Issue 2188: Documentation hint about disabling proxy detection.

[python.git] / Doc / library / mailbox.rst

: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>


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:`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` iterator iterates over message representations, not
keys as the default dictionary 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
   Maildir; try to avoid using single-file formats such as 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:: Mailbox.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, or a file-like object (which
   should be open in text 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.


.. method:: Mailbox.remove(key)
            Mailbox.__delitem__(key)
            Mailbox.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:: Mailbox.__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, or a file-like object (which
   should be open in text 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:: Mailbox.iterkeys()
            Mailbox.keys()

   Return an iterator over all keys if called as :meth:`iterkeys` or return a list
   of keys if called as :meth:`keys`.


.. method:: Mailbox.itervalues()
            Mailbox.__iter__()
            Mailbox.values()

   Return an iterator over representations of all messages if called as
   :meth:`itervalues` or :meth:`__iter__` or return a list of such representations
   if called as :meth:`values`. 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:: Mailbox.iteritems()
            Mailbox.items()

   Return an iterator over (*key*, *message*) pairs, where *key* is a key and
   *message* is a message representation, if called as :meth:`iteritems` or return
   a list of such pairs if called as :meth:`items`. 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:: Mailbox.get(key[, default=None])
            Mailbox.__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:: Mailbox.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:: Mailbox.get_string(key)

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


.. method:: Mailbox.get_file(key)

   Return a file-like 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.

   .. note::

      Unlike other representations of messages, file-like 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:: Mailbox.has_key(key)
            Mailbox.__contains__(key)

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


.. method:: Mailbox.__len__()

   Return a count of messages in the mailbox.


.. method:: Mailbox.clear()

   Delete all messages from the mailbox.


.. method:: Mailbox.pop(key[, default])

   Return a representation of the message corresponding to *key* and delete the
   message. If no such message exists, return *default* if it was supplied or else
   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:: Mailbox.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:: Mailbox.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:: Mailbox.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:: Mailbox.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:: Mailbox.unlock()

   Release the lock on the mailbox, if any.


.. method:: Mailbox.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`
^^^^^^^^^^^^^^^^


.. class:: Maildir(dirname[, factory=rfc822.Message[, 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.

   It is for historical reasons that *factory* defaults to :class:`rfc822.Message`
   and that *dirname* is named as such rather than *path*. For a :class:`Maildir`
   instance that behaves like instances of other :class:`Mailbox` subclasses, set
   *factory* to ``None``.

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".

.. note::

   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.

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


.. method:: Maildir.list_folders()

   Return a list of the names of all folders.


.. method:: Maildir.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:: Maildir.add_folder(folder)

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


.. method:: Maildir.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:: Maildir.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.

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


.. method:: Maildir.add(message)
            Maildir.__setitem__(key, message)
            Maildir.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:: Maildir.flush()

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


.. method:: Maildir.lock()
            Maildir.unlock()

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


.. method:: Maildir.close()

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


.. method:: Maildir.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 qmail <http://www.qmail.org/man/man5/maildir.html>`_
      The original specification of the format.

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

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


.. _mailbox-mbox:

:class:`mbox`
^^^^^^^^^^^^^


.. 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:: mbox.get_file(key)

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


.. method:: mbox.lock()
            mbox.unlock()

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


.. seealso::

   `mbox man page from qmail <http://www.qmail.org/man/man5/mbox.html>`_
      A specification of the format and its variations.

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

   `Configuring Netscape Mail on Unix: Why The Content-Length Format is Bad <http://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 <http://homepages.tesco.net./~J.deBoynePollard/FGA/mail-mbox-formats.html>`_
      A history of mbox variations.


.. _mailbox-mh:

:class:`MH`
^^^^^^^^^^^


.. 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:


.. method:: MH.list_folders()

   Return a list of the names of all folders.


.. method:: MH.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:: MH.add_folder(folder)

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


.. method:: MH.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:: MH.get_sequences()

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


.. method:: MH.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:: MH.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:: MH.remove(key)
            MH.__delitem__(key)
            MH.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:: MH.lock()
            MH.unlock()

   Three locking mechanisms are used---dot locking and, if available, the
   :cfunc:`flock` and :cfunc:`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:: MH.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:: MH.flush()

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


.. method:: MH.close()

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


.. seealso::

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

   `MH & nmh: Email for Users & Programmers <http://www.ics.uci.edu/~mh/book/>`_
      A GPL-licensed book on :program:`mh` and :program:`nmh`, with some information
      on the mailbox format.


.. _mailbox-babyl:

:class:`Babyl`
^^^^^^^^^^^^^^


.. 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:: Babyl.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:: Babyl.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 a :class:`StringIO` instance (from the
   :mod:`StringIO` module), 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:: Babyl.lock()
            Babyl.unlock()

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


.. seealso::

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

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


.. _mailbox-mmdf:

:class:`MMDF`
^^^^^^^^^^^^^


.. 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:: MMDF.get_file(key)

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


.. method:: MMDF.lock()
            MMDF.unlock()

   Three locking mechanisms are used---dot locking and, if available, the
   :cfunc:`flock` and :cfunc:`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 <http://en.wikipedia.org/wiki/MMDF>`_
      A Wikipedia article describing the Multichannel Memorandum Distribution
      Facility.


.. _mailbox-message-objects:

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


.. class:: Message([message])

   A subclass of the :mod:`email.Message` module's :class:`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
   or a file, it should contain an :rfc:`2822`\ -compliant message, which is read
   and parsed.

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
not 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`
^^^^^^^^^^^^^^^^^^^^^^^


.. class:: MaildirMessage([message])

   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:: MaildirMessage.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 is has been read. A message
      ``msg`` has been read if ``"S" in msg.get_flags()`` is ``True``.


.. method:: MaildirMessage.set_subdir(subdir)

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


.. method:: MaildirMessage.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:: MaildirMessage.set_flags(flags)

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


.. method:: MaildirMessage.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:: MaildirMessage.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:: MaildirMessage.get_date()

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


.. method:: MaildirMessage.set_date(date)

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


.. method:: MaildirMessage.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:: MaildirMessage.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`
^^^^^^^^^^^^^^^^^^^^


.. class:: mboxMessage([message])

   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:: mboxMessage.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:: mboxMessage.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:`struct_time` instance, a tuple suitable for passing to
   :meth:`time.strftime`, or ``True`` (to use :meth:`time.gmtime`).


.. method:: mboxMessage.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:: mboxMessage.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:: mboxMessage.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:: mboxMessage.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:`Message` 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`
^^^^^^^^^^^^^^^^^^


.. class:: MHMessage([message])

   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:: MHMessage.get_sequences()

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


.. method:: MHMessage.set_sequences(sequences)

   Set the list of sequences that include this message.


.. method:: MHMessage.add_sequence(sequence)

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


.. method:: MHMessage.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`
^^^^^^^^^^^^^^^^^^^^^


.. class:: BabylMessage([message])

   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:: BabylMessage.get_labels()

   Return a list of labels on the message.


.. method:: BabylMessage.set_labels(labels)

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


.. method:: BabylMessage.add_label(label)

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


.. method:: BabylMessage.remove_label(label)

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


.. method:: BabylMessage.get_visible()

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


.. method:: BabylMessage.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:: BabylMessage.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`
^^^^^^^^^^^^^^^^^^^^


.. class:: MMDFMessage([message])

   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:: MMDFMessage.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:: MMDFMessage.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:`struct_time` instance, a tuple suitable for passing to
   :meth:`time.strftime`, or ``True`` (to use :meth:`time.gmtime`).


.. method:: MMDFMessage.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:: MMDFMessage.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:: MMDFMessage.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:: MMDFMessage.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:


.. class:: Error()

   The based class for all other module-specific exceptions.


.. class:: 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.


.. class:: NotEmptyErrorError()

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


.. class:: 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.


.. class:: 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-deprecated:

Deprecated classes and methods
------------------------------

Older versions of the :mod:`mailbox` module do not support modification of
mailboxes, such as adding or removing message, and do not provide classes to
represent format-specific message properties. For backward compatibility, the
older mailbox classes are still available, but the newer classes should be used
in preference to them.

Older mailbox objects support only iteration and provide a single public method:


.. method:: oldmailbox.next()

   Return the next message in the mailbox, created with the optional *factory*
   argument passed into the mailbox object's constructor. By default this is an
   :class:`rfc822.Message` object (see the :mod:`rfc822` module).  Depending on the
   mailbox implementation the *fp* attribute of this object may be a true file
   object or a class instance simulating a file object, taking care of things like
   message boundaries if multiple mail messages are contained in a single file,
   etc.  If no more messages are available, this method returns ``None``.

Most of the older mailbox classes have names that differ from the current
mailbox class names, except for :class:`Maildir`. For this reason, the new
:class:`Maildir` class defines a :meth:`next` method and its constructor differs
slightly from those of the other new mailbox classes.

The older mailbox classes whose names are not the same as their newer
counterparts are as follows:


.. class:: UnixMailbox(fp[, factory])

   Access to a classic Unix-style mailbox, where all messages are contained in a
   single file and separated by ``From`` (a.k.a. ``From_``) lines.  The file object
   *fp* points to the mailbox file.  The optional *factory* parameter is a callable
   that should create new message objects.  *factory* is called with one argument,
   *fp* by the :meth:`next` method of the mailbox object.  The default is the
   :class:`rfc822.Message` class (see the :mod:`rfc822` module -- and the note
   below).

   .. note::

      For reasons of this module's internal implementation, you will probably want to
      open the *fp* object in binary mode.  This is especially important on Windows.

   For maximum portability, messages in a Unix-style mailbox are separated by any
   line that begins exactly with the string ``'From '`` (note the trailing space)
   if preceded by exactly two newlines. Because of the wide-range of variations in
   practice, nothing else on the ``From_`` line should be considered.  However, the
   current implementation doesn't check for the leading two newlines.  This is
   usually fine for most applications.

   The :class:`UnixMailbox` class implements a more strict version of ``From_``
   line checking, using a regular expression that usually correctly matched
   ``From_`` delimiters.  It considers delimiter line to be separated by ``From
   name time`` lines.  For maximum portability, use the
   :class:`PortableUnixMailbox` class instead.  This class is identical to
   :class:`UnixMailbox` except that individual messages are separated by only
   ``From`` lines.


.. class:: PortableUnixMailbox(fp[, factory])

   A less-strict version of :class:`UnixMailbox`, which considers only the ``From``
   at the beginning of the line separating messages.  The "*name* *time*" portion
   of the From line is ignored, to protect against some variations that are
   observed in practice.  This works since lines in the message which begin with
   ``'From '`` are quoted by mail handling software at delivery-time.


.. class:: MmdfMailbox(fp[, factory])

   Access an MMDF-style mailbox, where all messages are contained in a single file
   and separated by lines consisting of 4 control-A characters.  The file object
   *fp* points to the mailbox file. Optional *factory* is as with the
   :class:`UnixMailbox` class.


.. class:: MHMailbox(dirname[, factory])

   Access an MH mailbox, a directory with each message in a separate file with a
   numeric name. The name of the mailbox directory is passed in *dirname*.
   *factory* is as with the :class:`UnixMailbox` class.


.. class:: BabylMailbox(fp[, factory])

   Access a Babyl mailbox, which is similar to an MMDF mailbox.  In Babyl format,
   each message has two sets of headers, the *original* headers and the *visible*
   headers.  The original headers appear before a line containing only ``'*** EOOH
   ***'`` (End-Of-Original-Headers) and the visible headers appear after the
   ``EOOH`` line.  Babyl-compliant mail readers will show you only the visible
   headers, and :class:`BabylMailbox` objects will return messages containing only
   the visible headers.  You'll have to do your own parsing of the mailbox file to
   get at the original headers.  Mail messages start with the EOOH line and end
   with a line containing only ``'\037\014'``.  *factory* is as with the
   :class:`UnixMailbox` class.

If you wish to use the older mailbox classes with the :mod:`email` module rather
than the deprecated :mod:`rfc822` module, you can do so as follows::

   import email
   import email.Errors
   import mailbox

   def msgfactory(fp):
       try:
           return email.message_from_file(fp)
       except email.Errors.MessageParseError:
           # Don't return None since that will
           # stop the mailbox iterator
           return ''

   mbox = mailbox.UnixMailbox(fp, msgfactory)

Alternatively, if you know your mailbox contains only well-formed MIME messages,
you can simplify this to::

   import email
   import mailbox

   mbox = mailbox.UnixMailbox(fp, email.message_from_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 = dict((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()