23.2. "shlex" — Simple lexical analysis
***************************************

New in version 1.5.2.

**Source code:** Lib/shlex.py

======================================================================

The "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, (for example, in run control files for
Python applications) or for parsing quoted strings.

Prior to Python 2.7.3, this module did not support Unicode input.

The "shlex" module defines the following functions:

shlex.split(s[, comments[, posix]])

   Split the string *s* using shell-like syntax. If *comments* is
   "False" (the default), the parsing of comments in the given string
   will be disabled (setting the "commenters" attribute of the "shlex"
   instance to the empty string).  This function operates in POSIX
   mode by default, but uses non-POSIX mode if the *posix* argument is
   false.

   New in version 2.3.

   Changed in version 2.6: Added the *posix* parameter.

   Note: Since the "split()" function instantiates a "shlex"
     instance, passing "None" for *s* will read the string to split
     from standard input.

The "shlex" module defines the following class:

class shlex.shlex([instream[, infile[, posix]]])

   A "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-/stream-like object with
   "read()" and "readline()" methods, or a string (strings are
   accepted since Python 2.3).  If no argument is given, input will be
   taken from "sys.stdin".  The second optional argument is a filename
   string, which sets the initial value of the "infile" attribute.  If
   the *instream* argument is omitted or equal to "sys.stdin", this
   second argument defaults to “stdin”.  The *posix* argument was
   introduced in Python 2.3, and defines the operational mode.  When
   *posix* is not true (default), the "shlex" instance will operate in
   compatibility mode.  When operating in POSIX mode, "shlex" will try
   to be as close as possible to the POSIX shell parsing rules.

See also:

  Module "ConfigParser"
     Parser for configuration files similar to the Windows ".ini"
     files.


23.2.1. shlex Objects
=====================

A "shlex" instance has the following methods:

shlex.get_token()

   Return a token.  If tokens have been stacked using "push_token()",
   pop a token off the stack.  Otherwise, read one from the input
   stream.  If reading encounters an immediate end-of-file, "eof" is
   returned (the empty string ("''") in non-POSIX mode, and "None" in
   POSIX mode).

shlex.push_token(str)

   Push the argument onto the token stack.

shlex.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.)

shlex.sourcehook(filename)

   When "shlex" detects a source request (see "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 (such as
   "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 "#include
   "file.h"").

   The result of the manipulations is treated as a filename, and
   returned as the first component of the tuple, with "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 "close()" method of the sourced input stream when it
   returns EOF.

   For more explicit control of source stacking, use the
   "push_source()" and "pop_source()" methods.

shlex.push_source(stream[, 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
   "sourcehook()" method.

   New in version 2.1.

shlex.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 version 2.1.

shlex.error_leader([file[, 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 "%s" is replaced with the name of the current source file
   and the "%d" with the current input line number (the optional
   arguments can be used to override these).

   This convenience is provided to encourage "shlex" users to generate
   error messages in the standard, parseable format understood by
   Emacs and other Unix tools.

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

shlex.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 "'#'" by default.

shlex.wordchars

   The string of characters that will accumulate into multi-character
   tokens.  By default, includes all ASCII alphanumerics and
   underscore.

shlex.whitespace

   Characters that will be considered whitespace and skipped.
   Whitespace bounds tokens.  By default, includes space, tab,
   linefeed and carriage-return.

shlex.escape

   Characters that will be considered as escape. This will be only
   used in POSIX mode, and includes just "'\'" by default.

   New in version 2.3.

shlex.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.

shlex.escapedquotes

   Characters in "quotes" that will interpret escape characters
   defined in "escape".  This is only used in POSIX mode, and includes
   just "'"'" by default.

   New in version 2.3.

shlex.whitespace_split

   If "True", tokens will only be split in whitespaces. This is
   useful, for example, for parsing command lines with "shlex",
   getting tokens in a similar way to shell arguments.

   New in version 2.3.

shlex.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.

shlex.instream

   The input stream from which this "shlex" instance is reading
   characters.

shlex.source

   This attribute is "None" by default.  If you assign a string to it,
   that string will be recognized as a lexical-level inclusion request
   similar to the "source" keyword in various shells.  That is, the
   immediately following token will be opened as a filename and input
   will be taken from that stream until EOF, at which point the
   "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.

shlex.debug

   If this attribute is numeric and "1" or more, a "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.

shlex.lineno

   Source line number (count of newlines seen so far plus one).

shlex.token

   The token buffer.  It may be useful to examine this when catching
   exceptions.

shlex.eof

   Token used to determine end of file. This will be set to the empty
   string ("''"), in non-POSIX mode, and to "None" in POSIX mode.

   New in version 2.3.


23.2.2. Parsing Rules
=====================

When operating in non-POSIX mode, "shlex" will try to obey to the
following rules.

* Quote characters are not recognized within words
  ("Do"Not"Separate" is parsed as the single word "Do"Not"Separate");

* Escape characters are not recognized;

* Enclosing characters in quotes preserve the literal value of all
  characters within the quotes;

* Closing quotes separate words (""Do"Separate" is parsed as ""Do""
  and "Separate");

* If "whitespace_split" is "False", any character not declared to be
  a word character, whitespace, or a quote will be returned as a
  single- character token. If it is "True", "shlex" will only split
  words in whitespaces;

* EOF is signaled with an empty string ("''");

* It’s not possible to parse empty strings, even if quoted.

When operating in POSIX mode, "shlex" will try to obey to the
following parsing rules.

* Quotes are stripped out, and do not separate words
  (""Do"Not"Separate"" is parsed as the single word "DoNotSeparate");

* Non-quoted escape characters (e.g. "'\'") preserve the literal
  value of the next character that follows;

* Enclosing characters in quotes which are not part of
  "escapedquotes" (e.g. ""'"") preserve the literal value of all
  characters within the quotes;

* Enclosing characters in quotes which are part of "escapedquotes"
  (e.g. "'"'") preserves the literal value of all characters within
  the quotes, with the exception of the characters mentioned in
  "escape".  The escape characters retain its special meaning only
  when followed by the quote in use, or the escape character itself.
  Otherwise the escape character will be considered a normal
  character.

* EOF is signaled with a "None" value;

* Quoted empty strings ("''") are allowed;
