18.11. "rfc822" — Parse RFC 2822 mail headers
*********************************************

2.3 版後已棄用: The "email" package should be used in preference to
the "rfc822" module.  This module is present only to maintain backward
compatibility, and has been removed in Python 3.

This module defines a class, "Message", which represents an 「email
message」 as defined by the Internet standard **RFC 2822**. [1]  Such
messages consist of a collection of message headers, and a message
body.  This module also defines a helper class "AddressList" for
parsing **RFC 2822** addresses.  Please refer to the RFC for
information on the specific syntax of **RFC 2822** messages.

The "mailbox" module provides classes  to read mailboxes produced by
various end-user mail programs.

class rfc822.Message(file[, seekable])

   A "Message" instance is instantiated with an input object as
   parameter. Message relies only on the input object having a
   "readline()" method; in particular, ordinary file objects qualify.
   Instantiation reads headers from the input object up to a delimiter
   line (normally a blank line) and stores them in the instance.  The
   message body, following the headers, is not consumed.

   This class can work with any input object that supports a
   "readline()" method.  If the input object has seek and tell
   capability, the "rewindbody()" method will work; also, illegal
   lines will be pushed back onto the input stream.  If the input
   object lacks seek but has an "unread()" method that can push back a
   line of input, "Message" will use that to push back illegal lines.
   Thus this class can be used to parse messages coming from a
   buffered stream.

   The optional *seekable* argument is provided as a workaround for
   certain stdio libraries in which "tell()" discards buffered data
   before discovering that the "lseek()" system call doesn’t work.
   For maximum portability, you should set the seekable argument to
   zero to prevent that initial "tell()" when passing in an unseekable
   object such as a file object created from a socket object.

   Input lines as read from the file may either be terminated by CR-LF
   or by a single linefeed; a terminating CR-LF is replaced by a
   single linefeed before the line is stored.

   All header matching is done independent of upper or lower case;
   e.g. "m['From']", "m['from']" and "m['FROM']" all yield the same
   result.

class rfc822.AddressList(field)

   You may instantiate the "AddressList" helper class using a single
   string parameter, a comma-separated list of **RFC 2822** addresses
   to be parsed.  (The parameter "None" yields an empty list.)

rfc822.quote(str)

   Return a new string with backslashes in *str* replaced by two
   backslashes and double quotes replaced by backslash-double quote.

rfc822.unquote(str)

   Return a new string which is an *unquoted* version of *str*. If
   *str* ends and begins with double quotes, they are stripped off.
   Likewise if *str* ends and begins with angle brackets, they are
   stripped off.

rfc822.parseaddr(address)

   Parse *address*, which should be the value of some address-
   containing field such as *To* or *Cc*, into its constituent 「
   realname」 and 「email address」 parts. Returns a tuple of that
   information, unless the parse fails, in which case a 2-tuple
   "(None, None)" is returned.

rfc822.dump_address_pair(pair)

   The inverse of "parseaddr()", this takes a 2-tuple of the form
   "(realname, email_address)" and returns the string value suitable
   for a *To* or *Cc* header.  If the first element of *pair* is
   false, then the second element is returned unmodified.

rfc822.parsedate(date)

   Attempts to parse a date according to the rules in **RFC 2822**.
   however, some mailers don’t follow that format as specified, so
   "parsedate()" tries to guess correctly in such cases.  *date* is a
   string containing an **RFC 2822** date, such as  "'Mon, 20 Nov 1995
   19:12:08 -0500'".  If it succeeds in parsing the date,
   "parsedate()" returns a 9-tuple that can be passed directly to
   "time.mktime()"; otherwise "None" will be returned.  Note that
   indexes 6, 7, and 8 of the result tuple are not usable.

rfc822.parsedate_tz(date)

   Performs the same function as "parsedate()", but returns either
   "None" or a 10-tuple; the first 9 elements make up a tuple that can
   be passed directly to "time.mktime()", and the tenth is the offset
   of the date’s timezone from UTC (which is the official term for
   Greenwich Mean Time).  (Note that the sign of the timezone offset
   is the opposite of the sign of the "time.timezone" variable for the
   same timezone; the latter variable follows the POSIX standard while
   this module follows **RFC 2822**.)  If the input string has no
   timezone, the last element of the tuple returned is "None".  Note
   that indexes 6, 7, and 8 of the result tuple are not usable.

rfc822.mktime_tz(tuple)

   Turn a 10-tuple as returned by "parsedate_tz()" into a UTC
   timestamp.  If the timezone item in the tuple is "None", assume
   local time.  Minor deficiency: this first interprets the first 8
   elements as a local time and then compensates for the timezone
   difference; this may yield a slight error around daylight savings
   time switch dates.  Not enough to worry about for common use.

也參考:

  Module "email"
     Comprehensive email handling package; supersedes the "rfc822"
     module.

  Module "mailbox"
     Classes to read various mailbox formats produced  by end-user
     mail programs.

  Module "mimetools"
     Subclass of "rfc822.Message" that handles MIME encoded messages.


18.11.1. Message Objects
========================

A "Message" instance has the following methods:

Message.rewindbody()

   Seek to the start of the message body.  This only works if the file
   object is seekable.

Message.isheader(line)

   Returns a line’s canonicalized fieldname (the dictionary key that
   will be used to index it) if the line is a legal **RFC 2822**
   header; otherwise returns "None" (implying that parsing should stop
   here and the line be pushed back on the input stream).  It is
   sometimes useful to override this method in a subclass.

Message.islast(line)

   Return true if the given line is a delimiter on which Message
   should stop.  The delimiter line is consumed, and the file object’s
   read location positioned immediately after it.  By default this
   method just checks that the line is blank, but you can override it
   in a subclass.

Message.iscomment(line)

   Return "True" if the given line should be ignored entirely, just
   skipped. By default this is a stub that always returns "False", but
   you can override it in a subclass.

Message.getallmatchingheaders(name)

   Return a list of lines consisting of all headers matching *name*,
   if any.  Each physical line, whether it is a continuation line or
   not, is a separate list item.  Return the empty list if no header
   matches *name*.

Message.getfirstmatchingheader(name)

   Return a list of lines comprising the first header matching *name*,
   and its continuation line(s), if any.  Return "None" if there is no
   header matching *name*.

Message.getrawheader(name)

   Return a single string consisting of the text after the colon in
   the first header matching *name*.  This includes leading
   whitespace, the trailing linefeed, and internal linefeeds and
   whitespace if there any continuation line(s) were present.  Return
   "None" if there is no header matching *name*.

Message.getheader(name[, default])

   Return a single string consisting of the last header matching
   *name*, but strip leading and trailing whitespace. Internal
   whitespace is not stripped.  The optional *default* argument can be
   used to specify a different default to be returned when there is no
   header matching *name*; it defaults to "None". This is the
   preferred way to get parsed headers.

Message.get(name[, default])

   An alias for "getheader()", to make the interface more compatible
   with regular dictionaries.

Message.getaddr(name)

   Return a pair "(full name, email address)" parsed from the string
   returned by "getheader(name)".  If no header matching *name*
   exists, return "(None, None)"; otherwise both the full name and the
   address are (possibly empty) strings.

   Example: If *m*’s first *From* header contains the string
   "'jack@cwi.nl (Jack Jansen)'", then "m.getaddr('From')" will yield
   the pair "('Jack Jansen', 'jack@cwi.nl')". If the header contained
   "'Jack Jansen <jack@cwi.nl>'" instead, it would yield the exact
   same result.

Message.getaddrlist(name)

   This is similar to "getaddr(list)", but parses a header containing
   a list of email addresses (e.g. a *To* header) and returns a list
   of "(full name, email address)" pairs (even if there was only one
   address in the header). If there is no header matching *name*,
   return an empty list.

   If multiple headers exist that match the named header (e.g. if
   there are several *Cc* headers), all are parsed for addresses. Any
   continuation lines the named headers contain are also parsed.

Message.getdate(name)

   Retrieve a header using "getheader()" and parse it into a 9-tuple
   compatible with "time.mktime()"; note that fields 6, 7, and 8  are
   not usable.  If there is no header matching *name*, or it is
   unparsable, return "None".

   Date parsing appears to be a black art, and not all mailers adhere
   to the standard.  While it has been tested and found correct on a
   large collection of email from many sources, it is still possible
   that this function may occasionally yield an incorrect result.

Message.getdate_tz(name)

   Retrieve a header using "getheader()" and parse it into a 10-tuple;
   the first 9 elements will make a tuple compatible with
   "time.mktime()", and the 10th is a number giving the offset of the
   date’s timezone from UTC.  Note that fields 6, 7, and 8  are not
   usable.  Similarly to "getdate()", if there is no header matching
   *name*, or it is unparsable, return "None".

"Message" instances also support a limited mapping interface. In
particular: "m[name]" is like "m.getheader(name)" but raises
"KeyError" if there is no matching header; and "len(m)", "m.get(name[,
default])", "name in m", "m.keys()", "m.values()" "m.items()", and
"m.setdefault(name[, default])" act as expected, with the one
difference that "setdefault()" uses an empty string as the default
value. "Message" instances also support the mapping writable interface
"m[name] = value" and "del m[name]".  "Message" objects do not support
the "clear()", "copy()", "popitem()", or "update()" methods of the
mapping interface.  (Support for "get()" and "setdefault()" was only
added in Python 2.2.)

Finally, "Message" instances have some public instance variables:

Message.headers

   A list containing the entire set of header lines, in the order in
   which they were read (except that setitem calls may disturb this
   order). Each line contains a trailing newline.  The blank line
   terminating the headers is not contained in the list.

Message.fp

   The file or file-like object passed at instantiation time.  This
   can be used to read the message content.

Message.unixfrom

   The Unix "From" line, if the message had one, or an empty string.
   This is needed to regenerate the message in some contexts, such as
   an "mbox"-style mailbox file.


18.11.2. AddressList Objects
============================

An "AddressList" instance has the following methods:

AddressList.__len__()

   Return the number of addresses in the address list.

AddressList.__str__()

   Return a canonicalized string representation of the address list.
   Addresses are rendered in 「name」 <host@domain> form, comma-
   separated.

AddressList.__add__(alist)

   Return a new "AddressList" instance that contains all addresses in
   both "AddressList" operands, with duplicates removed (set union).

AddressList.__iadd__(alist)

   In-place version of "__add__()"; turns this "AddressList" instance
   into the union of itself and the right-hand instance, *alist*.

AddressList.__sub__(alist)

   Return a new "AddressList" instance that contains every address in
   the left-hand "AddressList" operand that is not present in the
   right-hand address operand (set difference).

AddressList.__isub__(alist)

   In-place version of "__sub__()", removing addresses in this list
   which are also in *alist*.

Finally, "AddressList" instances have one public instance variable:

AddressList.addresslist

   A list of tuple string pairs, one per address.  In each member, the
   first is the canonicalized name part, the second is the actual
   route-address ("'@'"-separated username-host.domain pair).

-[ Footnotes ]-

[1] This module originally conformed to **RFC 822**, hence the
    name. Since then, **RFC 2822** has been released as an update to
    **RFC 822**.  This module should be considered **RFC
    2822**-conformant, especially in cases where the syntax or
    semantics have changed since **RFC 822**.
