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

**源代码：** Lib/mailbox.py

======================================================================

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.

参见:

  模块 "email"
     Represent and manipulate messages.


"Mailbox" 对象
==============

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.

   警告:

     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, a byte string, or a
      file-like object (which should be open in binary mode). If
      *message* is an instance of the appropriate format-specific
      "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.

      在 3.2 版更改: Support for binary input was added.

   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, a byte
      string, or a file-like object (which should be open in binary
      mode). If *message* is an instance of the appropriate format-
      specific "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.

      注解:

        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_bytes(key)

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

      3.2 新版功能.

   get_string(key)

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

   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.

      在 3.2 版更改: The file object really is a binary file;
      previously it was incorrectly returned in text mode.  Also, the
      file-like object now supports the context management protocol:
      you can use a "with" statement to automatically close it.

      注解:

        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.

      注解:

        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 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.

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

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

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

   Maildir mailboxes contain three subdirectories, namely: "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".

   注解:

     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)

      警告:

        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.

参见:

  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 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.

参见:

  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 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.

      注解:

        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()".

参见:

  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 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.

      注解:

        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 an "io.BytesIO" instance, which has an API identical to
      that of a file. As a result, the file-like object is truly
      independent of the underlying mailbox but does not save memory
      compared to a string representation.

   lock()
   unlock()

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

参见:

  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 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.

参见:

  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 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, a byte string, or a
   file, it should contain an **RFC 2822**-compliant message, which is
   read and parsed.  Files should be open in binary mode, but text
   mode files are accepted for backward compatibility.

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

   There is no requirement that "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 be 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 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:

   +--------+-----------+----------------------------------+
   | 标记   | 意义      | 解释                             |
   |========|===========|==================================|
   | D      | 草稿      | Under composition                |
   +--------+-----------+----------------------------------+
   | F      | 已标记    | 标记为重要                       |
   +--------+-----------+----------------------------------+
   | P      | 已读      | 转发，重新发送或退回             |
   +--------+-----------+----------------------------------+
   | R      | 已回复    | 回复给                           |
   +--------+-----------+----------------------------------+
   | S      | 查看      | 读取                             |
   +--------+-----------+----------------------------------+
   | T      | 已删除    | 标记为以后删除                   |
   +--------+-----------+----------------------------------+

   "MaildirMessage" 实例提供以下方法：

   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).

      注解:

        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:

+----------------------+------------------------------------------------+
| 结果状态             | "mboxMessage" 或 "MMDFMessage" 状态            |
|======================|================================================|
| "cur" 子目录         | O 标记                                         |
+----------------------+------------------------------------------------+
| F 标记               | F 标记                                         |
+----------------------+------------------------------------------------+
| R 标记               | A 标记                                         |
+----------------------+------------------------------------------------+
| S 标记               | R 标记                                         |
+----------------------+------------------------------------------------+
| T 标记               | D 标记                                         |
+----------------------+------------------------------------------------+

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

+---------------------------------+----------------------------+
| 结果状态                        | "MHMessage" 状态           |
|=================================|============================|
| "cur" 子目录                    | "unseen" 序列              |
+---------------------------------+----------------------------+
| "cur" subdirectory and S flag   | 非 "unseen" 序列           |
+---------------------------------+----------------------------+
| F 标记                          | "flagged" 序列             |
+---------------------------------+----------------------------+
| R 标记                          | "replied" 序列             |
+---------------------------------+----------------------------+

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

+---------------------------------+---------------------------------+
| 结果状态                        | "BabylMessage" 状态             |
|=================================|=================================|
| "cur" 子目录                    | "unseen" 标签                   |
+---------------------------------+---------------------------------+
| "cur" subdirectory and S flag   | 非 "unseen" 标签                |
+---------------------------------+---------------------------------+
| P 标记                          | "forwarded" 或 "resent" 标签    |
+---------------------------------+---------------------------------+
| R 标记                          | "answered" 标签                 |
+---------------------------------+---------------------------------+
| T 标记                          | "deleted" 标签                  |
+---------------------------------+---------------------------------+


"mboxMessage"
-------------

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:

   +--------+------------+----------------------------------+
   | 标记   | 意义       | 解释                             |
   |========|============|==================================|
   | R      | 读取       | 读取                             |
   +--------+------------+----------------------------------+
   | O      | Old        | 以前由MUA检测                    |
   +--------+------------+----------------------------------+
   | D      | 已删除     | 标记为以后删除                   |
   +--------+------------+----------------------------------+
   | F      | 已标记     | 标记为重要                       |
   +--------+------------+----------------------------------+
   | A      | 已回复     | 回复给                           |
   +--------+------------+----------------------------------+

   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
      "time.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:

+-------------------+---------------------------------+
| 结果状态          | "MaildirMessage" 状态           |
|===================|=================================|
| R 标记            | S 标记                          |
+-------------------+---------------------------------+
| O 标记            | "cur" 子目录                    |
+-------------------+---------------------------------+
| D 标记            | T 标记                          |
+-------------------+---------------------------------+
| F 标记            | F 标记                          |
+-------------------+---------------------------------+
| A 标记            | R 标记                          |
+-------------------+---------------------------------+

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

+---------------------+----------------------------+
| 结果状态            | "MHMessage" 状态           |
|=====================|============================|
| R 标记 和 O 标记    | 非 "unseen" 序列           |
+---------------------+----------------------------+
| O 标记              | "unseen" 序列              |
+---------------------+----------------------------+
| F 标记              | "flagged" 序列             |
+---------------------+----------------------------+
| A 标记              | "replied" 序列             |
+---------------------+----------------------------+

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

+---------------------+-------------------------------+
| 结果状态            | "BabylMessage" 状态           |
|=====================|===============================|
| R 标记 和 O 标记    | 非 "unseen" 标签              |
+---------------------+-------------------------------+
| O 标记              | "unseen" 标签                 |
+---------------------+-------------------------------+
| D 标记              | "deleted" 标签                |
+---------------------+-------------------------------+
| A 标记              | "answered" 标签               |
+---------------------+-------------------------------+

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

+-------------------+------------------------------+
| 结果状态          | "MMDFMessage" 状态           |
|===================|==============================|
| R 标记            | R 标记                       |
+-------------------+------------------------------+
| O 标记            | O 标记                       |
+-------------------+------------------------------+
| D 标记            | D 标记                       |
+-------------------+------------------------------+
| F 标记            | F 标记                       |
+-------------------+------------------------------+
| A 标记            | A 标记                       |
+-------------------+------------------------------+


"MHMessage"
-----------

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:

   +------------+--------------------------------------------+
   | 序列       | 解释                                       |
   |============|============================================|
   | 未读       | 未读取，但先前被MUA检测到                  |
   +------------+--------------------------------------------+
   | 已回复     | 回复给                                     |
   +------------+--------------------------------------------+
   | 已标记     | 标记为重要                                 |
   +------------+--------------------------------------------+

   "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:

+----------------------+---------------------------------+
| 结果状态             | "MaildirMessage" 状态           |
|======================|=================================|
| "unseen" 序列        | 非 S 标记                       |
+----------------------+---------------------------------+
| "replied" 序列       | R 标记                          |
+----------------------+---------------------------------+
| "flagged" 序列       | F 标记                          |
+----------------------+---------------------------------+

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:

+----------------------+------------------------------------------------+
| 结果状态             | "mboxMessage" 或 "MMDFMessage" 状态            |
|======================|================================================|
| "unseen" 序列        | 非 R 标记                                      |
+----------------------+------------------------------------------------+
| "replied" 序列       | A 标记                                         |
+----------------------+------------------------------------------------+
| "flagged" 序列       | F 标记                                         |
+----------------------+------------------------------------------------+

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

+----------------------+-------------------------------+
| 结果状态             | "BabylMessage" 状态           |
|======================|===============================|
| "unseen" 序列        | "unseen" 标签                 |
+----------------------+-------------------------------+
| "replied" 序列       | "answered" 标签               |
+----------------------+-------------------------------+


"BabylMessage"
--------------

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:

   +-------------+--------------------------------------------+
   | 标签        | 解释                                       |
   |=============|============================================|
   | 未读        | 未读取，但先前被MUA检测到                  |
   +-------------+--------------------------------------------+
   | deleted     | 标记为以后删除                             |
   +-------------+--------------------------------------------+
   | filed       | 复制到另一个文件或邮箱                     |
   +-------------+--------------------------------------------+
   | answered    | 回复给                                     |
   +-------------+--------------------------------------------+
   | forwarded   | 已转发                                     |
   +-------------+--------------------------------------------+
   | edited      | 由用户修改                                 |
   +-------------+--------------------------------------------+
   | resent      | 已重发                                     |
   +-------------+--------------------------------------------+

   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()

      返回邮件上的标签列表。

   set_labels(labels)

      将消息上的标签列表设置为 *labels* 。

   add_label(label)

      将 *label* 添加到消息上的标签列表中。

   remove_label(label)

      从消息上的标签列表中删除 *label* 。

   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:

+---------------------+---------------------------------+
| 结果状态            | "MaildirMessage" 状态           |
|=====================|=================================|
| "unseen" 标签       | 非 S 标记                       |
+---------------------+---------------------------------+
| "deleted" 标签      | T 标记                          |
+---------------------+---------------------------------+
| "answered" 标签     | R 标记                          |
+---------------------+---------------------------------+
| "forwarded" 标签    | P 标记                          |
+---------------------+---------------------------------+

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:

+--------------------+------------------------------------------------+
| 结果状态           | "mboxMessage" 或 "MMDFMessage" 状态            |
|====================|================================================|
| "unseen" 标签      | 非 R 标记                                      |
+--------------------+------------------------------------------------+
| "deleted" 标签     | D 标记                                         |
+--------------------+------------------------------------------------+
| "answered" 标签    | A 标记                                         |
+--------------------+------------------------------------------------+

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

+--------------------+----------------------------+
| 结果状态           | "MHMessage" 状态           |
|====================|============================|
| "unseen" 标签      | "unseen" 序列              |
+--------------------+----------------------------+
| "answered" 标签    | "replied" 序列             |
+--------------------+----------------------------+


"MMDFMessage"
-------------

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:

   +--------+------------+----------------------------------+
   | 标记   | 意义       | 解释                             |
   |========|============|==================================|
   | R      | 读取       | 读取                             |
   +--------+------------+----------------------------------+
   | O      | Old        | 以前由MUA检测                    |
   +--------+------------+----------------------------------+
   | D      | 已删除     | 标记为以后删除                   |
   +--------+------------+----------------------------------+
   | F      | 已标记     | 标记为重要                       |
   +--------+------------+----------------------------------+
   | A      | 已回复     | 回复给                           |
   +--------+------------+----------------------------------+

   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
      "time.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:

+-------------------+---------------------------------+
| 结果状态          | "MaildirMessage" 状态           |
|===================|=================================|
| R 标记            | S 标记                          |
+-------------------+---------------------------------+
| O 标记            | "cur" 子目录                    |
+-------------------+---------------------------------+
| D 标记            | T 标记                          |
+-------------------+---------------------------------+
| F 标记            | F 标记                          |
+-------------------+---------------------------------+
| A 标记            | R 标记                          |
+-------------------+---------------------------------+

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

+---------------------+----------------------------+
| 结果状态            | "MHMessage" 状态           |
|=====================|============================|
| R 标记 和 O 标记    | 非 "unseen" 序列           |
+---------------------+----------------------------+
| O 标记              | "unseen" 序列              |
+---------------------+----------------------------+
| F 标记              | "flagged" 序列             |
+---------------------+----------------------------+
| A 标记              | "replied" 序列             |
+---------------------+----------------------------+

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

+---------------------+-------------------------------+
| 结果状态            | "BabylMessage" 状态           |
|=====================|===============================|
| R 标记 和 O 标记    | 非 "unseen" 标签              |
+---------------------+-------------------------------+
| O 标记              | "unseen" 标签                 |
+---------------------+-------------------------------+
| D 标记              | "deleted" 标签                |
+---------------------+-------------------------------+
| A 标记              | "answered" 标签               |
+---------------------+-------------------------------+

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

+-------------------+------------------------------+
| 结果状态          | "mboxMessage" 状态           |
|===================|==============================|
| R 标记            | R 标记                       |
+-------------------+------------------------------+
| O 标记            | O 标记                       |
+-------------------+------------------------------+
| D 标记            | D 标记                       |
+-------------------+------------------------------+
| F 标记            | F 标记                       |
+-------------------+------------------------------+
| A 标记            | A 标记                       |
+-------------------+------------------------------+


异常
====

The following exception classes are defined in the "mailbox" module:

exception mailbox.Error

   The based class for all other module-specific exceptions.

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 mailbox.NotEmptyError

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

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 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.


示例
====

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()
