18.10. "multifile" — Support for files containing distinct parts
****************************************************************

2.5 版後已棄用: The "email" package should be used in preference to
the "multifile" module. This module is present only to maintain
backward compatibility.

The "MultiFile" object enables you to treat sections of a text file as
file-like input objects, with "''" being returned by "readline()" when
a given delimiter pattern is encountered.  The defaults of this class
are designed to make it useful for parsing MIME multipart messages,
but by subclassing it and overriding methods  it can be easily adapted
for more general use.

class multifile.MultiFile(fp[, seekable])

   Create a multi-file.  You must instantiate this class with an input
   object argument for the "MultiFile" instance to get lines from,
   such as a file object returned by "open()".

   "MultiFile" only ever looks at the input object’s "readline()",
   "seek()" and "tell()" methods, and the latter two are only needed
   if you want random access to the individual MIME parts. To use
   "MultiFile" on a non-seekable stream object, set the optional
   *seekable* argument to false; this will prevent using the input
   object’s "seek()" and "tell()" methods.

It will be useful to know that in "MultiFile"’s view of the world,
text is composed of three kinds of lines: data, section-dividers, and
end-markers. MultiFile is designed to support parsing of messages that
may have multiple nested message parts, each with its own pattern for
section-divider and end-marker lines.

也參考:

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


18.10.1. MultiFile Objects
==========================

A "MultiFile" instance has the following methods:

MultiFile.readline(str)

   Read a line.  If the line is data (not a section-divider or end-
   marker or real EOF) return it.  If the line matches the most-
   recently-stacked boundary, return "''" and set "self.last" to 1 or
   0 according as the match is or is not an end-marker.  If the line
   matches any other stacked boundary, raise an error.  On
   encountering end-of-file on the underlying stream object, the
   method raises "Error" unless all boundaries have been popped.

MultiFile.readlines(str)

   Return all lines remaining in this part as a list of strings.

MultiFile.read()

   Read all lines, up to the next section.  Return them as a single
   (multiline) string.  Note that this doesn’t take a size argument!

MultiFile.seek(pos[, whence])

   Seek.  Seek indices are relative to the start of the current
   section. The *pos* and *whence* arguments are interpreted as for a
   file seek.

MultiFile.tell()

   Return the file position relative to the start of the current
   section.

MultiFile.next()

   Skip lines to the next section (that is, read lines until a
   section-divider or end-marker has been consumed).  Return true if
   there is such a section, false if an end-marker is seen.  Re-enable
   the most-recently-pushed boundary.

MultiFile.is_data(str)

   Return true if *str* is data and false if it might be a section
   boundary.  As written, it tests for a prefix other than "'-""-'" at
   start of line (which all MIME boundaries have) but it is declared
   so it can be overridden in derived classes.

   Note that this test is used intended as a fast guard for the real
   boundary tests; if it always returns false it will merely slow
   processing, not cause it to fail.

MultiFile.push(str)

   Push a boundary string.  When a decorated version of this boundary
   is found as an input line, it will be interpreted as a section-
   divider  or end-marker (depending on the decoration, see **RFC
   2045**).  All subsequent reads will return the empty string to
   indicate end-of-file, until a call to "pop()" removes the boundary
   a or "next()" call reenables it.

   It is possible to push more than one boundary.  Encountering the
   most-recently-pushed boundary will return EOF; encountering any
   other boundary will raise an error.

MultiFile.pop()

   Pop a section boundary.  This boundary will no longer be
   interpreted as EOF.

MultiFile.section_divider(str)

   Turn a boundary into a section-divider line.  By default, this
   method prepends "'--'" (which MIME section boundaries have) but it
   is declared so it can be overridden in derived classes.  This
   method need not append LF or CR-LF, as comparison with the result
   ignores trailing whitespace.

MultiFile.end_marker(str)

   Turn a boundary string into an end-marker line.  By default, this
   method prepends "'--'" and appends "'--'" (like a MIME-multipart
   end-of-message marker) but it is declared so it can be overridden
   in derived classes.  This method need not append LF or CR-LF, as
   comparison with the result ignores trailing whitespace.

Finally, "MultiFile" instances have two public instance variables:

MultiFile.level

   Nesting depth of the current part.

MultiFile.last

   True if the last end-of-file was for an end-of-message marker.


18.10.2. "MultiFile" Example
============================

   import mimetools
   import multifile
   import StringIO

   def extract_mime_part_matching(stream, mimetype):
       """Return the first element in a multipart MIME message on stream
       matching mimetype."""

       msg = mimetools.Message(stream)
       msgtype = msg.gettype()
       params = msg.getplist()

       data = StringIO.StringIO()
       if msgtype[:10] == "multipart/":

           file = multifile.MultiFile(stream)
           file.push(msg.getparam("boundary"))
           while file.next():
               submsg = mimetools.Message(file)
               try:
                   data = StringIO.StringIO()
                   mimetools.decode(file, data, submsg.getencoding())
               except ValueError:
                   continue
               if submsg.gettype() == mimetype:
                   break
           file.pop()
       return data.getvalue()
