161 lines
6.1 KiB
TeX
161 lines
6.1 KiB
TeX
\section{\module{multifile} ---
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Support for reading files which contain distinct parts.}
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\declaremodule{standard}{multifile}
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\sectionauthor{Eric S. Raymond}{esr@snark.thyrsus.com}
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\modulesynopsis{Support for reading files which contain distinct
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parts, such as some MIME data.}
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The \class{MultiFile} object enables you to treat sections of a text
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file as file-like input objects, with \code{''} being returned by
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\method{readline()} when a given delimiter pattern is encountered. The
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defaults of this class are designed to make it useful for parsing
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MIME multipart messages, but by subclassing it and overriding methods
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it can be easily adapted for more general use.
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\begin{classdesc}{MultiFile}{fp\optional{, seekable}}
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Create a multi-file. You must instantiate this class with an input
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object argument for the \class{MultiFile} instance to get lines from,
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such as as a file object returned by \function{open()}.
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\class{MultiFile} only ever looks at the input object's
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\method{readline()}, \method{seek()} and \method{tell()} methods, and
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the latter two are only needed if you want random access to the
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individual MIME parts. To use \class{MultiFile} on a non-seekable
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stream object, set the optional \var{seekable} argument to false; this
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will prevent using the input object's \method{seek()} and
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\method{tell()} methods.
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\end{classdesc}
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It will be useful to know that in \class{MultiFile}'s view of the world, text
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is composed of three kinds of lines: data, section-dividers, and
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end-markers. MultiFile is designed to support parsing of
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messages that may have multiple nested message parts, each with its
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own pattern for section-divider and end-marker lines.
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\subsection{MultiFile Objects \label{MultiFile-objects}}
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A \class{MultiFile} instance has the following methods:
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\begin{methoddesc}{push}{str}
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Push a boundary string. When an appropriately decorated version of
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this boundary is found as an input line, it will be interpreted as a
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section-divider or end-marker. All subsequent
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reads will return the empty string to indicate end-of-file, until a
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call to \method{pop()} removes the boundary a or \method{next()} call
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reenables it.
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It is possible to push more than one boundary. Encountering the
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most-recently-pushed boundary will return EOF; encountering any other
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boundary will raise an error.
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\end{methoddesc}
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\begin{methoddesc}{readline}{str}
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Read a line. If the line is data (not a section-divider or end-marker
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or real EOF) return it. If the line matches the most-recently-stacked
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boundary, return \code{''} and set \code{self.last} to 1 or 0 according as
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the match is or is not an end-marker. If the line matches any other
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stacked boundary, raise an error. On encountering end-of-file on the
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underlying stream object, the method raises \exception{Error} unless
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all boundaries have been popped.
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\end{methoddesc}
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\begin{methoddesc}{readlines}{str}
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Return all lines remaining in this part as a list of strings.
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\end{methoddesc}
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\begin{methoddesc}{read}{}
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Read all lines, up to the next section. Return them as a single
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(multiline) string. Note that this doesn't take a size argument!
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\end{methoddesc}
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\begin{methoddesc}{next}{}
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Skip lines to the next section (that is, read lines until a
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section-divider or end-marker has been consumed). Return true if
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there is such a section, false if an end-marker is seen. Re-enable
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the most-recently-pushed boundary.
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\end{methoddesc}
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\begin{methoddesc}{pop}{}
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Pop a section boundary. This boundary will no longer be interpreted
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as EOF.
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\end{methoddesc}
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\begin{methoddesc}{seek}{pos\optional{, whence}}
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Seek. Seek indices are relative to the start of the current section.
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The \var{pos} and \var{whence} arguments are interpreted as for a file
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seek.
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\end{methoddesc}
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\begin{methoddesc}{tell}{}
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Return the file position relative to the start of the current section.
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\end{methoddesc}
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\begin{methoddesc}{is_data}{str}
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Return true if \var{str} is data and false if it might be a section
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boundary. As written, it tests for a prefix other than \code{'--'} at
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start of line (which all MIME boundaries have) but it is declared so
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it can be overridden in derived classes.
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Note that this test is used intended as a fast guard for the real
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boundary tests; if it always returns false it will merely slow
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processing, not cause it to fail.
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\end{methoddesc}
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\begin{methoddesc}{section_divider}{str}
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Turn a boundary into a section-divider line. By default, this
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method prepends \code{'--'} (which MIME section boundaries have) but
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it is declared so it can be overridden in derived classes. This
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method need not append LF or CR-LF, as comparison with the result
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ignores trailing whitespace.
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\end{methoddesc}
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\begin{methoddesc}{end_marker}{str}
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Turn a boundary string into an end-marker line. By default, this
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method prepends \code{'--'} and appends \code{'--'} (like a
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MIME-multipart end-of-message marker) but it is declared so it can be
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be overridden in derived classes. This method need not append LF or
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CR-LF, as comparison with the result ignores trailing whitespace.
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\end{methoddesc}
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Finally, \class{MultiFile} instances have two public instance variables:
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\begin{memberdesc}{level}
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Nesting depth of the current part.
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\end{memberdesc}
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\begin{memberdesc}{last}
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True if the last end-of-file was for an end-of-message marker.
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\end{memberdesc}
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\subsection{\class{MultiFile} Example \label{multifile-example}}
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% This is almost unreadable; should be re-written when someone gets time.
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\begin{verbatim}
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fp = MultiFile(sys.stdin, 0)
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fp.push(outer_boundary)
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message1 = fp.readlines()
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# We should now be either at real EOF or stopped on a message
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# boundary. Re-enable the outer boundary.
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fp.next()
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# Read another message with the same delimiter
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message2 = fp.readlines()
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# Re-enable that delimiter again
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fp.next()
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# Now look for a message subpart with a different boundary
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fp.push(inner_boundary)
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sub_header = fp.readlines()
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# If no exception has been thrown, we're looking at the start of
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# the message subpart. Reset and grab the subpart
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fp.next()
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sub_body = fp.readlines()
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# Got it. Now pop the inner boundary to re-enable the outer one.
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fp.pop()
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# Read to next outer boundary
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message3 = fp.readlines()
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\end{verbatim}
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