"cmd" --- 以列為導向的指令直譯器支援
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**原始碼：**Lib/cmd.py

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

The "Cmd" class provides a simple framework for writing line-oriented
command interpreters.  These are often useful for test harnesses,
administrative tools, and prototypes that will later be wrapped in a
more sophisticated interface.

class cmd.Cmd(completekey='tab', stdin=None, stdout=None)

   A "Cmd" instance or subclass instance is a line-oriented
   interpreter framework.  There is no good reason to instantiate
   "Cmd" itself; rather, it's useful as a superclass of an interpreter
   class you define yourself in order to inherit "Cmd"'s methods and
   encapsulate action methods.

   The optional argument *completekey* is the "readline" name of a
   completion key; it defaults to "Tab". If *completekey* is not
   "None" and "readline" is available, command completion is done
   automatically.

   The optional arguments *stdin* and *stdout* specify the  input and
   output file objects that the Cmd instance or subclass  instance
   will use for input and output. If not specified, they will default
   to "sys.stdin" and "sys.stdout".

   If you want a given *stdin* to be used, make sure to set the
   instance's "use_rawinput" attribute to "False", otherwise *stdin*
   will be ignored.


Cmd 物件
========

A "Cmd" instance has the following methods:

Cmd.cmdloop(intro=None)

   Repeatedly issue a prompt, accept input, parse an initial prefix
   off the received input, and dispatch to action methods, passing
   them the remainder of the line as argument.

   The optional argument is a banner or intro string to be issued
   before the first prompt (this overrides the "intro" class
   attribute).

   If the "readline" module is loaded, input will automatically
   inherit **bash**-like history-list editing (e.g. "Control"-"P"
   scrolls back to the last command, "Control"-"N" forward to the next
   one, "Control"-"F" moves the cursor to the right non-destructively,
   "Control"-"B" moves the cursor to the left non-destructively,
   etc.).

   An end-of-file on input is passed back as the string "'EOF'".

   An interpreter instance will recognize a command name "foo" if and
   only if it has a method "do_foo()".  As a special case, a line
   beginning with the character "'?'" is dispatched to the method
   "do_help()".  As another special case, a line beginning with the
   character "'!'" is dispatched to the method "do_shell()" (if such a
   method is defined).

   This method will return when the "postcmd()" method returns a true
   value. The *stop* argument to "postcmd()" is the return value from
   the command's corresponding "do_*()" method.

   If completion is enabled, completing commands will be done
   automatically, and completing of commands args is done by calling
   "complete_foo()" with arguments *text*, *line*, *begidx*, and
   *endidx*.  *text* is the string prefix we are attempting to match:
   all returned matches must begin with it. *line* is the current
   input line with leading whitespace removed, *begidx* and *endidx*
   are the beginning and ending indexes of the prefix text, which
   could be used to provide different completion depending upon which
   position the argument is in.

Cmd.do_help(arg)

   All subclasses of "Cmd" inherit a predefined "do_help()".  This
   method, called with an argument "'bar'", invokes the corresponding
   method "help_bar()", and if that is not present, prints the
   docstring of "do_bar()", if available.  With no argument,
   "do_help()" lists all available help topics (that is, all commands
   with corresponding "help_*()" methods or commands that have
   docstrings), and also lists any undocumented commands.

Cmd.onecmd(str)

   Interpret the argument as though it had been typed in response to
   the prompt. This may be overridden, but should not normally need to
   be; see the "precmd()" and "postcmd()" methods for useful execution
   hooks.  The return value is a flag indicating whether
   interpretation of commands by the interpreter should stop.  If
   there is a "do_*()" method for the command *str*, the return value
   of that method is returned, otherwise the return value from the
   "default()" method is returned.

Cmd.emptyline()

   Method called when an empty line is entered in response to the
   prompt. If this method is not overridden, it repeats the last
   nonempty command entered.

Cmd.default(line)

   Method called on an input line when the command prefix is not
   recognized. If this method is not overridden, it prints an error
   message and returns.

Cmd.completedefault(text, line, begidx, endidx)

   Method called to complete an input line when no command-specific
   "complete_*()" method is available.  By default, it returns an
   empty list.

Cmd.columnize(list, displaywidth=80)

   Method called to display a list of strings as a compact set of
   columns. Each column is only as wide as necessary. Columns are
   separated by two spaces for readability.

Cmd.precmd(line)

   Hook method executed just before the command line *line* is
   interpreted, but after the input prompt is generated and issued.
   This method is a stub in "Cmd"; it exists to be overridden by
   subclasses.  The return value is used as the command which will be
   executed by the "onecmd()" method; the "precmd()" implementation
   may re-write the command or simply return *line* unchanged.

Cmd.postcmd(stop, line)

   Hook method executed just after a command dispatch is finished.
   This method is a stub in "Cmd"; it exists to be overridden by
   subclasses.  *line* is the command line which was executed, and
   *stop* is a flag which indicates whether execution will be
   terminated after the call to "postcmd()"; this will be the return
   value of the "onecmd()" method.  The return value of this method
   will be used as the new value for the internal flag which
   corresponds to *stop*; returning false will cause interpretation to
   continue.

Cmd.preloop()

   Hook method executed once when "cmdloop()" is called.  This method
   is a stub in "Cmd"; it exists to be overridden by subclasses.

Cmd.postloop()

   Hook method executed once when "cmdloop()" is about to return. This
   method is a stub in "Cmd"; it exists to be overridden by
   subclasses.

Instances of "Cmd" subclasses have some public instance variables:

Cmd.prompt

   The prompt issued to solicit input.

Cmd.identchars

   The string of characters accepted for the command prefix.

Cmd.lastcmd

   The last nonempty command prefix seen.

Cmd.cmdqueue

   A list of queued input lines.  The cmdqueue list is checked in
   "cmdloop()" when new input is needed; if it is nonempty, its
   elements will be processed in order, as if entered at the prompt.

Cmd.intro

   A string to issue as an intro or banner.  May be overridden by
   giving the "cmdloop()" method an argument.

Cmd.doc_header

   The header to issue if the help output has a section for documented
   commands.

Cmd.misc_header

   The header to issue if the help output has a section for
   miscellaneous  help topics (that is, there are "help_*()" methods
   without corresponding "do_*()" methods).

Cmd.undoc_header

   The header to issue if the help output has a section for
   undocumented  commands (that is, there are "do_*()" methods without
   corresponding "help_*()" methods).

Cmd.ruler

   The character used to draw separator lines under the help-message
   headers.  If empty, no ruler line is drawn.  It defaults to "'='".

Cmd.use_rawinput

   A flag, defaulting to true.  If true, "cmdloop()" uses "input()" to
   display a prompt and read the next command; if false,
   "sys.stdout.write()" and "sys.stdin.readline()" are used. (This
   means that by importing "readline", on systems that support it, the
   interpreter will automatically support **Emacs**-like line editing
   and command-history keystrokes.)


Cmd Example
===========

The "cmd" module is mainly useful for building custom shells that let
a user work with a program interactively.

This section presents a simple example of how to build a shell around
a few of the commands in the "turtle" module.

Basic turtle commands such as "forward()" are added to a "Cmd"
subclass with method named "do_forward()".  The argument is converted
to a number and dispatched to the turtle module.  The docstring is
used in the help utility provided by the shell.

The example also includes a basic record and playback facility
implemented with the "precmd()" method which is responsible for
converting the input to lowercase and writing the commands to a file.
The "do_playback()" method reads the file and adds the recorded
commands to the "cmdqueue" for immediate playback:

   import cmd, sys
   from turtle import *

   class TurtleShell(cmd.Cmd):
       intro = 'Welcome to the turtle shell.   Type help or ? to list commands.\n'
       prompt = '(turtle) '
       file = None

       # ----- basic turtle commands -----
       def do_forward(self, arg):
           'Move the turtle forward by the specified distance:  FORWARD 10'
           forward(*parse(arg))
       def do_right(self, arg):
           'Turn turtle right by given number of degrees:  RIGHT 20'
           right(*parse(arg))
       def do_left(self, arg):
           'Turn turtle left by given number of degrees:  LEFT 90'
           left(*parse(arg))
       def do_goto(self, arg):
           'Move turtle to an absolute position with changing orientation.  GOTO 100 200'
           goto(*parse(arg))
       def do_home(self, arg):
           'Return turtle to the home position:  HOME'
           home()
       def do_circle(self, arg):
           'Draw circle with given radius an options extent and steps:  CIRCLE 50'
           circle(*parse(arg))
       def do_position(self, arg):
           'Print the current turtle position:  POSITION'
           print('Current position is %d %d\n' % position())
       def do_heading(self, arg):
           'Print the current turtle heading in degrees:  HEADING'
           print('Current heading is %d\n' % (heading(),))
       def do_color(self, arg):
           'Set the color:  COLOR BLUE'
           color(arg.lower())
       def do_undo(self, arg):
           'Undo (repeatedly) the last turtle action(s):  UNDO'
       def do_reset(self, arg):
           'Clear the screen and return turtle to center:  RESET'
           reset()
       def do_bye(self, arg):
           'Stop recording, close the turtle window, and exit:  BYE'
           print('Thank you for using Turtle')
           self.close()
           bye()
           return True

       # ----- record and playback -----
       def do_record(self, arg):
           'Save future commands to filename:  RECORD rose.cmd'
           self.file = open(arg, 'w')
       def do_playback(self, arg):
           'Playback commands from a file:  PLAYBACK rose.cmd'
           self.close()
           with open(arg) as f:
               self.cmdqueue.extend(f.read().splitlines())
       def precmd(self, line):
           line = line.lower()
           if self.file and 'playback' not in line:
               print(line, file=self.file)
           return line
       def close(self):
           if self.file:
               self.file.close()
               self.file = None

   def parse(arg):
       'Convert a series of zero or more numbers to an argument tuple'
       return tuple(map(int, arg.split()))

   if __name__ == '__main__':
       TurtleShell().cmdloop()

Here is a sample session with the turtle shell showing the help
functions, using blank lines to repeat commands, and the simple record
and playback facility:

   Welcome to the turtle shell.   Type help or ? to list commands.

   (turtle) ?

   Documented commands (type help <topic>):
   ========================================
   bye     color    goto     home  playback  record  right
   circle  forward  heading  left  position  reset   undo

   (turtle) help forward
   Move the turtle forward by the specified distance:  FORWARD 10
   (turtle) record spiral.cmd
   (turtle) position
   Current position is 0 0

   (turtle) heading
   Current heading is 0

   (turtle) reset
   (turtle) circle 20
   (turtle) right 30
   (turtle) circle 40
   (turtle) right 30
   (turtle) circle 60
   (turtle) right 30
   (turtle) circle 80
   (turtle) right 30
   (turtle) circle 100
   (turtle) right 30
   (turtle) circle 120
   (turtle) right 30
   (turtle) circle 120
   (turtle) heading
   Current heading is 180

   (turtle) forward 100
   (turtle)
   (turtle) right 90
   (turtle) forward 100
   (turtle)
   (turtle) right 90
   (turtle) forward 400
   (turtle) right 90
   (turtle) forward 500
   (turtle) right 90
   (turtle) forward 400
   (turtle) right 90
   (turtle) forward 300
   (turtle) playback spiral.cmd
   Current position is 0 0

   Current heading is 0

   Current heading is 180

   (turtle) bye
   Thank you for using Turtle
