Doc/lib/libmultifile.tex

   1 \section{\module{multifile} ---
   2          Support for files containing distinct parts}
   3
   4 \declaremodule{standard}{multifile}
   5 \modulesynopsis{Support for reading files which contain distinct
   6                 parts, such as some MIME data.}
   7 \sectionauthor{Eric S. Raymond}{esr@snark.thyrsus.com}
   8
   9
  10 The \class{MultiFile} object enables you to treat sections of a text
  11 file as file-like input objects, with \code{''} being returned by
  12 \method{readline()} when a given delimiter pattern is encountered.  The
  13 defaults of this class are designed to make it useful for parsing
  14 MIME multipart messages, but by subclassing it and overriding methods
  15 it can be easily adapted for more general use.
  16
  17 \begin{classdesc}{MultiFile}{fp\optional{, seekable}}
  18 Create a multi-file.  You must instantiate this class with an input
  19 object argument for the \class{MultiFile} instance to get lines from,
  20 such as as a file object returned by \function{open()}.
  21
  22 \class{MultiFile} only ever looks at the input object's
  23 \method{readline()}, \method{seek()} and \method{tell()} methods, and
  24 the latter two are only needed if you want random access to the
  25 individual MIME parts. To use \class{MultiFile} on a non-seekable
  26 stream object, set the optional \var{seekable} argument to false; this
  27 will prevent using the input object's \method{seek()} and
  28 \method{tell()} methods.
  29 \end{classdesc}
  30
  31 It will be useful to know that in \class{MultiFile}'s view of the world, text
  32 is composed of three kinds of lines: data, section-dividers, and
  33 end-markers.  MultiFile is designed to support parsing of
  34 messages that may have multiple nested message parts, each with its
  35 own pattern for section-divider and end-marker lines.
  36
  37
  38 \subsection{MultiFile Objects \label{MultiFile-objects}}
  39
  40 A \class{MultiFile} instance has the following methods:
  41
  42 \begin{methoddesc}{readline}{str}
  43 Read a line.  If the line is data (not a section-divider or end-marker
  44 or real EOF) return it.  If the line matches the most-recently-stacked
  45 boundary, return \code{''} and set \code{self.last} to 1 or 0 according as
  46 the match is or is not an end-marker.  If the line matches any other
  47 stacked boundary, raise an error.  On encountering end-of-file on the
  48 underlying stream object, the method raises \exception{Error} unless
  49 all boundaries have been popped.
  50 \end{methoddesc}
  51
  52 \begin{methoddesc}{readlines}{str}
  53 Return all lines remaining in this part as a list of strings.
  54 \end{methoddesc}
  55
  56 \begin{methoddesc}{read}{}
  57 Read all lines, up to the next section.  Return them as a single
  58 (multiline) string.  Note that this doesn't take a size argument!
  59 \end{methoddesc}
  60
  61 \begin{methoddesc}{seek}{pos\optional{, whence}}
  62 Seek.  Seek indices are relative to the start of the current section.
  63 The \var{pos} and \var{whence} arguments are interpreted as for a file
  64 seek.
  65 \end{methoddesc}
  66
  67 \begin{methoddesc}{tell}{}
  68 Return the file position relative to the start of the current section.
  69 \end{methoddesc}
  70
  71 \begin{methoddesc}{next}{}
  72 Skip lines to the next section (that is, read lines until a
  73 section-divider or end-marker has been consumed).  Return true if
  74 there is such a section, false if an end-marker is seen.  Re-enable
  75 the most-recently-pushed boundary.
  76 \end{methoddesc}
  77
  78 \begin{methoddesc}{is_data}{str}
  79 Return true if \var{str} is data and false if it might be a section
  80 boundary.  As written, it tests for a prefix other than \code{'-}\code{-'} at
  81 start of line (which all MIME boundaries have) but it is declared so
  82 it can be overridden in derived classes.
  83
  84 Note that this test is used intended as a fast guard for the real
  85 boundary tests; if it always returns false it will merely slow
  86 processing, not cause it to fail.
  87 \end{methoddesc}
  88
  89 \begin{methoddesc}{push}{str}
  90 Push a boundary string.  When an appropriately decorated version of
  91 this boundary is found as an input line, it will be interpreted as a
  92 section-divider or end-marker.  All subsequent
  93 reads will return the empty string to indicate end-of-file, until a
  94 call to \method{pop()} removes the boundary a or \method{next()} call
  95 reenables it.
  96
  97 It is possible to push more than one boundary.  Encountering the
  98 most-recently-pushed boundary will return EOF; encountering any other
  99 boundary will raise an error.
 100 \end{methoddesc}
 101
 102 \begin{methoddesc}{pop}{}
 103 Pop a section boundary.  This boundary will no longer be interpreted
 104 as EOF.
 105 \end{methoddesc}
 106
 107 \begin{methoddesc}{section_divider}{str}
 108 Turn a boundary into a section-divider line.  By default, this
 109 method prepends \code{'-}\code{-'} (which MIME section boundaries have) but
 110 it is declared so it can be overridden in derived classes.  This
 111 method need not append LF or CR-LF, as comparison with the result
 112 ignores trailing whitespace.
 113 \end{methoddesc}
 114
 115 \begin{methoddesc}{end_marker}{str}
 116 Turn a boundary string into an end-marker line.  By default, this
 117 method prepends \code{'-}\code{-'} and appends \code{'-}\code{-'} (like a
 118 MIME-multipart end-of-message marker) but it is declared so it can be
 119 be overridden in derived classes.  This method need not append LF or
 120 CR-LF, as comparison with the result ignores trailing whitespace.
 121 \end{methoddesc}
 122
 123 Finally, \class{MultiFile} instances have two public instance variables:
 124
 125 \begin{memberdesc}{level}
 126 Nesting depth of the current part.
 127 \end{memberdesc}
 128
 129 \begin{memberdesc}{last}
 130 True if the last end-of-file was for an end-of-message marker.
 131 \end{memberdesc}
 132
 133
 134 \subsection{\class{MultiFile} Example \label{multifile-example}}
 135 \sectionauthor{Skip Montanaro}{skip@mojam.com}
 136
 137 \begin{verbatim}
 138 import mimetools
 139 import multifile
 140 import StringIO
 141
 142 def extract_mime_part_matching(stream, mimetype):
 143     """Return the first element in a multipart MIME message on stream
 144     matching mimetype."""
 145
 146     msg = mimetools.Message(stream)
 147     msgtype = msg.gettype()
 148     params = msg.getplist()
 149
 150     data = StringIO.StringIO()
 151     if msgtype[:10] == "multipart/":
 152
 153         file = multifile.MultiFile(stream)
 154         file.push(msg.getparam("boundary"))
 155         while file.next():
 156             submsg = mimetools.Message(file)
 157             try:
 158                 data = StringIO.StringIO()
 159                 mimetools.decode(file, data, submsg.getencoding())
 160             except ValueError:
 161                 continue
 162             if submsg.gettype() == mimetype:
 163                 break
 164         file.pop()
 165     return data.getvalue()
 166 \end{verbatim}