
``mailbox`` --- Manipulate mailboxes in various formats
*******************************************************

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

See also:

   Module ``email``
      Represent and manipulate messages.


``Mailbox`` objects
===================

class class mailbox.Mailbox

   A mailbox, which may be inspected and modified.

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

   The ``Mailbox`` interface is dictionary-like, with small keys
   corresponding to messages. Keys are issued by the ``Mailbox``
   instance with which they will be used and are only meaningful to
   that ``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 ``Mailbox`` instance using the set-like
   method ``add()`` and removed using a ``del`` statement or the set-
   like methods ``remove()`` and ``discard()``.

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

   The default ``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 ``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
     ``lock()`` and ``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.

   ``Mailbox`` instances have the following methods:

   add(message)

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

      Parameter *message* may be a ``Message`` instance, an
      ``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 ``Message`` subclass
      (e.g., if it's an ``mboxMessage`` instance and this is an
      ``mbox`` instance), its format-specific information is used.
      Otherwise, reasonable defaults for format-specific information
      are used.

   remove(key)
   __delitem__(key)
   discard(key)

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

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

   __setitem__(key, message)

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

      As with ``add()``, parameter *message* may be a ``Message``
      instance, an ``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
      ``Message`` subclass (e.g., if it's an ``mboxMessage`` instance
      and this is an ``mbox`` instance), its format-specific
      information is used. Otherwise, the format-specific information
      of the message that currently corresponds to *key* is left
      unchanged.

   iterkeys()
   keys()

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

   itervalues()
   __iter__()
   values()

      Return an iterator over representations of all messages if
      called as ``itervalues()`` or ``__iter__()`` or return a list of
      such representations if called as ``values()``. The messages are
      represented as instances of the appropriate format-specific
      ``Message`` subclass unless a custom message factory was
      specified when the ``Mailbox`` instance was initialized.

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

   iteritems()
   items()

      Return an iterator over (*key*, *message*) pairs, where *key* is
      a key and *message* is a message representation, if called as
      ``iteritems()`` or return a list of such pairs if called as
      ``items()``. The messages are represented as instances of the
      appropriate format-specific ``Message`` subclass unless a custom
      message factory was specified when the ``Mailbox`` instance was
      initialized.

   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 ``get()`` and a ``KeyError`` exception is raised
      if the method was called as ``__getitem__()``. The message is
      represented as an instance of the appropriate format-specific
      ``Message`` subclass unless a custom message factory was
      specified when the ``Mailbox`` instance was initialized.

   get_message(key)

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

   get_string(key)

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

   get_file(key)

      Return a file-like representation of the message corresponding
      to *key*, or raise a ``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
        ``Mailbox`` instance that created them or of the underlying
        mailbox. More specific documentation is provided by each
        subclass.

   __contains__(key)

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

   __len__()

      Return a count of messages in the mailbox.

   clear()

      Delete all messages from the mailbox.

   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 ``Message`` subclass unless a custom
      message factory was specified when the ``Mailbox`` instance was
      initialized.

   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
      ``KeyError`` exception. The message is represented as an
      instance of the appropriate format-specific ``Message`` subclass
      unless a custom message factory was specified when the
      ``Mailbox`` instance was initialized.

   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
      ``__setitem__()``. As with ``__setitem__()``, each *key* must
      already correspond to a message in the mailbox or else a
      ``KeyError`` exception will be raised, so in general it is
      incorrect for *arg* to be a ``Mailbox`` instance.

      Note: Unlike with dictionaries, keyword arguments are not supported.

   flush()

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

   lock()

      Acquire an exclusive advisory lock on the mailbox so that other
      processes know not to modify it. An ``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.

   unlock()

      Release the lock on the mailbox, if any.

   close()

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


``Maildir``
-----------

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

   A subclass of ``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``,
   ``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 *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: ``tmp``,
   ``new``, and ``cur``. Messages are created momentarily in the
   ``tmp`` subdirectory and then moved to the ``new`` subdirectory to
   finalize delivery. A mail user agent may subsequently move the
   message to the ``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 ``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 ``colon`` attribute may also be set on a per-instance basis.

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

   list_folders()

      Return a list of the names of all folders.

   get_folder(folder)

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

   add_folder(folder)

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

   remove_folder(folder)

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

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

   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.

   flush()

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

   lock()
   unlock()

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

   close()

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

   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.

See also:

   maildir man page from qmail
      The original specification of the format.

   Using maildir format
      Notes on Maildir by its inventor. Includes an updated name-
      creation scheme and details on "info" semantics.

   maildir man page from Courier
      Another specification of the format. Describes a common
      extension for supporting folders.


``mbox``
--------

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

   A subclass of ``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``,
   ``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,
   ``mbox`` implements the original format, which is sometimes
   referred to as *mboxo*. This means that the *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 ``Mailbox`` methods implemented by ``mbox`` deserve special
   remarks:

   get_file(key)

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

   lock()
   unlock()

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

See also:

   mbox man page from qmail
      A specification of the format and its variations.

   mbox man page from tin
      Another specification of the format, with details on locking.

   Configuring Netscape Mail on Unix: Why The Content-Length Format is
   Bad
      An argument for using the original mbox format rather than a
      variation.

   "mbox" is a family of several mutually incompatible mailbox formats
      A history of mbox variations.


``MH``
------

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

   A subclass of ``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``,
   ``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 *folders*) in addition to messages. Folders may
   be nested indefinitely. MH mailboxes also support *sequences*,
   which are named lists used to logically group messages without
   moving them to sub-folders. Sequences are defined in a file called
   ``.mh_sequences`` in each folder.

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

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

   list_folders()

      Return a list of the names of all folders.

   get_folder(folder)

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

   add_folder(folder)

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

   remove_folder(folder)

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

   get_sequences()

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

   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 ``get_sequences()``.

   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 ``Mailbox`` methods implemented by ``MH`` deserve special
   remarks:

   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.

   lock()
   unlock()

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

   get_file(key)

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

   flush()

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

   close()

      ``MH`` instances do not keep any open files, so this method is
      equivalent to ``unlock()``.

See also:

   nmh - Message Handling System
      Home page of **nmh**, an updated version of the original **mh**.

   MH & nmh: Email for Users & Programmers
      A GPL-licensed book on **mh** and **nmh**, with some information
      on the mailbox format.


``Babyl``
---------

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

   A subclass of ``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``,
   ``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 *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.

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

   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 ``Mailbox`` methods implemented by ``Babyl`` deserve special
   remarks:

   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 ``StringIO`` instance (from the ``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.

   lock()
   unlock()

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

See also:

   Format of Version 5 Babyl Files
      A specification of the Babyl format.

   Reading Mail with Rmail
      The Rmail manual, with some information on Babyl semantics.


``MMDF``
--------

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

   A subclass of ``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``,
   ``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 ``Mailbox`` methods implemented by ``MMDF`` deserve special
   remarks:

   get_file(key)

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

   lock()
   unlock()

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

See also:

   mmdf man page from tin
      A specification of MMDF format from the documentation of tin, a
      newsreader.

   MMDF
      A Wikipedia article describing the Multichannel Memorandum
      Distribution Facility.


``Message`` objects
===================

class class mailbox.Message(message=None)

   A subclass of the ``email.Message`` module's ``Message``.
   Subclasses of ``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 ``email.Message.Message`` instance,
   its contents are copied; furthermore, any format-specific
   information is converted insofar as possible if *message* is a
   ``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 ``Message`` instances be used to
   represent messages retrieved using ``Mailbox`` instances. In some
   situations, the time and memory required to generate ``Message``
   representations might not not acceptable. For such situations,
   ``Mailbox`` instances also offer string and file-like
   representations, and a custom message factory may be specified when
   a ``Mailbox`` instance is initialized.


``MaildirMessage``
------------------

class class mailbox.MaildirMessage(message=None)

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

   Typically, a mail user agent application moves all of the messages
   in the ``new`` subdirectory to the ``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 ``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 ``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   |
   +--------+-----------+----------------------------------+

   ``MaildirMessage`` instances offer the following methods:

   get_subdir()

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

      Note: A message is typically moved from ``new`` to ``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``.

   set_subdir(subdir)

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

   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.

   set_flags(flags)

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

   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.

   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.

   get_date()

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

   set_date(date)

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

   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).

   set_info(info)

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

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

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

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

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

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

+---------------------------------+---------------------------------+
| Resulting state                 | ``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                 |
+---------------------------------+---------------------------------+


``mboxMessage``
---------------

class class mailbox.mboxMessage(message=None)

   A message with mbox-specific behaviors. Parameter *message* has the
   same meaning as with the ``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 *Status* and *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 *Status* header, and the
   "D", "F", and "A" flags are stored in the *X-Status* header. The
   flags and headers typically appear in the order mentioned.

   ``mboxMessage`` instances offer the following methods:

   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.

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

   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'``.

   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'``.

   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.

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

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

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

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

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

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

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

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


``MHMessage``
-------------

class class mailbox.MHMessage(message=None)

   A message with MH-specific behaviors. Parameter *message* has the
   same meaning as with the ``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 **mh** and **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                        |
   +------------+--------------------------------------------+

   ``MHMessage`` instances offer the following methods:

   get_sequences()

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

   set_sequences(sequences)

      Set the list of sequences that include this message.

   add_sequence(sequence)

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

   remove_sequence(sequence)

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

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

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

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

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

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

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


``BabylMessage``
----------------

class class mailbox.BabylMessage(message=None)

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

   Certain message labels, called *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
   ``BabylMessage`` class, though, uses the original headers because
   they are more complete. Visible headers may be accessed explicitly
   if desired.

   ``BabylMessage`` instances offer the following methods:

   get_labels()

      Return a list of labels on the message.

   set_labels(labels)

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

   add_label(label)

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

   remove_label(label)

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

   get_visible()

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

   set_visible(visible)

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

   update_visible()

      When a ``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 *Date*,
      *From*, *Reply-To*, *To*, *CC*, and *Subject* that are present
      in the original headers but not the visible headers are added to
      the visible headers.

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

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

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

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

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

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


``MMDFMessage``
---------------

class class mailbox.MMDFMessage(message=None)

   A message with MMDF-specific behaviors. Parameter *message* has the
   same meaning as with the ``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 *Status* and *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 *Status* header, and the
   "D", "F", and "A" flags are stored in the *X-Status* header. The
   flags and headers typically appear in the order mentioned.

   ``MMDFMessage`` instances offer the following methods, which are
   identical to those offered by ``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.

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

   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'``.

   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'``.

   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.

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

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

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

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

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

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

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

+-------------------+------------------------------+
| Resulting state   | ``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 ``mailbox`` module:

exception exception mailbox.Error

   The based class for all other module-specific exceptions.

exception exception mailbox.NoSuchMailboxError

   Raised when a mailbox is expected but is not found, such as when
   instantiating a ``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 exception mailbox.NotEmptyError

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

exception exception mailbox.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 exception mailbox.FormatError

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


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()
