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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

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<title>25.2. cmd — Support for line-oriented command interpreters — Python 3.7.0 說明文件</title>

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<h1>25.2. <a class="reference internal" href="#module-cmd" title="cmd: Build line-oriented command interpreters."><code class="xref py py-mod docutils literal notranslate">cmd</code></a> — Support for line-oriented command interpreters<a class="headerlink" href="#module-cmd" title="本標題的永久連結">¶</a></h1>

Source code: <a class="reference external" href="https://github.com/python/cpython/tree/3.7/Lib/cmd.py">Lib/cmd.py</a>

The <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code class="xref py py-class docutils literal notranslate">Cmd</code></a> 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 <code class="descclassname">cmd.</code><code class="descname">Cmd</code>(completekey='tab', stdin=None, stdout=None)<a class="headerlink" href="#cmd.Cmd" title="本定義的永久連結">¶</a></dt>

<dd>A <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code class="xref py py-class docutils literal notranslate">Cmd</code></a> instance or subclass instance is a line-oriented interpreter

framework. There is no good reason to instantiate <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code class="xref py py-class docutils literal notranslate">Cmd</code></a> itself; rather,

it’s useful as a superclass of an interpreter class you define yourself in order

to inherit <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code class="xref py py-class docutils literal notranslate">Cmd</code></a>’s methods and encapsulate action methods.

The optional argument completekey is the <a class="reference internal" href="readline.html#module-readline" title="readline: GNU readline support for Python. (Unix)"><code class="xref py py-mod docutils literal notranslate">readline</code></a> name of a completion

key; it defaults to <kbd class="kbd docutils literal notranslate">Tab</kbd>. If completekey is not <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a> and

<a class="reference internal" href="readline.html#module-readline" title="readline: GNU readline support for Python. (Unix)"><code class="xref py py-mod docutils literal notranslate">readline</code></a> 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 <a class="reference internal" href="sys.html#sys.stdin" title="sys.stdin"><code class="xref py py-data docutils literal notranslate">sys.stdin</code></a> and

<a class="reference internal" href="sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate">sys.stdout</code></a>.

If you want a given stdin to be used, make sure to set the instance’s

<a class="reference internal" href="#cmd.Cmd.use_rawinput" title="cmd.Cmd.use_rawinput"><code class="xref py py-attr docutils literal notranslate">use_rawinput</code></a> attribute to <code class="docutils literal notranslate">False</code>, otherwise stdin will be

ignored.

</dd></dl>

<h2>25.2.1. Cmd Objects<a class="headerlink" href="#cmd-objects" title="本標題的永久連結">¶</a></h2>

A <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code class="xref py py-class docutils literal notranslate">Cmd</code></a> instance has the following methods:

<code class="descclassname">Cmd.</code><code class="descname">cmdloop</code>(intro=None)<a class="headerlink" href="#cmd.Cmd.cmdloop" title="本定義的永久連結">¶</a></dt>

<dd>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 <a class="reference internal" href="#cmd.Cmd.intro" title="cmd.Cmd.intro"><code class="xref py py-attr docutils literal notranslate">intro</code></a> class attribute).

If the <a class="reference internal" href="readline.html#module-readline" title="readline: GNU readline support for Python. (Unix)"><code class="xref py py-mod docutils literal notranslate">readline</code></a> module is loaded, input will automatically inherit

bash-like history-list editing (e.g. <kbd class="kbd docutils literal notranslate">Control-P</kbd> scrolls back

to the last command, <kbd class="kbd docutils literal notranslate">Control-N</kbd> forward to the next one, <kbd class="kbd docutils literal notranslate">Control-F</kbd>

moves the cursor to the right non-destructively, <kbd class="kbd docutils literal notranslate">Control-B</kbd> moves the

cursor to the left non-destructively, etc.).

An end-of-file on input is passed back as the string <code class="docutils literal notranslate">'EOF'</code>.

An interpreter instance will recognize a command name <code class="docutils literal notranslate">foo</code> if and only if it

has a method <code class="xref py py-meth docutils literal notranslate">do_foo()</code>. As a special case, a line beginning with the

character <code class="docutils literal notranslate">'?'</code> is dispatched to the method <code class="xref py py-meth docutils literal notranslate">do_help()</code>. As another

special case, a line beginning with the character <code class="docutils literal notranslate">'!'</code> is dispatched to the

method <code class="xref py py-meth docutils literal notranslate">do_shell()</code> (if such a method is defined).

This method will return when the <a class="reference internal" href="#cmd.Cmd.postcmd" title="cmd.Cmd.postcmd"><code class="xref py py-meth docutils literal notranslate">postcmd()</code></a> method returns a true value.

The stop argument to <a class="reference internal" href="#cmd.Cmd.postcmd" title="cmd.Cmd.postcmd"><code class="xref py py-meth docutils literal notranslate">postcmd()</code></a> is the return value from the command’s

corresponding <code class="xref py py-meth docutils literal notranslate">do_*()</code> method.

If completion is enabled, completing commands will be done automatically, and

completing of commands args is done by calling <code class="xref py py-meth docutils literal notranslate">complete_foo()</code> 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.

All subclasses of <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code class="xref py py-class docutils literal notranslate">Cmd</code></a> inherit a predefined <code class="xref py py-meth docutils literal notranslate">do_help()</code>. This

method, called with an argument <code class="docutils literal notranslate">'bar'</code>, invokes the corresponding method

<code class="xref py py-meth docutils literal notranslate">help_bar()</code>, and if that is not present, prints the docstring of

<code class="xref py py-meth docutils literal notranslate">do_bar()</code>, if available. With no argument, <code class="xref py py-meth docutils literal notranslate">do_help()</code> lists all

available help topics (that is, all commands with corresponding

<code class="xref py py-meth docutils literal notranslate">help_*()</code> methods or commands that have docstrings), and also lists any

undocumented commands.

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">onecmd</code>(str)<a class="headerlink" href="#cmd.Cmd.onecmd" title="本定義的永久連結">¶</a></dt>

<dd>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

<a class="reference internal" href="#cmd.Cmd.precmd" title="cmd.Cmd.precmd"><code class="xref py py-meth docutils literal notranslate">precmd()</code></a> and <a class="reference internal" href="#cmd.Cmd.postcmd" title="cmd.Cmd.postcmd"><code class="xref py py-meth docutils literal notranslate">postcmd()</code></a> 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 <code class="xref py py-meth docutils literal notranslate">do_*()</code> method for the command

str, the return value of that method is returned, otherwise the return value

from the <a class="reference internal" href="#cmd.Cmd.default" title="cmd.Cmd.default"><code class="xref py py-meth docutils literal notranslate">default()</code></a> method is returned.

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">emptyline</code>()<a class="headerlink" href="#cmd.Cmd.emptyline" title="本定義的永久連結">¶</a></dt>

<dd>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.

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">default</code>(line)<a class="headerlink" href="#cmd.Cmd.default" title="本定義的永久連結">¶</a></dt>

<dd>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.

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">completedefault</code>(text, line, begidx, endidx)<a class="headerlink" href="#cmd.Cmd.completedefault" title="本定義的永久連結">¶</a></dt>

<dd>Method called to complete an input line when no command-specific

<code class="xref py py-meth docutils literal notranslate">complete_*()</code> method is available. By default, it returns an empty list.

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">precmd</code>(line)<a class="headerlink" href="#cmd.Cmd.precmd" title="本定義的永久連結">¶</a></dt>

<dd>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

<a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code class="xref py py-class docutils literal notranslate">Cmd</code></a>; it exists to be overridden by subclasses. The return value is

used as the command which will be executed by the <a class="reference internal" href="#cmd.Cmd.onecmd" title="cmd.Cmd.onecmd"><code class="xref py py-meth docutils literal notranslate">onecmd()</code></a> method; the

unchanged.

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">postcmd</code>(stop, line)<a class="headerlink" href="#cmd.Cmd.postcmd" title="本定義的永久連結">¶</a></dt>

<dd>Hook method executed just after a command dispatch is finished. This method is

a stub in <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code class="xref py py-class docutils literal notranslate">Cmd</code></a>; 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 <a class="reference internal" href="#cmd.Cmd.postcmd" title="cmd.Cmd.postcmd"><code class="xref py py-meth docutils literal notranslate">postcmd()</code></a>; this will be the

return value of the <a class="reference internal" href="#cmd.Cmd.onecmd" title="cmd.Cmd.onecmd"><code class="xref py py-meth docutils literal notranslate">onecmd()</code></a> 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.

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">preloop</code>()<a class="headerlink" href="#cmd.Cmd.preloop" title="本定義的永久連結">¶</a></dt>

<dd>Hook method executed once when <a class="reference internal" href="#cmd.Cmd.cmdloop" title="cmd.Cmd.cmdloop"><code class="xref py py-meth docutils literal notranslate">cmdloop()</code></a> is called. This method is a stub

in <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code class="xref py py-class docutils literal notranslate">Cmd</code></a>; it exists to be overridden by subclasses.

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">postloop</code>()<a class="headerlink" href="#cmd.Cmd.postloop" title="本定義的永久連結">¶</a></dt>

is a stub in <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code class="xref py py-class docutils literal notranslate">Cmd</code></a>; it exists to be overridden by subclasses.

</dd></dl>

Instances of <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code class="xref py py-class docutils literal notranslate">Cmd</code></a> subclasses have some public instance variables:

<code class="descclassname">Cmd.</code><code class="descname">prompt</code><a class="headerlink" href="#cmd.Cmd.prompt" title="本定義的永久連結">¶</a></dt>

<dd>The prompt issued to solicit input.

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">identchars</code><a class="headerlink" href="#cmd.Cmd.identchars" title="本定義的永久連結">¶</a></dt>

<dd>The string of characters accepted for the command prefix.

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">lastcmd</code><a class="headerlink" href="#cmd.Cmd.lastcmd" title="本定義的永久連結">¶</a></dt>

<dd>The last nonempty command prefix seen.

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">cmdqueue</code><a class="headerlink" href="#cmd.Cmd.cmdqueue" title="本定義的永久連結">¶</a></dt>

<dd>A list of queued input lines. The cmdqueue list is checked in

<a class="reference internal" href="#cmd.Cmd.cmdloop" title="cmd.Cmd.cmdloop"><code class="xref py py-meth docutils literal notranslate">cmdloop()</code></a> when new input is needed; if it is nonempty, its elements

will be processed in order, as if entered at the prompt.

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">intro</code><a class="headerlink" href="#cmd.Cmd.intro" title="本定義的永久連結">¶</a></dt>

<dd>A string to issue as an intro or banner. May be overridden by giving the

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">doc_header</code><a class="headerlink" href="#cmd.Cmd.doc_header" title="本定義的永久連結">¶</a></dt>

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

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">misc_header</code><a class="headerlink" href="#cmd.Cmd.misc_header" title="本定義的永久連結">¶</a></dt>

<dd>The header to issue if the help output has a section for miscellaneous help

topics (that is, there are <code class="xref py py-meth docutils literal notranslate">help_*()</code> methods without corresponding

<code class="xref py py-meth docutils literal notranslate">do_*()</code> methods).

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">undoc_header</code><a class="headerlink" href="#cmd.Cmd.undoc_header" title="本定義的永久連結">¶</a></dt>

<dd>The header to issue if the help output has a section for undocumented commands

(that is, there are <code class="xref py py-meth docutils literal notranslate">do_*()</code> methods without corresponding <code class="xref py py-meth docutils literal notranslate">help_*()</code>

methods).

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">ruler</code><a class="headerlink" href="#cmd.Cmd.ruler" title="本定義的永久連結">¶</a></dt>

<dd>The character used to draw separator lines under the help-message headers. If

empty, no ruler line is drawn. It defaults to <code class="docutils literal notranslate">'='</code>.

</dd></dl>

<code class="descclassname">Cmd.</code><code class="descname">use_rawinput</code><a class="headerlink" href="#cmd.Cmd.use_rawinput" title="本定義的永久連結">¶</a></dt>

<dd>A flag, defaulting to true. If true, <a class="reference internal" href="#cmd.Cmd.cmdloop" title="cmd.Cmd.cmdloop"><code class="xref py py-meth docutils literal notranslate">cmdloop()</code></a> uses <a class="reference internal" href="functions.html#input" title="input"><code class="xref py py-func docutils literal notranslate">input()</code></a> to

display a prompt and read the next command; if false, <code class="xref py py-meth docutils literal notranslate">sys.stdout.write()</code>

and <code class="xref py py-meth docutils literal notranslate">sys.stdin.readline()</code> are used. (This means that by importing

<a class="reference internal" href="readline.html#module-readline" title="readline: GNU readline support for Python. (Unix)"><code class="xref py py-mod docutils literal notranslate">readline</code></a>, on systems that support it, the interpreter will automatically

support Emacs-like line editing and command-history keystrokes.)

</dd></dl>

</div>

<h2>25.2.2. Cmd Example<a class="headerlink" href="#cmd-example" title="本標題的永久連結">¶</a></h2>

The <a class="reference internal" href="#module-cmd" title="cmd: Build line-oriented command interpreters."><code class="xref py py-mod docutils literal notranslate">cmd</code></a> 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 <a class="reference internal" href="turtle.html#module-turtle" title="turtle: An educational framework for simple graphics applications"><code class="xref py py-mod docutils literal notranslate">turtle</code></a> module.

Basic turtle commands such as <a class="reference internal" href="turtle.html#turtle.forward" title="turtle.forward"><code class="xref py py-meth docutils literal notranslate">forward()</code></a> are added to a

<a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code class="xref py py-class docutils literal notranslate">Cmd</code></a> subclass with method named <code class="xref py py-meth docutils literal notranslate">do_forward()</code>. 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 <a class="reference internal" href="#cmd.Cmd.precmd" title="cmd.Cmd.precmd"><code class="xref py py-meth docutils literal notranslate">precmd()</code></a> method which is responsible for converting the input to

lowercase and writing the commands to a file. The <code class="xref py py-meth docutils literal notranslate">do_playback()</code> method

reads the file and adds the recorded commands to the <code class="xref py py-attr docutils literal notranslate">cmdqueue</code> for

immediate playback:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>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()

</pre></div>

</div>

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:

<div class="highlight-none notranslate"><div class="highlight"><pre>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

</pre></div>

</div>

<ul>

<li><a class="reference internal" href="#">25.2. <code class="docutils literal notranslate">cmd</code> — Support for line-oriented command interpreters</a><ul>

<li><a class="reference internal" href="#cmd-objects">25.2.1. Cmd Objects</a></li>

<li><a class="reference internal" href="#cmd-example">25.2.2. Cmd Example</a></li>

</ul>

</li>

</ul>

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title="上一章">25.1. <code class="docutils literal notranslate">turtle</code> — 龜圖學</a>

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Last updated on 8月 22, 2018.

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