maildir flags
        if sync_maildir_flags:
            msg.maildir_flags_to_tags()
        return (msg, status)

    _remove_message = nmlib.notmuch_database_remove_message
    _remove_message.argtypes = [NotmuchDatabaseP, c_char_p]
    _remove_message.restype = c_uint

    def remove_message(self, filename):
        """Removes a message (filename) from the given notmuch database

        Note that only this particular filename association is removed from
        the database. If the same message (as determined by the message ID)
        is still available via other filenames, then the message will
        persist in the database for those filenames. When the last filename
        is removed for a particular message, the database content for that
        message will be entirely removed.

        :returns: A :attr:`STATUS` value with the following meaning:

             :attr:`STATUS`.SUCCESS
               The last filename was removed and the message was removed
               from the database.
             :attr:`STATUS`.DUPLICATE_MESSAGE_ID
               This filename was removed but the message persists in the
               database with at least one other filename.

        :raises: Raises a :exc:`NotmuchError` with the following meaning.
             If such an exception occurs, nothing was removed from the
             database.

             :attr:`STATUS`.READ_ONLY_DATABASE
               Database was opened in read-only mode so no message can be
               removed.
        """
        self._assert_db_is_initialized()
        return self._remove_message(self._db, _str(filename))

    def find_message(self, msgid):
        """Returns a :class:`Message` as identified by its message ID

        Wraps the underlying *notmuch_database_find_message* function.

        :param msgid: The message ID
        :type msgid: unicode or str
        :returns: :class:`Message` or `None` if no message is found.
        :raises:
            :exc:`OutOfMemoryError`
                  If an Out-of-memory occured while constructing the message.
            :exc:`XapianError`
                  In case of a Xapian Exception. These exceptions
                  include "Database modified" situations, e.g. when the
                  notmuch database has been modified by another program
                  in the meantime. In this case, you should close and
                  reopen the database and retry.
            :exc:`NotInitializedError` if
                    the database was not intitialized.
        """
        self._assert_db_is_initialized()
        msg_p = NotmuchMessageP()
        status = Database._find_message(self._db, _str(msgid), byref(msg_p))
        if status != STATUS.SUCCESS:
            raise NotmuchError(status)
        return msg_p and Message(msg_p, self) or None

    def find_message_by_filename(self, filename):
        """Find a message with the given filename

        .. warning::

            This call needs a writeable database in
            :attr:`Database.MODE`.READ_WRITE mode. The underlying library will
            exit the program if this method is used on a read-only database!

        :returns: If the database contains a message with the given
            filename, then a class:`Message:` is returned.  This
            function returns None if no message is found with the given
            filename.

        :raises:
            :exc:`OutOfMemoryError`
                  If an Out-of-memory occured while constructing the message.
            :exc:`XapianError`
                  In case of a Xapian Exception. These exceptions
                  include "Database modified" situations, e.g. when the
                  notmuch database has been modified by another program
                  in the meantime. In this case, you should close and
                  reopen the database and retry.
            :exc:`NotInitializedError` if
                    the database was not intitialized.

        *Added in notmuch 0.9*"""
        self._assert_db_is_initialized()
        msg_p = NotmuchMessageP()
        status = Database._find_message_by_filename(self._db, _str(filename),
                                                    byref(msg_p))
        if status != STATUS.SUCCESS:
            raise NotmuchError(status)
        return msg_p and Message(msg_p, self) or None

    def get_all_tags(self):
        """Returns :class:`Tags` with a list of all tags found in the database

        :returns: :class:`Tags`
        :execption: :exc:`NotmuchError` with :attr:`STATUS`.NULL_POINTER
                    on error
        """
        self._assert_db_is_initialized()
        tags_p = Database._get_all_tags(self._db)
        if tags_p == None:
            raise NullPointerError()
        return Tags(tags_p, self)

    def create_query(self, querystring):
        """Returns a :class:`Query` derived from this database

        This is a shorthand method for doing::

          # short version
          # Automatically frees the Database() when 'q' is deleted

          q  = Database(dbpath).create_query('from:"Biene Maja"')

          # long version, which is functionally equivalent but will keep the
          # Database in the 'db' variable around after we delete 'q':

          db = Database(dbpath)
          q  = Query(db,'from:"Biene Maja"')

        This function is a python extension and not in the underlying C API.
        """
        return Query(self, querystring)

    def __repr__(self):
        return "'Notmuch DB " + self.get_path() + "'"

    def _get_user_default_db(self):
        """ Reads a user's notmuch config and returns his db location

        Throws a NotmuchError if it cannot find it"""
        try:
            # python3.x
            from configparser import SafeConfigParser
        except ImportError:
            # python2.x
            from ConfigParser import SafeConfigParser

        config = SafeConfigParser()
        conf_f = os.getenv('NOTMUCH_CONFIG',
                           os.path.expanduser('~/.notmuch-config'))
        config.readfp(codecs.open(conf_f, 'r', 'utf-8'))
        if not config.has_option('database', 'path'):
            raise NotmuchError(message="No DB path specified"
                                       " and no user default found")
        return config.get('database', 'path')

    @property
    def db_p(self):
        """Property returning a pointer to `notmuch_database_t` or `None`

        This should normally not be needed by a user (and is not yet
        guaranteed to remain stable in future versions).
        """
        return self._db


class Directory(object):
    """Represents a directory entry in the notmuch directory

    Modifying attributes of this object will modify the
    database, not the real directory attributes.

    The Directory object is usually derived from another object
    e.g. via :meth:`Database.get_directory`, and will automatically be
    become invalid whenever that parent is deleted. You should
    therefore initialized this object handing it a reference to the
    parent, preventing the parent from automatically being garbage
    collected.
    """

    """notmuch_directory_get_mtime"""
    _get_mtime = nmlib.notmuch_directory_get_mtime
    _get_mtime.argtypes = [NotmuchDirectoryP]
    _get_mtime.restype = c_long

    """notmuch_directory_set_mtime"""
    _set_mtime = nmlib.notmuch_directory_set_mtime
    _set_mtime.argtypes = [NotmuchDirectoryP, c_long]
    _set_mtime.restype = c_uint

    """notmuch_directory_get_child_files"""
    _get_child_files = nmlib.notmuch_directory_get_child_files
    _get_child_files.argtypes = [NotmuchDirectoryP]
    _get_child_files.restype = NotmuchFilenamesP

    """notmuch_directory_get_child_directories"""
    _get_child_directories = nmlib.notmuch_directory_get_child_directories
    _get_child_directories.argtypes = [NotmuchDirectoryP]
    _get_child_directories.restype = NotmuchFilenamesP

    def _assert_dir_is_initialized(self):
        """Raises a NotmuchError(:attr:`STATUS`.NOT_INITIALIZED)
        if dir_p is None"""
        if not self._dir_p:
            raise NotInitializedError()

    def __init__(self, path, dir_p, parent):
        """
        :param path:   The absolute path of the directory object.
        :param dir_p:  The pointer to an internal notmuch_directory_t object.
        :param parent: The object this Directory is derived from
                       (usually a :class:`Database`). We do not directly use
                       this, but store a reference to it as long as
                       this Directory object lives. This keeps the
                       parent object alive.
        """
        self._path = path
        self._dir_p = dir_p
        self._parent = parent

    def set_mtime(self, mtime):
        """Sets the mtime value of this directory in the database

        The intention is for the caller to use the mtime to allow efficient
        identification of new messages to be added to the database. The
        recommended usage is as follows:

        * Read the mtime of a directory from the filesystem

        * Call :meth:`Database.add_message` for all mail files in
          the directory

        * Call notmuch_directory_set_mtime with the mtime read from the
          filesystem.  Then, when wanting to check for updates to the
          directory in the future, the client can call :meth:`get_mtime`
          and know that it only needs to add files if the mtime of the
          directory and files are newer than the stored timestamp.

          .. note::

                :meth:`get_mtime` function does not allow the caller to
                distinguish a timestamp of 0 from a non-existent timestamp. So
                don't store a timestamp of 0 unless you are comfortable with
                that.

        :param mtime: A (time_t) timestamp
        :raises: :exc:`XapianError` a Xapian exception occurred, mtime
                 not stored
        :raises: :exc:`ReadOnlyDatabaseError` the database was opened
                 in read-only mode so directory mtime cannot be modified
        :raises: :exc:`NotInitializedError` the directory object has not
                 been initialized
        """
        self._assert_dir_is_initialized()
        status = Directory._set_mtime(self._dir_p, mtime)

        if status != STATUS.SUCCESS:
            raise NotmuchError(status)

    def get_mtime(self):
        """Gets the mtime value of this directory in the database

        Retrieves a previously stored mtime for this directory.

        :param mtime: A (time_t) timestamp
        :raises: :exc:`NotmuchError`:

                        :attr:`STATUS`.NOT_INITIALIZED
                          The directory has not been initialized
        """
        self._assert_dir_is_initialized()
        return Directory._get_mtime(self._dir_p)

    # Make mtime attribute a property of Directory()
    mtime = property(get_mtime, set_mtime, doc="""Property that allows getting
                     and setting of the Directory *mtime* (read-write)

                     See :meth:`get_mtime` and :meth:`set_mtime` for usage and
                     possible exceptions.""")

    def get_child_files(self):
        """Gets a Filenames iterator listing all the filenames of
        messages in the database within the given directory.

        The returned filenames will be the basename-entries only (not
        complete paths.
        """
        self._assert_dir_is_initialized()
        files_p = Directory._get_child_files(self._dir_p)
        return Filenames(files_p, self)

    def get_child_directories(self):
        """Gets a :class:`Filenames` iterator listing all the filenames of
        sub-directories in the database within the given directory

        The returned filenames will be the basename-entries only (not
        complete paths.
        """
        self._assert_dir_is_initialized()
        files_p = Directory._get_child_directories(self._dir_p)
        return Filenames(files_p, self)

    @property
    def path(self):
        """Returns the absolute path of this Directory (read-only)"""
        return self._path

    def __repr__(self):
        """Object representation"""
        return "<notmuch Directory object '%s'>" % self._path

    _destroy = nmlib.notmuch_directory_destroy
    _destroy.argtypes = [NotmuchDirectoryP]
    _destroy.argtypes = None

    def __del__(self):
        """Close and free the Directory"""
        if self._dir_p is not None:
            self._destroy(self._dir_p)


class Filenames(object):
    """An iterator over File- or Directory names stored in the database"""

    #notmuch_filenames_get
    _get = nmlib.notmuch_filenames_get
    _get.argtypes = [NotmuchFilenamesP]
    _get.restype = c_char_p

    def __init__(self, files_p, parent):
        """
        :param files_p: The pointer to an internal notmuch_filenames_t object.
        :param parent: The object this Directory is derived from
                       (usually a Directory()). We do not directly use
                       this, but store a reference to it as long as
                       this Directory object lives. This keeps the
                       parent object alive.
        """
        self._files_p = files_p
        self._parent = parent

    def __iter__(self):
        """ Make Filenames an iterator """
        return self

    _valid = nmlib.notmuch_filenames_valid
    _valid.argtypes = [NotmuchFilenamesP]
    _valid.restype = bool

    _move_to_next = nmlib.notmuch_filenames_move_to_next
    _move_to_next.argtypes = [NotmuchFilenamesP]
    _move_to_next.restype = None

    def __next__(self):
        if not self._files_p:
            raise NotInitializedError()

        if not self._valid(self._files_p):
            self._files_p = None
            raise StopIteration

        file_ = Filenames._get(self._files_p)
        self._move_to_next(self._files_p)
        return file_.decode('utf-8', 'ignore')
    next = __next__ # python2.x iterator protocol compatibility

    def __len__(self):
        """len(:class:`Filenames`) returns the number of contained files

        .. note::

            As this iterates over the files, we will not be able to
            iterate over them again! So this will fail::

                 #THIS FAILS
                 files = Database().get_directory('').get_child_files()
                 if len(files) > 0:  # this 'exhausts' msgs
                     # next line raises
                     # NotmuchError(:attr:`STATUS`.NOT_INITIALIZED)
                     for file in files: print file
        """
        if not self._files_p:
            raise NotInitializedError()

        i = 0
        while self._valid(self._files_p):
            self._move_to_next(self._files_p)
            i += 1
        self._files_p = None
        return i

    _destroy = nmlib.notmuch_filenames_destroy
    _destroy.argtypes = [NotmuchFilenamesP]
    _destroy.restype = None

    def __del__(self):
        """Close and free Filenames"""
        if self._files_p is not None:
            self._destroy(self._files_p)