:mod:`mailbox` --- Manipulate mailboxes in various formats
==========================================================

.. module:: mailbox
   :synopsis: Manipulate mailboxes in various formats

.. moduleauthor:: Gregory K. Johnson <gkj@gregorykjohnson.com>
.. sectionauthor:: Gregory K. Johnson <gkj@gregorykjohnson.com>

**Source code:** :source:`Lib/mailbox.py`

--------------

This module defines two classes, :class:`Mailbox` and :class:`Message`, for
accessing and manipulating on-disk mailboxes and the messages they contain.
:class:`!Mailbox` offers a dictionary-like mapping from keys to messages.
:class:`!Message` extends the :mod:`email.message` module's
:class:`~email.message.Message` class with format-specific state and behavior.
Supported mailbox formats are Maildir, mbox, MH, Babyl, and MMDF.


.. seealso::

   Module :mod:`email`
      Represent and manipulate messages.


.. _mailbox-objects:

:class:`!Mailbox` objects
-------------------------

.. class:: Mailbox

   A mailbox, which may be inspected and modified.

   The :class:`!Mailbox` class defines an interface and is not intended to be
   instantiated.  Instead, format-specific subclasses should inherit from
   :class:`!Mailbox` and your code should instantiate a particular subclass.

   The :class:`!Mailbox` interface is dictionary-like, with small keys
   corresponding to messages. Keys are issued by the :class:`!Mailbox` instance
   with which they will be used and are only meaningful to that :class:`!Mailbox`
   instance. A key continues to identify a message even if the corresponding
   message is modified, such as by replacing it with another message.

   Messages may be added to a :class:`!Mailbox` instance using the set-like
   method :meth:`add` and removed using a ``del`` statement or the set-like
   methods :meth:`remove` and :meth:`discard`.

   :class:`!Mailbox` interface semantics differ from dictionary semantics in some
   noteworthy ways. Each time a message is requested, a new representation
   (typically a :class:`Message` instance) is generated based upon the current
   state of the mailbox. Similarly, when a message is added to a
   :class:`!Mailbox` instance, the provided message representation's contents are
   copied. In neither case is a reference to the message representation kept by
   the :class:`!Mailbox` instance.

   The default :class:`!Mailbox` :term:`iterator` iterates over message
   representations, not keys as the default :class:`dictionary <dict>`
   iterator does. Moreover, modification of a
   mailbox during iteration is safe and well-defined. Messages added to the
   mailbox after an iterator is created will not be seen by the
   iterator. Messages removed from the mailbox before the iterator yields them
   will be silently skipped, though using a key from an iterator may result in a
   :exc:`KeyError` exception if the corresponding message is subsequently
   removed.

   .. warning::

      Be very cautious when modifying mailboxes that might be simultaneously
      changed by some other process.  The safest mailbox format to use for such
      tasks is :class:`Maildir`; try to avoid using single-file formats such as
      :class:`mbox` for
      concurrent writing.  If you're modifying a mailbox, you *must* lock it by
      calling the :meth:`lock` and :meth:`unlock` methods *before* reading any
      messages in the file or making any changes by adding or deleting a
      message.  Failing to lock the mailbox runs the risk of losing messages or
      corrupting the entire mailbox.

   :class:`!Mailbox` instances have the following methods:


   .. method:: add(message)

      Add *message* to the mailbox and return the key that has been assigned to
      it.

      Parameter *message* may be a :class:`Message` instance, an
      :class:`email.message.Message` instance, a string, a byte string, or a
      file-like object (which should be open in binary mode). If *message* is
      an instance of the
      appropriate format-specific :class:`Message` subclass (e.g., if it's an
      :class:`mboxMessage` instance and this is an :class:`mbox` instance), its
      format-specific information is used. Otherwise, reasonable defaults for
      format-specific information are used.

      .. versionchanged:: 3.2
         Support for binary input was added.


   .. method:: remove(key)
               __delitem__(key)
               discard(key)

      Delete the message corresponding to *key* from the mailbox.

      If no such message exists, a :exc:`KeyError` exception is raised if the
      method was called as :meth:`remove` or :meth:`__delitem__` but no
      exception is raised if the method was called as :meth:`discard`. The
      behavior of :meth:`discard` may be preferred if the underlying mailbox
      format supports concurrent modification by other processes.


   .. method:: __setitem__(key, message)

      Replace the message corresponding to *key* with *message*. Raise a
      :exc:`KeyError` exception if no message already corresponds to *key*.

      As with :meth:`add`, parameter *message* may be a :class:`Message`
      instance, an :class:`email.message.Message` instance, a string, a byte
      string, or a file-like object (which should be open in binary mode). If
      *message* is an
      instance of the appropriate format-specific :class:`Message` subclass
      (e.g., if it's an :class:`mboxMessage` instance and this is an
      :class:`mbox` instance), its format-specific information is
      used. Otherwise, the format-specific information of the message that
      currently corresponds to *key* is left unchanged.


   .. method:: iterkeys()

      Return an :term:`iterator` over all keys


   .. method:: keys()

      The same as :meth:`iterkeys`, except that a :class:`list` is returned
      rather than an :term:`iterator`


   .. method:: itervalues()
               __iter__()

      Return an :term:`iterator` over representations of all messages.
      The messages are represented
      as instances of the appropriate format-specific :class:`Message` subclass
      unless a custom message factory was specified when the :class:`!Mailbox`
      instance was initialized.

      .. note::

         The behavior of :meth:`__iter__` is unlike that of dictionaries, which
         iterate over keys.


   .. method:: values()

      The same as :meth:`itervalues`, except that a :class:`list` is returned
      rather than an :term:`iterator`


   .. method:: iteritems()

      Return an :term:`iterator` over (*key*, *message*) pairs, where *key* is
      a key and *message* is a message representation. The messages are
      represented as instances of the appropriate format-specific
      :class:`Message` subclass unless a custom message factory was specified
      when the :class:`!Mailbox` instance was initialized.


   .. method:: items()

      The same as :meth:`iteritems`, except that a :class:`list` of pairs is
      returned rather than an :term:`iterator` of pairs.


   .. method:: get(key, default=None)
               __getitem__(key)

      Return a representation of the message corresponding to *key*. If no such
      message exists, *default* is returned if the method was called as
      :meth:`get` and a :exc:`KeyError` exception is raised if the method was
      called as :meth:`!__getitem__`. The message is represented as an instance
      of the appropriate format-specific :class:`Message` subclass unless a
      custom message factory was specified when the :class:`!Mailbox` instance
      was initialized.


   .. method:: get_message(key)

      Return a representation of the message corresponding to *key* as an
      instance of the appropriate format-specific :class:`Message` subclass, or
      raise a :exc:`KeyError` exception if no such message exists.


   .. method:: get_bytes(key)

      Return a byte representation of the message corresponding to *key*, or
      raise a :exc:`KeyError` exception if no such message exists.

      .. versionadded:: 3.2


   .. method:: get_string(key)

      Return a string representation of the message corresponding to *key*, or
      raise a :exc:`KeyError` exception if no such message exists.  The
      message is processed through :class:`email.message.Message` to
      convert it to a 7bit clean representation.


   .. method:: get_file(key)

      Return a :term:`file-like <file-like object>` representation of the
      message corresponding to *key*,
      or raise a :exc:`KeyError` exception if no such message exists.  The
      file-like object behaves as if open in binary mode.  This file should be
      closed once it is no longer needed.

      .. versionchanged:: 3.2
         The file object really is a :term:`binary file`; previously it was
         incorrectly returned in text mode.  Also, the :term:`file-like object`
         now supports the :term:`context manager` protocol: you can use a
         :keyword:`with` statement to automatically close it.

      .. note::

         Unlike other representations of messages,
         :term:`file-like <file-like object>` representations are not
         necessarily independent of the :class:`!Mailbox` instance that
         created them or of the underlying mailbox.  More specific documentation
         is provided by each subclass.


   .. method:: __contains__(key)

      Return ``True`` if *key* corresponds to a message, ``False`` otherwise.


   .. method:: __len__()

      Return a count of messages in the mailbox.


   .. method:: clear()

      Delete all messages from the mailbox.


   .. method:: pop(key, default=None)

      Return a representation of the message corresponding to *key* and delete
      the message. If no such message exists, return *default*. The message is
      represented as an instance of the appropriate format-specific
      :class:`Message` subclass unless a custom message factory was specified
      when the :class:`!Mailbox` instance was initialized.


   .. method:: popitem()

      Return an arbitrary (*key*, *message*) pair, where *key* is a key and
      *message* is a message representation, and delete the corresponding
      message. If the mailbox is empty, raise a :exc:`KeyError` exception. The
      message is represented as an instance of the appropriate format-specific
      :class:`Message` subclass unless a custom message factory was specified
      when the :class:`!Mailbox` instance was initialized.


   .. method:: update(arg)

      Parameter *arg* should be a *key*-to-*message* mapping or an iterable of
      (*key*, *message*) pairs. Updates the mailbox so that, for each given
      *key* and *message*, the message corresponding to *key* is set to
      *message* as if by using :meth:`__setitem__`. As with :meth:`__setitem__`,
      each *key* must already correspond to a message in the mailbox or else a
      :exc:`KeyError` exception will be raised, so in general it is incorrect
      for *arg* to be a :class:`!Mailbox` instance.

      .. note::

         Unlike with dictionaries, keyword arguments are not supported.


   .. method:: flush()

      Write any pending changes to the filesystem. For some :class:`Mailbox`
      subclasses, changes are always written immediately and :meth:`!flush` does
      nothing, but you should still make a habit of calling this method.


   .. method:: lock()

      Acquire an exclusive advisory lock on the mailbox so that other processes
      know not to modify it. An :exc:`ExternalClashError` is raised if the lock
      is not available. The particular locking mechanisms used depend upon the
      mailbox format.  You should *always* lock the mailbox before making any
      modifications to its contents.


   .. method:: unlock()

      Release the lock on the mailbox, if any.


   .. method:: close()

      Flush the mailbox, unlock it if necessary, and close any open files. For
      some :class:`!Mailbox` subclasses, this method does nothing.


.. _mailbox-maildir:

:class:`!Maildir` objects
^^^^^^^^^^^^^^^^^^^^^^^^^


.. class:: Maildir(dirname, factory=None, create=True)

   A subclass of :class:`Mailbox` for mailboxes in Maildir format. Parameter
   *factory* is a callable object that accepts a file-like message representation
   (which behaves as if opened in binary mode) and returns a custom representation.
   If *factory* is ``None``, :class:`MaildirMessage` is used as the default message
   representation. If *create* is ``True``, the mailbox is created if it does not
   exist.

   If *create* is ``True`` and the *dirname* path exists, it will be treated as
   an existing maildir without attempting to verify its directory layout.

   It is for historical reasons that *dirname* is named as such rather than *path*.

   Maildir is a directory-based mailbox format invented for the qmail mail
   transfer agent and now widely supported by other programs. Messages in a
   Maildir mailbox are stored in separate files within a common directory
   structure. This design allows Maildir mailboxes to be accessed and modified
   by multiple unrelated programs without data corruption, so file locking is
   unnecessary.

   Maildir mailboxes contain three subdirectories, namely: :file:`tmp`,
   :file:`new`, and :file:`cur`. Messages are created momentarily in the
   :file:`tmp` subdirectory and then moved to the :file:`new` subdirectory to
   finalize delivery. A mail user agent may subsequently move the message to the
   :file:`cur` subdirectory and store information about the state of the message
   in a special "info" section appended to its file name.

   Folders of the style introduced by the Courier mail transfer agent are also
   supported. Any subdirectory of the main mailbox is considered a folder if
   ``'.'`` is the first character in its name. Folder names are represented by
   :class:`!Maildir` without the leading ``'.'``. Each folder is itself a Maildir
   mailbox but should not contain other folders. Instead, a logical nesting is
   indicated using ``'.'`` to delimit levels, e.g., "Archived.2005.07".

   .. attribute:: Maildir.colon

      The Maildir specification requires the use of a colon (``':'``) in certain
      message file names. However, some operating systems do not permit this
      character in file names, If you wish to use a Maildir-like format on such
      an operating system, you should specify another character to use
      instead. The exclamation point (``'!'``) is a popular choice. For
      example::

         import mailbox
         mailbox.Maildir.colon = '!'

      The :attr:`!colon` attribute may also be set on a per-instance basis.

   .. versionchanged:: 3.13
      :class:`Maildir` now ignores files with a leading dot.

   :class:`!Maildir` instances have all of the methods of :class:`Mailbox` in
   addition to the following:


   .. method:: list_folders()

      Return a list of the names of all folders.


   .. method:: get_folder(folder)

      Return a :class:`!Maildir` instance representing the folder whose name is
      *folder*. A :exc:`NoSuchMailboxError` exception is raised if the folder
      does not exist.


   .. method:: add_folder(folder)

      Create a folder whose name is *folder* and return a :class:`!Maildir`
      instance representing it.


   .. method:: remove_folder(folder)

      Delete the folder whose name is *folder*. If the folder contains any
      messages, a :exc:`NotEmptyError` exception will be raised and the folder
      will not be deleted.


   .. method:: clean()

      Delete temporary files from the mailbox that have not been accessed in the
      last 36 hours. The Maildir specification says that mail-reading programs
      should do this occasionally.


   .. method:: get_flags(key)

      Return as a string the flags that are set on the message
      corresponding to *key*.
      This is the same as ``get_message(key).get_flags()`` but much
      faster, because it does not open the message file.
      Use this method when iterating over the keys to determine which
      messages are interesting to get.

      If you do have a :class:`MaildirMessage` object, use
      its :meth:`~MaildirMessage.get_flags` method instead, because
      changes made by the message's :meth:`~MaildirMessage.set_flags`,
      :meth:`~MaildirMessage.add_flag` and :meth:`~MaildirMessage.remove_flag`
      methods are not reflected here until the mailbox's
      :meth:`__setitem__` method is called.

      .. versionadded:: 3.13


   .. method:: set_flags(key, flags)

      On the message corresponding to *key*, set the flags specified
      by *flags* and unset all others.
      Calling ``some_mailbox.set_flags(key, flags)`` is similar to ::

         one_message = some_mailbox.get_message(key)
         one_message.set_flags(flags)
         some_mailbox[key] = one_message

      but faster, because it does not open the message file.

      If you do have a :class:`MaildirMessage` object, use
      its :meth:`~MaildirMessage.set_flags` method instead, because
      changes made with this mailbox method will not be visible to the
      message object's method, :meth:`~MaildirMessage.get_flags`.

      .. versionadded:: 3.13


   .. method:: add_flag(key, flag)

      On the message corresponding to *key*, set the flags specified
      by *flag* without changing other flags. To add more than one
      flag at a time, *flag* may be a string of more than one character.

      Considerations for using this method versus the message object's
      :meth:`~MaildirMessage.add_flag` method are similar to
      those for :meth:`set_flags`; see the discussion there.

      .. versionadded:: 3.13


   .. method:: remove_flag(key, flag)

      On the message corresponding to *key*, unset the flags specified
      by *flag* without changing other flags. To remove more than one
      flag at a time, *flag* may be a string of more than one character.

      Considerations for using this method versus the message object's
      :meth:`~MaildirMessage.remove_flag` method are similar to
      those for :meth:`set_flags`; see the discussion there.

      .. versionadded:: 3.13


   .. method:: get_info(key)

      Return a string containing the info for the message
      corresponding to *key*.
      This is the same as ``get_message(key).get_info()`` but much
      faster, because it does not open the message file.
      Use this method when iterating over the keys to determine which
      messages are interesting to get.

      If you do have a :class:`MaildirMessage` object, use
      its :meth:`~MaildirMessage.get_info` method instead, because
      changes made by the message's :meth:`~MaildirMessage.set_info` method
      are not reflected here until the mailbox's :meth:`__setitem__` method
      is called.

      .. versionadded:: 3.13


   .. method:: set_info(key, info)

      Set the info of the message corresponding to *key* to *info*.
      Calling ``some_mailbox.set_info(key, flags)`` is similar to ::

         one_message = some_mailbox.get_message(key)
         one_message.set_info(info)
         some_mailbox[key] = one_message

      but faster, because it does not open the message file.

      If you do have a :class:`MaildirMessage` object, use
      its :meth:`~MaildirMessage.set_info` method instead, because
      changes made with this mailbox method will not be visible to the
      message object's method, :meth:`~MaildirMessage.get_info`.

      .. versionadded:: 3.13

   Some :class:`Mailbox` methods implemented by :class:`!Maildir` deserve special
   remarks:


   .. method:: add(message)
               __setitem__(key, message)
               update(arg)

      .. warning::

         These methods generate unique file names based upon the current process
         ID. When using multiple threads, undetected name clashes may occur and
         cause corruption of the mailbox unless threads are coordinated to avoid
         using these methods to manipulate the same mailbox simultaneously.


   .. method:: flush()

      All changes to Maildir mailboxes are immediately applied, so this method
      does nothing.


   .. method:: lock()
               unlock()

      Maildir mailboxes do not support (or require) locking, so these methods do
      nothing.


   .. method:: close()

      :class:`!Maildir` instances do not keep any open files and the underlying
      mailboxes do not support locking, so this method does nothing.


   .. method:: get_file(key)

      Depending upon the host platform, it may not be possible to modify or
      remove the underlying message while the returned file remains open.


.. seealso::

   `maildir man page from Courier <https://www.courier-mta.org/maildir.html>`_
      A specification of the format. Describes a common extension for
      supporting folders.

   `Using maildir format <https://cr.yp.to/proto/maildir.html>`_
      Notes on Maildir by its inventor. Includes an updated name-creation scheme and
      details on "info" semantics.


.. _mailbox-mbox:

:class:`!mbox` objects
^^^^^^^^^^^^^^^^^^^^^^


.. class:: mbox(path, factory=None, create=True)

   A subclass of :class:`Mailbox` for mailboxes in mbox format. Parameter *factory*
   is a callable object that accepts a file-like message representation (which
   behaves as if opened in binary mode) and returns a custom representation. If
   *factory* is ``None``, :class:`mboxMessage` is used as the default message
   representation. If *create* is ``True``, the mailbox is created if it does not
   exist.

   The mbox format is the classic format for storing mail on Unix systems. All
   messages in an mbox mailbox are stored in a single file with the beginning of
   each message indicated by a line whose first five characters are "From ".

   Several variations of the mbox format exist to address perceived shortcomings in
   the original. In the interest of compatibility, :class:`!mbox` implements the
   original format, which is sometimes referred to as :dfn:`mboxo`. This means that
   the :mailheader:`Content-Length` header, if present, is ignored and that any
   occurrences of "From " at the beginning of a line in a message body are
   transformed to ">From " when storing the message, although occurrences of ">From
   " are not transformed to "From " when reading the message.

   Some :class:`Mailbox` methods implemented by :class:`!mbox` deserve special
   remarks:


   .. method:: get_file(key)

      Using the file after calling :meth:`~Mailbox.flush` or
      :meth:`~Mailbox.close` on the :class:`!mbox` instance may yield
      unpredictable results or raise an exception.


   .. method:: lock()
               unlock()

      Three locking mechanisms are used---dot locking and, if available, the
      :c:func:`!flock` and :c:func:`!lockf` system calls.


.. seealso::

   `mbox man page from tin <http://www.tin.org/bin/man.cgi?section=5&topic=mbox>`_
      A specification of the format, with details on locking.

   `Configuring Netscape Mail on Unix: Why The Content-Length Format is Bad <https://www.jwz.org/doc/content-length.html>`_
      An argument for using the original mbox format rather than a variation.

   `"mbox" is a family of several mutually incompatible mailbox formats <https://www.loc.gov/preservation/digital/formats/fdd/fdd000383.shtml>`_
      A history of mbox variations.


.. _mailbox-mh:

:class:`!MH` objects
^^^^^^^^^^^^^^^^^^^^


.. class:: MH(path, factory=None, create=True)

   A subclass of :class:`Mailbox` for mailboxes in MH format. Parameter *factory*
   is a callable object that accepts a file-like message representation (which
   behaves as if opened in binary mode) and returns a custom representation. If
   *factory* is ``None``, :class:`MHMessage` is used as the default message
   representation. If *create* is ``True``, the mailbox is created if it does not
   exist.

   MH is a directory-based mailbox format invented for the MH Message Handling
   System, a mail user agent. Each message in an MH mailbox resides in its own
   file. An MH mailbox may contain other MH mailboxes (called :dfn:`folders`) in
   addition to messages. Folders may be nested indefinitely. MH mailboxes also
   support :dfn:`sequences`, which are named lists used to logically group
   messages without moving them to sub-folders. Sequences are defined in a file
   called :file:`.mh_sequences` in each folder.

   The :class:`!MH` class manipulates MH mailboxes, but it does not attempt to
   emulate all of :program:`mh`'s behaviors. In particular, it does not modify
   and is not affected by the :file:`context` or :file:`.mh_profile` files that
   are used by :program:`mh` to store its state and configuration.

   :class:`!MH` instances have all of the methods of :class:`Mailbox` in addition
   to the following:

   .. versionchanged:: 3.13

      Supported folders that don't contain a :file:`.mh_sequences` file.


   .. method:: list_folders()

      Return a list of the names of all folders.


   .. method:: get_folder(folder)

      Return an :class:`!MH` instance representing the folder whose name is
      *folder*. A :exc:`NoSuchMailboxError` exception is raised if the folder
      does not exist.


   .. method:: add_folder(folder)

      Create a folder whose name is *folder* and return an :class:`!MH` instance
      representing it.


   .. method:: remove_folder(folder)

      Delete the folder whose name is *folder*. If the folder contains any
      messages, a :exc:`NotEmptyError` exception will be raised and the folder
      will not be deleted.


   .. method:: get_sequences()

      Return a dictionary of sequence names mapped to key lists. If there are no
      sequences, the empty dictionary is returned.


   .. method:: set_sequences(sequences)

      Re-define the sequences that exist in the mailbox based upon *sequences*,
      a dictionary of names mapped to key lists, like returned by
      :meth:`get_sequences`.


   .. method:: pack()

      Rename messages in the mailbox as necessary to eliminate gaps in
      numbering.  Entries in the sequences list are updated correspondingly.

      .. note::

         Already-issued keys are invalidated by this operation and should not be
         subsequently used.

   Some :class:`Mailbox` methods implemented by :class:`!MH` deserve special
   remarks:


   .. method:: remove(key)
               __delitem__(key)
               discard(key)

      These methods immediately delete the message. The MH convention of marking
      a message for deletion by prepending a comma to its name is not used.


   .. method:: lock()
               unlock()

      Three locking mechanisms are used---dot locking and, if available, the
      :c:func:`!flock` and :c:func:`!lockf` system calls. For MH mailboxes, locking
      the mailbox means locking the :file:`.mh_sequences` file and, only for the
      duration of any operations that affect them, locking individual message
      files.


   .. method:: get_file(key)

      Depending upon the host platform, it may not be possible to remove the
      underlying message while the returned file remains open.


   .. method:: flush()

      All changes to MH mailboxes are immediately applied, so this method does
      nothing.


   .. method:: close()

      :class:`!MH` instances do not keep any open files, so this method is
      equivalent to :meth:`unlock`.


.. seealso::

   `nmh - Message Handling System <https://www.nongnu.org/nmh/>`_
      Home page of :program:`nmh`, an updated version of the original :program:`mh`.

   `MH & nmh: Email for Users & Programmers <https://rand-mh.sourceforge.io/book/>`_
      A GPL-licensed book on :program:`mh` and :program:`nmh`, with some information
      on the mailbox format.


.. _mailbox-babyl:

:class:`!Babyl` objects
^^^^^^^^^^^^^^^^^^^^^^^


.. class:: Babyl(path, factory=None, create=True)

   A subclass of :class:`Mailbox` for mailboxes in Babyl format. Parameter
   *factory* is a callable object that accepts a file-like message representation
   (which behaves as if opened in binary mode) and returns a custom representation.
   If *factory* is ``None``, :class:`BabylMessage` is used as the default message
   representation. If *create* is ``True``, the mailbox is created if it does not
   exist.

   Babyl is a single-file mailbox format used by the Rmail mail user agent
   included with Emacs. The beginning of a message is indicated by a line
   containing the two characters Control-Underscore (``'\037'``) and Control-L
   (``'\014'``). The end of a message is indicated by the start of the next
   message or, in the case of the last message, a line containing a
   Control-Underscore (``'\037'``) character.

   Messages in a Babyl mailbox have two sets of headers, original headers and
   so-called visible headers. Visible headers are typically a subset of the
   original headers that have been reformatted or abridged to be more
   attractive. Each message in a Babyl mailbox also has an accompanying list of
   :dfn:`labels`, or short strings that record extra information about the
   message, and a list of all user-defined labels found in the mailbox is kept
   in the Babyl options section.

   :class:`!Babyl` instances have all of the methods of :class:`Mailbox` in
   addition to the following:


   .. method:: get_labels()

      Return a list of the names of all user-defined labels used in the mailbox.

      .. note::

         The actual messages are inspected to determine which labels exist in
         the mailbox rather than consulting the list of labels in the Babyl
         options section, but the Babyl section is updated whenever the mailbox
         is modified.

   Some :class:`Mailbox` methods implemented by :class:`!Babyl` deserve special
   remarks:


   .. method:: get_file(key)

      In Babyl mailboxes, the headers of a message are not stored contiguously
      with the body of the message. To generate a file-like representation, the
      headers and body are copied together into an :class:`io.BytesIO` instance,
      which has an API identical to that of a
      file. As a result, the file-like object is truly independent of the
      underlying mailbox but does not save memory compared to a string
      representation.


   .. method:: lock()
               unlock()

      Three locking mechanisms are used---dot locking and, if available, the
      :c:func:`!flock` and :c:func:`!lockf` system calls.


.. seealso::

   `Format of Version 5 Babyl Files <https://quimby.gnus.org/notes/BABYL>`_
      A specification of the Babyl format.

   `Reading Mail with Rmail <https://www.gnu.org/software/emacs/manual/html_node/emacs/Rmail.html>`_
      The Rmail manual, with some information on Babyl semantics.


.. _mailbox-mmdf:

:class:`!MMDF` objects
^^^^^^^^^^^^^^^^^^^^^^


.. class:: MMDF(path, factory=None, create=True)

   A subclass of :class:`Mailbox` for mailboxes in MMDF format. Parameter *factory*
   is a callable object that accepts a file-like message representation (which
   behaves as if opened in binary mode) and returns a custom representation. If
   *factory* is ``None``, :class:`MMDFMessage` is used as the default message
   representation. If *create* is ``True``, the mailbox is created if it does not
   exist.

   MMDF is a single-file mailbox format invented for the Multichannel Memorandum
   Distribution Facility, a mail transfer agent. Each message is in the same
   form as an mbox message but is bracketed before and after by lines containing
   four Control-A (``'\001'``) characters. As with the mbox format, the
   beginning of each message is indicated by a line whose first five characters
   are "From ", but additional occurrences of "From " are not transformed to
   ">From " when storing messages because the extra message separator lines
   prevent mistaking such occurrences for the starts of subsequent messages.

   Some :class:`Mailbox` methods implemented by :class:`!MMDF` deserve special
   remarks:


   .. method:: get_file(key)

      Using the file after calling :meth:`~Mailbox.flush` or
      :meth:`~Mailbox.close` on the :class:`!MMDF` instance may yield
      unpredictable results or raise an exception.


   .. method:: lock()
               unlock()

      Three locking mechanisms are used---dot locking and, if available, the
      :c:func:`!flock` and :c:func:`!lockf` system calls.


.. seealso::

   `mmdf man page from tin <http://www.tin.org/bin/man.cgi?section=5&topic=mmdf>`_
      A specification of MMDF format from the documentation of tin, a newsreader.

   `MMDF <https://en.wikipedia.org/wiki/MMDF>`_
      A Wikipedia article describing the Multichannel Memorandum Distribution
      Facility.


.. _mailbox-message-objects:

:class:`!Message` objects
-------------------------


.. class:: Message(message=None)

   A subclass of the :mod:`email.message` module's
   :class:`~email.message.Message`. Subclasses of :class:`!mailbox.Message` add
   mailbox-format-specific state and behavior.

   If *message* is omitted, the new instance is created in a default, empty state.
   If *message* is an :class:`email.message.Message` instance, its contents are
   copied; furthermore, any format-specific information is converted insofar as
   possible if *message* is a :class:`!Message` instance. If *message* is a string,
   a byte string,
   or a file, it should contain an :rfc:`2822`\ -compliant message, which is read
   and parsed.  Files should be open in binary mode, but text mode files
   are accepted for backward compatibility.

   The format-specific state and behaviors offered by subclasses vary, but in
   general it is only the properties that are not specific to a particular
   mailbox that are supported (although presumably the properties are specific
   to a particular mailbox format). For example, file offsets for single-file
   mailbox formats and file names for directory-based mailbox formats are not
   retained, because they are only applicable to the original mailbox. But state
   such as whether a message has been read by the user or marked as important is
   retained, because it applies to the message itself.

   There is no requirement that :class:`!Message` instances be used to represent
   messages retrieved using :class:`Mailbox` instances. In some situations, the
   time and memory required to generate :class:`!Message` representations might
   not be acceptable. For such situations, :class:`!Mailbox` instances also
   offer string and file-like representations, and a custom message factory may
   be specified when a :class:`!Mailbox` instance is initialized.


.. _mailbox-maildirmessage:

:class:`!MaildirMessage` objects
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


.. class:: MaildirMessage(message=None)

   A message with Maildir-specific behaviors. Parameter *message* has the same
   meaning as with the :class:`Message` constructor.

   Typically, a mail user agent application moves all of the messages in the
   :file:`new` subdirectory to the :file:`cur` subdirectory after the first time
   the user opens and closes the mailbox, recording that the messages are old
   whether or not they've actually been read. Each message in :file:`cur` has an
   "info" section added to its file name to store information about its state.
   (Some mail readers may also add an "info" section to messages in
   :file:`new`.)  The "info" section may take one of two forms: it may contain
   "2," followed by a list of standardized flags (e.g., "2,FR") or it may
   contain "1," followed by so-called experimental information. Standard flags
   for Maildir messages are as follows:

   +------+---------+--------------------------------+
   | Flag | Meaning | Explanation                    |
   +======+=========+================================+
   | D    | Draft   | Under composition              |
   +------+---------+--------------------------------+
   | F    | Flagged | Marked as important            |
   +------+---------+--------------------------------+
   | P    | Passed  | Forwarded, resent, or bounced  |
   +------+---------+--------------------------------+
   | R    | Replied | Replied to                     |
   +------+---------+--------------------------------+
   | S    | Seen    | Read                           |
   +------+---------+--------------------------------+
   | T    | Trashed | Marked for subsequent deletion |
   +------+---------+--------------------------------+

   :class:`!MaildirMessage` instances offer the following methods:


   .. method:: get_subdir()

      Return either "new" (if the message should be stored in the :file:`new`
      subdirectory) or "cur" (if the message should be stored in the :file:`cur`
      subdirectory).

      .. note::

         A message is typically moved from :file:`new` to :file:`cur` after its
         mailbox has been accessed, whether or not the message has been
         read. A message ``msg`` has been read if ``"S" in msg.get_flags()`` is
         ``True``.


   .. method:: set_subdir(subdir)

      Set the subdirectory the message should be stored in. Parameter *subdir*
      must be either "new" or "cur".


   .. method:: get_flags()

      Return a string specifying the flags that are currently set. If the
      message complies with the standard Maildir format, the result is the
      concatenation in alphabetical order of zero or one occurrence of each of
      ``'D'``, ``'F'``, ``'P'``, ``'R'``, ``'S'``, and ``'T'``. The empty string
      is returned if no flags are set or if "info" contains experimental
      semantics.


   .. method:: set_flags(flags)

      Set the flags specified by *flags* and unset all others.


   .. method:: add_flag(flag)

      Set the flag(s) specified by *flag* without changing other flags. To add
      more than one flag at a time, *flag* may be a string of more than one
      character. The current "info" is overwritten whether or not it contains
      experimental information rather than flags.


   .. method:: remove_flag(flag)

      Unset the flag(s) specified by *flag* without changing other flags. To
      remove more than one flag at a time, *flag* maybe a string of more than
      one character.  If "info" contains experimental information rather than
      flags, the current "info" is not modified.


   .. method:: get_date()

      Return the delivery date of the message as a floating-point number
      representing seconds since the epoch.


   .. method:: set_date(date)

      Set the delivery date of the message to *date*, a floating-point number
      representing seconds since the epoch.


   .. method:: get_info()

      Return a string containing the "info" for a message. This is useful for
      accessing and modifying "info" that is experimental (i.e., not a list of
      flags).


   .. method:: set_info(info)

      Set "info" to *info*, which should be a string.

When a :class:`!MaildirMessage` instance is created based upon an
:class:`mboxMessage` or :class:`MMDFMessage` instance, the :mailheader:`Status`
and :mailheader:`X-Status` headers are omitted and the following conversions
take place:

+--------------------+----------------------------------------------+
| Resulting state    | :class:`mboxMessage` or :class:`MMDFMessage` |
|                    | state                                        |
+====================+==============================================+
| "cur" subdirectory | O flag                                       |
+--------------------+----------------------------------------------+
| F flag             | F flag                                       |
+--------------------+----------------------------------------------+
| R flag             | A flag                                       |
+--------------------+----------------------------------------------+
| S flag             | R flag                                       |
+--------------------+----------------------------------------------+
| T flag             | D flag                                       |
+--------------------+----------------------------------------------+

When a :class:`!MaildirMessage` instance is created based upon an
:class:`MHMessage` instance, the following conversions take place:

+-------------------------------+--------------------------+
| Resulting state               | :class:`MHMessage` state |
+===============================+==========================+
| "cur" subdirectory            | "unseen" sequence        |
+-------------------------------+--------------------------+
| "cur" subdirectory and S flag | no "unseen" sequence     |
+-------------------------------+--------------------------+
| F flag                        | "flagged" sequence       |
+-------------------------------+--------------------------+
| R flag                        | "replied" sequence       |
+-------------------------------+--------------------------+

When a :class:`!MaildirMessage` instance is created based upon a
:class:`BabylMessage` instance, the following conversions take place:

+-------------------------------+-------------------------------+
| Resulting state               | :class:`BabylMessage` state   |
+===============================+===============================+
| "cur" subdirectory            | "unseen" label                |
+-------------------------------+-------------------------------+
| "cur" subdirectory and S flag | no "unseen" label             |
+-------------------------------+-------------------------------+
| P flag                        | "forwarded" or "resent" label |
+-------------------------------+-------------------------------+
| R flag                        | "answered" label              |
+-------------------------------+-------------------------------+
| T flag                        | "deleted" label               |
+-------------------------------+-------------------------------+


.. _mailbox-mboxmessage:

:class:`!mboxMessage` objects
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


.. class:: mboxMessage(message=None)

   A message with mbox-specific behaviors. Parameter *message* has the same meaning
   as with the :class:`Message` constructor.

   Messages in an mbox mailbox are stored together in a single file. The
   sender's envelope address and the time of delivery are typically stored in a
   line beginning with "From " that is used to indicate the start of a message,
   though there is considerable variation in the exact format of this data among
   mbox implementations. Flags that indicate the state of the message, such as
   whether it has been read or marked as important, are typically stored in
   :mailheader:`Status` and :mailheader:`X-Status` headers.

   Conventional flags for mbox messages are as follows:

   +------+----------+--------------------------------+
   | Flag | Meaning  | Explanation                    |
   +======+==========+================================+
   | R    | Read     | Read                           |
   +------+----------+--------------------------------+
   | O    | Old      | Previously detected by MUA     |
   +------+----------+--------------------------------+
   | D    | Deleted  | Marked for subsequent deletion |
   +------+----------+--------------------------------+
   | F    | Flagged  | Marked as important            |
   +------+----------+--------------------------------+
   | A    | Answered | Replied to                     |
   +------+----------+--------------------------------+

   The "R" and "O" flags are stored in the :mailheader:`Status` header, and the
   "D", "F", and "A" flags are stored in the :mailheader:`X-Status` header. The
   flags and headers typically appear in the order mentioned.

   :class:`!mboxMessage` instances offer the following methods:


   .. method:: get_from()

      Return a string representing the "From " line that marks the start of the
      message in an mbox mailbox. The leading "From " and the trailing newline
      are excluded.


   .. method:: set_from(from_, time_=None)

      Set the "From " line to *from_*, which should be specified without a
      leading "From " or trailing newline. For convenience, *time_* may be
      specified and will be formatted appropriately and appended to *from_*. If
      *time_* is specified, it should be a :class:`time.struct_time` instance, a
      tuple suitable for passing to :func:`time.strftime`, or ``True`` (to use
      :func:`time.gmtime`).


   .. method:: get_flags()

      Return a string specifying the flags that are currently set. If the
      message complies with the conventional format, the result is the
      concatenation in the following order of zero or one occurrence of each of
      ``'R'``, ``'O'``, ``'D'``, ``'F'``, and ``'A'``.


   .. method:: set_flags(flags)

      Set the flags specified by *flags* and unset all others. Parameter *flags*
      should be the concatenation in any order of zero or more occurrences of
      each of ``'R'``, ``'O'``, ``'D'``, ``'F'``, and ``'A'``.


   .. method:: add_flag(flag)

      Set the flag(s) specified by *flag* without changing other flags. To add
      more than one flag at a time, *flag* may be a string of more than one
      character.


   .. method:: remove_flag(flag)

      Unset the flag(s) specified by *flag* without changing other flags. To
      remove more than one flag at a time, *flag* maybe a string of more than
      one character.

When an :class:`!mboxMessage` instance is created based upon a
:class:`MaildirMessage` instance, a "From " line is generated based upon the
:class:`MaildirMessage` instance's delivery date, and the following conversions
take place:

+-----------------+-------------------------------+
| Resulting state | :class:`MaildirMessage` state |
+=================+===============================+
| R flag          | S flag                        |
+-----------------+-------------------------------+
| O flag          | "cur" subdirectory            |
+-----------------+-------------------------------+
| D flag          | T flag                        |
+-----------------+-------------------------------+
| F flag          | F flag                        |
+-----------------+-------------------------------+
| A flag          | R flag                        |
+-----------------+-------------------------------+

When an :class:`!mboxMessage` instance is created based upon an
:class:`MHMessage` instance, the following conversions take place:

+-------------------+--------------------------+
| Resulting state   | :class:`MHMessage` state |
+===================+==========================+
| R flag and O flag | no "unseen" sequence     |
+-------------------+--------------------------+
| O flag            | "unseen" sequence        |
+-------------------+--------------------------+
| F flag            | "flagged" sequence       |
+-------------------+--------------------------+
| A flag            | "replied" sequence       |
+-------------------+--------------------------+

When an :class:`!mboxMessage` instance is created based upon a
:class:`BabylMessage` instance, the following conversions take place:

+-------------------+-----------------------------+
| Resulting state   | :class:`BabylMessage` state |
+===================+=============================+
| R flag and O flag | no "unseen" label           |
+-------------------+-----------------------------+
| O flag            | "unseen" label              |
+-------------------+-----------------------------+
| D flag            | "deleted" label             |
+-------------------+-----------------------------+
| A flag            | "answered" label            |
+-------------------+-----------------------------+

When a :class:`!mboxMessage` instance is created based upon an
:class:`MMDFMessage`
instance, the "From " line is copied and all flags directly correspond:

+-----------------+----------------------------+
| Resulting state | :class:`MMDFMessage` state |
+=================+============================+
| R flag          | R flag                     |
+-----------------+----------------------------+
| O flag          | O flag                     |
+-----------------+----------------------------+
| D flag          | D flag                     |
+-----------------+----------------------------+
| F flag          | F flag                     |
+-----------------+----------------------------+
| A flag          | A flag                     |
+-----------------+----------------------------+


.. _mailbox-mhmessage:

:class:`!MHMessage` objects
^^^^^^^^^^^^^^^^^^^^^^^^^^^


.. class:: MHMessage(message=None)

   A message with MH-specific behaviors. Parameter *message* has the same meaning
   as with the :class:`Message` constructor.

   MH messages do not support marks or flags in the traditional sense, but they
   do support sequences, which are logical groupings of arbitrary messages. Some
   mail reading programs (although not the standard :program:`mh` and
   :program:`nmh`) use sequences in much the same way flags are used with other
   formats, as follows:

   +----------+------------------------------------------+
   | Sequence | Explanation                              |
   +==========+==========================================+
   | unseen   | Not read, but previously detected by MUA |
   +----------+------------------------------------------+
   | replied  | Replied to                               |
   +----------+------------------------------------------+
   | flagged  | Marked as important                      |
   +----------+------------------------------------------+

   :class:`!MHMessage` instances offer the following methods:


   .. method:: get_sequences()

      Return a list of the names of sequences that include this message.


   .. method:: set_sequences(sequences)

      Set the list of sequences that include this message.


   .. method:: add_sequence(sequence)

      Add *sequence* to the list of sequences that include this message.


   .. method:: remove_sequence(sequence)

      Remove *sequence* from the list of sequences that include this message.

When an :class:`!MHMessage` instance is created based upon a
:class:`MaildirMessage` instance, the following conversions take place:

+--------------------+-------------------------------+
| Resulting state    | :class:`MaildirMessage` state |
+====================+===============================+
| "unseen" sequence  | no S flag                     |
+--------------------+-------------------------------+
| "replied" sequence | R flag                        |
+--------------------+-------------------------------+
| "flagged" sequence | F flag                        |
+--------------------+-------------------------------+

When an :class:`!MHMessage` instance is created based upon an
:class:`mboxMessage` or :class:`MMDFMessage` instance, the :mailheader:`Status`
and :mailheader:`X-Status` headers are omitted and the following conversions
take place:

+--------------------+----------------------------------------------+
| Resulting state    | :class:`mboxMessage` or :class:`MMDFMessage` |
|                    | state                                        |
+====================+==============================================+
| "unseen" sequence  | no R flag                                    |
+--------------------+----------------------------------------------+
| "replied" sequence | A flag                                       |
+--------------------+----------------------------------------------+
| "flagged" sequence | F flag                                       |
+--------------------+----------------------------------------------+

When an :class:`!MHMessage` instance is created based upon a
:class:`BabylMessage` instance, the following conversions take place:

+--------------------+-----------------------------+
| Resulting state    | :class:`BabylMessage` state |
+====================+=============================+
| "unseen" sequence  | "unseen" label              |
+--------------------+-----------------------------+
| "replied" sequence | "answered" label            |
+--------------------+-----------------------------+


.. _mailbox-babylmessage:

:class:`!BabylMessage` objects
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


.. class:: BabylMessage(message=None)

   A message with Babyl-specific behaviors. Parameter *message* has the same
   meaning as with the :class:`Message` constructor.

   Certain message labels, called :dfn:`attributes`, are defined by convention
   to have special meanings. The attributes are as follows:

   +-----------+------------------------------------------+
   | Label     | Explanation                              |
   +===========+==========================================+
   | unseen    | Not read, but previously detected by MUA |
   +-----------+------------------------------------------+
   | deleted   | Marked for subsequent deletion           |
   +-----------+------------------------------------------+
   | filed     | Copied to another file or mailbox        |
   +-----------+------------------------------------------+
   | answered  | Replied to                               |
   +-----------+------------------------------------------+
   | forwarded | Forwarded                                |
   +-----------+------------------------------------------+
   | edited    | Modified by the user                     |
   +-----------+------------------------------------------+
   | resent    | Resent                                   |
   +-----------+------------------------------------------+

   By default, Rmail displays only visible headers. The :class:`!BabylMessage`
   class, though, uses the original headers because they are more
   complete. Visible headers may be accessed explicitly if desired.

   :class:`!BabylMessage` instances offer the following methods:


   .. method:: get_labels()

      Return a list of labels on the message.


   .. method:: set_labels(labels)

      Set the list of labels on the message to *labels*.


   .. method:: add_label(label)

      Add *label* to the list of labels on the message.


   .. method:: remove_label(label)

      Remove *label* from the list of labels on the message.


   .. method:: get_visible()

      Return an :class:`Message` instance whose headers are the message's
      visible headers and whose body is empty.


   .. method:: set_visible(visible)

      Set the message's visible headers to be the same as the headers in
      *message*.  Parameter *visible* should be a :class:`Message` instance, an
      :class:`email.message.Message` instance, a string, or a file-like object
      (which should be open in text mode).


   .. method:: update_visible()

      When a :class:`!BabylMessage` instance's original headers are modified, the
      visible headers are not automatically modified to correspond. This method
      updates the visible headers as follows: each visible header with a
      corresponding original header is set to the value of the original header,
      each visible header without a corresponding original header is removed,
      and any of :mailheader:`Date`, :mailheader:`From`, :mailheader:`Reply-To`,
      :mailheader:`To`, :mailheader:`CC`, and :mailheader:`Subject` that are
      present in the original headers but not the visible headers are added to
      the visible headers.

When a :class:`!BabylMessage` instance is created based upon a
:class:`MaildirMessage` instance, the following conversions take place:

+-------------------+-------------------------------+
| Resulting state   | :class:`MaildirMessage` state |
+===================+===============================+
| "unseen" label    | no S flag                     |
+-------------------+-------------------------------+
| "deleted" label   | T flag                        |
+-------------------+-------------------------------+
| "answered" label  | R flag                        |
+-------------------+-------------------------------+
| "forwarded" label | P flag                        |
+-------------------+-------------------------------+

When a :class:`!BabylMessage` instance is created based upon an
:class:`mboxMessage` or :class:`MMDFMessage` instance, the :mailheader:`Status`
and :mailheader:`X-Status` headers are omitted and the following conversions
take place:

+------------------+----------------------------------------------+
| Resulting state  | :class:`mboxMessage` or :class:`MMDFMessage` |
|                  | state                                        |
+==================+==============================================+
| "unseen" label   | no R flag                                    |
+------------------+----------------------------------------------+
| "deleted" label  | D flag                                       |
+------------------+----------------------------------------------+
| "answered" label | A flag                                       |
+------------------+----------------------------------------------+

When a :class:`!BabylMessage` instance is created based upon an
:class:`MHMessage` instance, the following conversions take place:

+------------------+--------------------------+
| Resulting state  | :class:`MHMessage` state |
+==================+==========================+
| "unseen" label   | "unseen" sequence        |
+------------------+--------------------------+
| "answered" label | "replied" sequence       |
+------------------+--------------------------+


.. _mailbox-mmdfmessage:

:class:`!MMDFMessage` objects
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


.. class:: MMDFMessage(message=None)

   A message with MMDF-specific behaviors. Parameter *message* has the same meaning
   as with the :class:`Message` constructor.

   As with message in an mbox mailbox, MMDF messages are stored with the
   sender's address and the delivery date in an initial line beginning with
   "From ".  Likewise, flags that indicate the state of the message are
   typically stored in :mailheader:`Status` and :mailheader:`X-Status` headers.

   Conventional flags for MMDF messages are identical to those of mbox message
   and are as follows:

   +------+----------+--------------------------------+
   | Flag | Meaning  | Explanation                    |
   +======+==========+================================+
   | R    | Read     | Read                           |
   +------+----------+--------------------------------+
   | O    | Old      | Previously detected by MUA     |
   +------+----------+--------------------------------+
   | D    | Deleted  | Marked for subsequent deletion |
   +------+----------+--------------------------------+
   | F    | Flagged  | Marked as important            |
   +------+----------+--------------------------------+
   | A    | Answered | Replied to                     |
   +------+----------+--------------------------------+

   The "R" and "O" flags are stored in the :mailheader:`Status` header, and the
   "D", "F", and "A" flags are stored in the :mailheader:`X-Status` header. The
   flags and headers typically appear in the order mentioned.

   :class:`!MMDFMessage` instances offer the following methods, which are
   identical to those offered by :class:`mboxMessage`:


   .. method:: get_from()

      Return a string representing the "From " line that marks the start of the
      message in an mbox mailbox. The leading "From " and the trailing newline
      are excluded.


   .. method:: set_from(from_, time_=None)

      Set the "From " line to *from_*, which should be specified without a
      leading "From " or trailing newline. For convenience, *time_* may be
      specified and will be formatted appropriately and appended to *from_*. If
      *time_* is specified, it should be a :class:`time.struct_time` instance, a
      tuple suitable for passing to :func:`time.strftime`, or ``True`` (to use
      :func:`time.gmtime`).


   .. method:: get_flags()

      Return a string specifying the flags that are currently set. If the
      message complies with the conventional format, the result is the
      concatenation in the following order of zero or one occurrence of each of
      ``'R'``, ``'O'``, ``'D'``, ``'F'``, and ``'A'``.


   .. method:: set_flags(flags)

      Set the flags specified by *flags* and unset all others. Parameter *flags*
      should be the concatenation in any order of zero or more occurrences of
      each of ``'R'``, ``'O'``, ``'D'``, ``'F'``, and ``'A'``.


   .. method:: add_flag(flag)

      Set the flag(s) specified by *flag* without changing other flags. To add
      more than one flag at a time, *flag* may be a string of more than one
      character.


   .. method:: remove_flag(flag)

      Unset the flag(s) specified by *flag* without changing other flags. To
      remove more than one flag at a time, *flag* maybe a string of more than
      one character.

When an :class:`!MMDFMessage` instance is created based upon a
:class:`MaildirMessage` instance, a "From " line is generated based upon the
:class:`MaildirMessage` instance's delivery date, and the following conversions
take place:

+-----------------+-------------------------------+
| Resulting state | :class:`MaildirMessage` state |
+=================+===============================+
| R flag          | S flag                        |
+-----------------+-------------------------------+
| O flag          | "cur" subdirectory            |
+-----------------+-------------------------------+
| D flag          | T flag                        |
+-----------------+-------------------------------+
| F flag          | F flag                        |
+-----------------+-------------------------------+
| A flag          | R flag                        |
+-----------------+-------------------------------+

When an :class:`!MMDFMessage` instance is created based upon an
:class:`MHMessage` instance, the following conversions take place:

+-------------------+--------------------------+
| Resulting state   | :class:`MHMessage` state |
+===================+==========================+
| R flag and O flag | no "unseen" sequence     |
+-------------------+--------------------------+
| O flag            | "unseen" sequence        |
+-------------------+--------------------------+
| F flag            | "flagged" sequence       |
+-------------------+--------------------------+
| A flag            | "replied" sequence       |
+-------------------+--------------------------+

When an :class:`!MMDFMessage` instance is created based upon a
:class:`BabylMessage` instance, the following conversions take place:

+-------------------+-----------------------------+
| Resulting state   | :class:`BabylMessage` state |
+===================+=============================+
| R flag and O flag | no "unseen" label           |
+-------------------+-----------------------------+
| O flag            | "unseen" label              |
+-------------------+-----------------------------+
| D flag            | "deleted" label             |
+-------------------+-----------------------------+
| A flag            | "answered" label            |
+-------------------+-----------------------------+

When an :class:`!MMDFMessage` instance is created based upon an
:class:`mboxMessage` instance, the "From " line is copied and all flags directly
correspond:

+-----------------+----------------------------+
| Resulting state | :class:`mboxMessage` state |
+=================+============================+
| R flag          | R flag                     |
+-----------------+----------------------------+
| O flag          | O flag                     |
+-----------------+----------------------------+
| D flag          | D flag                     |
+-----------------+----------------------------+
| F flag          | F flag                     |
+-----------------+----------------------------+
| A flag          | A flag                     |
+-----------------+----------------------------+


Exceptions
----------

The following exception classes are defined in the :mod:`!mailbox` module:


.. exception:: Error()

   The based class for all other module-specific exceptions.


.. exception:: NoSuchMailboxError()

   Raised when a mailbox is expected but is not found, such as when instantiating a
   :class:`Mailbox` subclass with a path that does not exist (and with the *create*
   parameter set to ``False``), or when opening a folder that does not exist.


.. exception:: NotEmptyError()

   Raised when a mailbox is not empty but is expected to be, such as when deleting
   a folder that contains messages.


.. exception:: ExternalClashError()

   Raised when some mailbox-related condition beyond the control of the program
   causes it to be unable to proceed, such as when failing to acquire a lock that
   another program already holds a lock, or when a uniquely generated file name
   already exists.


.. exception:: FormatError()

   Raised when the data in a file cannot be parsed, such as when an :class:`MH`
   instance attempts to read a corrupted :file:`.mh_sequences` file.


.. _mailbox-examples:

Examples
--------

A simple example of printing the subjects of all messages in a mailbox that seem
interesting::

   import mailbox
   for message in mailbox.mbox('~/mbox'):
       subject = message['subject']       # Could possibly be None.
       if subject and 'python' in subject.lower():
           print(subject)

To copy all mail from a Babyl mailbox to an MH mailbox, converting all of the
format-specific information that can be converted::

   import mailbox
   destination = mailbox.MH('~/Mail')
   destination.lock()
   for message in mailbox.Babyl('~/RMAIL'):
       destination.add(mailbox.MHMessage(message))
   destination.flush()
   destination.unlock()

This example sorts mail from several mailing lists into different mailboxes,
being careful to avoid mail corruption due to concurrent modification by other
programs, mail loss due to interruption of the program, or premature termination
due to malformed messages in the mailbox::

   import mailbox
   import email.errors

   list_names = ('python-list', 'python-dev', 'python-bugs')

   boxes = {name: mailbox.mbox('~/email/%s' % name) for name in list_names}
   inbox = mailbox.Maildir('~/Maildir', factory=None)

   for key in inbox.iterkeys():
       try:
           message = inbox[key]
       except email.errors.MessageParseError:
           continue                # The message is malformed. Just leave it.

       for name in list_names:
           list_id = message['list-id']
           if list_id and name in list_id:
               # Get mailbox to use
               box = boxes[name]

               # Write copy to disk before removing original.
               # If there's a crash, you might duplicate a message, but
               # that's better than losing a message completely.
               box.lock()
               box.add(message)
               box.flush()
               box.unlock()

               # Remove original message
               inbox.lock()
               inbox.discard(key)
               inbox.flush()
               inbox.unlock()
               break               # Found destination, so stop looking.

   for box in boxes.itervalues():
       box.close()