Doc/lib/libshlex.tex

\section{\module{shlex} ---
         Simple lexical analysis}

\declaremodule{standard}{shlex}
\modulesynopsis{Simple lexical analysis for \UNIX{} shell-like languages.}
\moduleauthor{Eric S. Raymond}{esr@snark.thyrsus.com}
\sectionauthor{Eric S. Raymond}{esr@snark.thyrsus.com}

\versionadded{1.5.2}

The \class{shlex} class makes it easy to write lexical analyzers for
simple syntaxes resembling that of the \UNIX{} shell.  This will often
be useful for writing minilanguages, e.g.\ in run control files for
Python applications.

\begin{classdesc}{shlex}{\optional{stream\optional{, file}}}
A \class{shlex} instance or subclass instance is a lexical analyzer
object.  The initialization argument, if present, specifies where to
read characters from. It must be a file- or stream-like object with
\method{read()} and \method{readline()} methods.  If no argument is given,
input will be taken from \code{sys.stdin}.  The second optional 
argument is a filename string, which sets the initial value of the
\member{infile} member.  If the stream argument is omitted or
equal to \code{sys.stdin}, this second argument defaults to ``stdin''.
\end{classdesc}


\begin{seealso}
  \seemodule{ConfigParser}{Parser for configuration files similar to the
                           Windows \file{.ini} files.}
\end{seealso}


\subsection{shlex Objects \label{shlex-objects}}

A \class{shlex} instance has the following methods:


\begin{methoddesc}{get_token}{}
Return a token.  If tokens have been stacked using
\method{push_token()}, pop a token off the stack.  Otherwise, read one
from the input stream.  If reading encounters an immediate
end-of-file, an empty string is returned. 
\end{methoddesc}

\begin{methoddesc}{push_token}{str}
Push the argument onto the token stack.
\end{methoddesc}

\begin{methoddesc}{read_token}{}
Read a raw token.  Ignore the pushback stack, and do not interpret source
requests.  (This is not ordinarily a useful entry point, and is
documented here only for the sake of completeness.)
\end{methoddesc}

\begin{methoddesc}{sourcehook}{filename}
When \class{shlex} detects a source request (see
\member{source} below) this method is given the following token as
argument, and expected to return a tuple consisting of a filename and
an open file-like object.

Normally, this method first strips any quotes off the argument.  If
the result is an absolute pathname, or there was no previous source
request in effect, or the previous source was a stream
(e.g. \code{sys.stdin}), the result is left alone.  Otherwise, if the
result is a relative pathname, the directory part of the name of the
file immediately before it on the source inclusion stack is prepended
(this behavior is like the way the C preprocessor handles
\code{\#include "file.h"}).

The result of the manipulations is treated as a filename, and returned
as the first component of the tuple, with
\function{open()} called on it to yield the second component. (Note:
this is the reverse of the order of arguments in instance initialization!)

This hook is exposed so that you can use it to implement directory
search paths, addition of file extensions, and other namespace hacks.
There is no corresponding `close' hook, but a shlex instance will call
the \method{close()} method of the sourced input stream when it
returns \EOF.

For more explicit control of source stacking, use the next two
methods. 
\end{methoddesc}

\begin{methoddesc}{push_source}{stream\optional{, filename}}
Push an input source stream onto the input stack.  If the filename
argument is specified it will later be available for use in error
messages.  This is the same method used internally by the
\method{sourcehook} method. (New in 2.1)
\end{methoddesc}

\begin{methoddesc}{pop_source}{}}
Pop the last-pushed input source from the input stack.
This is the same method used internally when the lexer reaches
\EOF on a stacked input stream. (New in 2.1)
\end{methoddesc}

\begin{methoddesc}{error_leader}{\optional{file\optional{, line}}}
This method generates an error message leader in the format of a
\UNIX{} C compiler error label; the format is '"\%s", line \%d: ',
where the \samp{\%s} is replaced with the name of the current source
file and the \samp{\%d} with the current input line number (the
optional arguments can be used to override these).

This convenience is provided to encourage \module{shlex} users to
generate error messages in the standard, parseable format understood
by Emacs and other \UNIX{} tools.
\end{methoddesc}

Instances of \class{shlex} subclasses have some public instance
variables which either control lexical analysis or can be used for
debugging:

\begin{memberdesc}{commenters}
The string of characters that are recognized as comment beginners.
All characters from the comment beginner to end of line are ignored.
Includes just \character{\#} by default.   
\end{memberdesc}

\begin{memberdesc}{wordchars}
The string of characters that will accumulate into multi-character
tokens.  By default, includes all \ASCII{} alphanumerics and
underscore.
\end{memberdesc}

\begin{memberdesc}{whitespace}
Characters that will be considered whitespace and skipped.  Whitespace
bounds tokens.  By default, includes space, tab, linefeed and
carriage-return.
\end{memberdesc}

\begin{memberdesc}{quotes}
Characters that will be considered string quotes.  The token
accumulates until the same quote is encountered again (thus, different
quote types protect each other as in the shell.)  By default, includes
\ASCII{} single and double quotes.
\end{memberdesc}

\begin{memberdesc}{infile}
The name of the current input file, as initially set at class
instantiation time or stacked by later source requests.  It may
be useful to examine this when constructing error messages.
\end{memberdesc}

\begin{memberdesc}{instream}
The input stream from which this \class{shlex} instance is reading
characters.
\end{memberdesc}

\begin{memberdesc}{source}
This member is \code{None} by default.  If you assign a string to it,
that string will be recognized as a lexical-level inclusion request
similar to the \samp{source} keyword in various shells.  That is, the
immediately following token will opened as a filename and input taken
from that stream until \EOF, at which point the \method{close()}
method of that stream will be called and the input source will again
become the original input stream. Source requests may be stacked any
number of levels deep.
\end{memberdesc}

\begin{memberdesc}{debug}
If this member is numeric and \code{1} or more, a \class{shlex}
instance will print verbose progress output on its behavior.  If you
need to use this, you can read the module source code to learn the
details.
\end{memberdesc}

Note that any character not declared to be a word character,
whitespace, or a quote will be returned as a single-character token.

Quote and comment characters are not recognized within words.  Thus,
the bare words \samp{ain't} and \samp{ain\#t} would be returned as single
tokens by the default parser.

\begin{memberdesc}{lineno}
Source line number (count of newlines seen so far plus one).
\end{memberdesc}

\begin{memberdesc}{token}
The token buffer.  It may be useful to examine this when catching
exceptions.
\end{memberdesc}