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gh-144345: Clarify subtle behaviors of cmd.Cmd in documentation #144383

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325 changes: 18 additions & 307 deletions Doc/library/cmd.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -45,6 +45,24 @@ interface.
.. versionchanged:: 3.13
``completekey='tab'`` is replaced by ``'^I'`` for ``editline``.

.. note::

Subtle behaviors of ``cmd.Cmd``:

* Command handler methods (``do_<command>``) should return ``True`` to
indicate that the command loop should terminate. Any other return
value continues the loop.

* If the user presses Enter on an empty line, the default behavior is
to repeat the last nonempty command entered. This can be disabled by
overriding :meth:`emptyline`.

* If no matching ``do_<command>`` method is found, the
:meth:`default` method is called.

* Exceptions raised inside command handlers are not caught by default
and will terminate the command loop unless handled explicitly.


.. _cmd-objects:

Expand Down Expand Up @@ -92,310 +110,3 @@ A :class:`Cmd` instance has the following methods:
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.


.. method:: Cmd.do_help(arg)

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


.. method:: 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
:meth:`precmd` and :meth:`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 :meth:`!do_\*` method for the command
*str*, the return value of that method is returned, otherwise the return value
from the :meth:`default` method is returned.


.. method:: 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.


.. method:: 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.


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

Method called to complete an input line when no command-specific
:meth:`!complete_\*` method is available. By default, it returns an empty list.


.. method:: 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.


.. method:: 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
:class:`Cmd`; it exists to be overridden by subclasses. The return value is
used as the command which will be executed by the :meth:`onecmd` method; the
:meth:`precmd` implementation may re-write the command or simply return *line*
unchanged.


.. method:: Cmd.postcmd(stop, line)

Hook method executed just after a command dispatch is finished. This method is
a stub in :class:`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 :meth:`postcmd`; this will be the
return value of the :meth:`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.


.. method:: Cmd.preloop()

Hook method executed once when :meth:`cmdloop` is called. This method is a stub
in :class:`Cmd`; it exists to be overridden by subclasses.


.. method:: Cmd.postloop()

Hook method executed once when :meth:`cmdloop` is about to return. This method
is a stub in :class:`Cmd`; it exists to be overridden by subclasses.


Instances of :class:`Cmd` subclasses have some public instance variables:

.. attribute:: Cmd.prompt

The prompt issued to solicit input.


.. attribute:: Cmd.identchars

The string of characters accepted for the command prefix.


.. attribute:: Cmd.lastcmd

The last nonempty command prefix seen.


.. attribute:: Cmd.cmdqueue

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


.. attribute:: Cmd.intro

A string to issue as an intro or banner. May be overridden by giving the
:meth:`cmdloop` method an argument.


.. attribute:: Cmd.doc_header

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


.. attribute:: Cmd.misc_header

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


.. attribute:: Cmd.undoc_header

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


.. attribute:: Cmd.ruler

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


.. attribute:: Cmd.use_rawinput

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


.. _cmd-example:

Cmd Example
-----------

.. sectionauthor:: Raymond Hettinger <python at rcn dot com>

The :mod:`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 :mod:`turtle` module.

Basic turtle commands such as :meth:`~turtle.forward` are added to a
:class:`Cmd` subclass with method named :meth:`!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 :meth:`~Cmd.precmd` method which is responsible for converting the input to
lowercase and writing the commands to a file. The :meth:`!do_playback` method
reads the file and adds the recorded commands to the :attr:`~Cmd.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:

.. code-block:: none

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