Doc/lib/libcmd.tex

   1 \section{\module{cmd} ---
   2          Support for line-oriented command interpreters}
   3
   4 \declaremodule{standard}{cmd}
   5 \sectionauthor{Eric S. Raymond}{esr@snark.thyrsus.com}
   6 \modulesynopsis{Build line-oriented command interpreters.}
   7
   8
   9 The \class{Cmd} class provides a simple framework for writing
  10 line-oriented command interpreters.  These are often useful for
  11 test harnesses, administrative tools, and prototypes that will
  12 later be wrapped in a more sophisticated interface.
  13
  14 \begin{classdesc}{Cmd}{\optional{completekey\optional{,
  15                        stdin\optional{, stdout}}}}
  16 A \class{Cmd} instance or subclass instance is a line-oriented
  17 interpreter framework.  There is no good reason to instantiate
  18 \class{Cmd} itself; rather, it's useful as a superclass of an
  19 interpreter class you define yourself in order to inherit
  20 \class{Cmd}'s methods and encapsulate action methods.
  21
  22 The optional argument \var{completekey} is the \refmodule{readline} name
  23 of a completion key; it defaults to \kbd{Tab}. If \var{completekey} is
  24 not \constant{None} and \refmodule{readline} is available, command completion
  25 is done automatically.
  26
  27 The optional arguments \var{stdin} and \var{stdout} specify the
  28 input and output file objects that the Cmd instance or subclass
  29 instance will use for input and output. If not specified, they
  30 will default to \var{sys.stdin} and \var{sys.stdout}.
  31
  32 \versionchanged[The \var{stdin} and \var{stdout} parameters were added]{2.3}
  33 \end{classdesc}
  34
  35 \subsection{Cmd Objects}
  36 \label{Cmd-objects}
  37
  38 A \class{Cmd} instance has the following methods:
  39
  40 \begin{methoddesc}{cmdloop}{\optional{intro}}
  41 Repeatedly issue a prompt, accept input, parse an initial prefix off
  42 the received input, and dispatch to action methods, passing them the
  43 remainder of the line as argument.
  44
  45 The optional argument is a banner or intro string to be issued before the
  46 first prompt (this overrides the \member{intro} class member).
  47
  48 If the \refmodule{readline} module is loaded, input will automatically
  49 inherit \program{bash}-like history-list editing (e.g. \kbd{Control-P}
  50 scrolls back to the last command, \kbd{Control-N} forward to the next
  51 one, \kbd{Control-F} moves the cursor to the right non-destructively,
  52 \kbd{Control-B} moves the cursor to the left non-destructively, etc.).
  53
  54 An end-of-file on input is passed back as the string \code{'EOF'}.
  55
  56 An interpreter instance will recognize a command name \samp{foo} if
  57 and only if it has a method \method{do_foo()}.  As a special case,
  58 a line beginning with the character \character{?} is dispatched to
  59 the method \method{do_help()}.  As another special case, a line
  60 beginning with the character \character{!} is dispatched to the
  61 method \method{do_shell()} (if such a method is defined).
  62
  63 This method will return when the \method{postcmd()} method returns a
  64 true value.  The \var{stop} argument to \method{postcmd()} is the
  65 return value from the command's corresponding \method{do_*()} method.
  66
  67 If completion is enabled, completing commands will be done
  68 automatically, and completing of commands args is done by calling
  69 \method{complete_foo()} with arguments \var{text}, \var{line},
  70 \var{begidx}, and \var{endidx}.  \var{text} is the string prefix we
  71 are attempting to match: all returned matches must begin with it.
  72 \var{line} is the current input line with leading whitespace removed,
  73 \var{begidx} and \var{endidx} are the beginning and ending indexes
  74 of the prefix text, which could be used to provide different
  75 completion depending upon which position the argument is in.
  76
  77 All subclasses of \class{Cmd} inherit a predefined \method{do_help()}.
  78 This method, called with an argument \code{'bar'}, invokes the
  79 corresponding method \method{help_bar()}.  With no argument,
  80 \method{do_help()} lists all available help topics (that is, all
  81 commands with corresponding \method{help_*()} methods), and also lists
  82 any undocumented commands.
  83 \end{methoddesc}
  84
  85 \begin{methoddesc}{onecmd}{str}
  86 Interpret the argument as though it had been typed in response to the
  87 prompt.  This may be overridden, but should not normally need to be;
  88 see the \method{precmd()} and \method{postcmd()} methods for useful
  89 execution hooks.  The return value is a flag indicating whether
  90 interpretation of commands by the interpreter should stop.  If there
  91 is a \method{do_*()} method for the command \var{str}, the return
  92 value of that method is returned, otherwise the return value from the
  93 \method{default()} method is returned.
  94 \end{methoddesc}
  95
  96 \begin{methoddesc}{emptyline}{}
  97 Method called when an empty line is entered in response to the prompt.
  98 If this method is not overridden, it repeats the last nonempty command
  99 entered.
 100 \end{methoddesc}
 101
 102 \begin{methoddesc}{default}{line}
 103 Method called on an input line when the command prefix is not
 104 recognized. If this method is not overridden, it prints an
 105 error message and returns.
 106 \end{methoddesc}
 107
 108 \begin{methoddesc}{completedefault}{text, line, begidx, endidx}
 109 Method called to complete an input line when no command-specific
 110 \method{complete_*()} method is available.  By default, it returns an
 111 empty list.
 112 \end{methoddesc}
 113
 114 \begin{methoddesc}{precmd}{line}
 115 Hook method executed just before the command line \var{line} is
 116 interpreted, but after the input prompt is generated and issued.  This
 117 method is a stub in \class{Cmd}; it exists to be overridden by
 118 subclasses.  The return value is used as the command which will be
 119 executed by the \method{onecmd()} method; the \method{precmd()}
 120 implementation may re-write the command or simply return \var{line}
 121 unchanged.
 122 \end{methoddesc}
 123
 124 \begin{methoddesc}{postcmd}{stop, line}
 125 Hook method executed just after a command dispatch is finished.  This
 126 method is a stub in \class{Cmd}; it exists to be overridden by
 127 subclasses.  \var{line} is the command line which was executed, and
 128 \var{stop} is a flag which indicates whether execution will be
 129 terminated after the call to \method{postcmd()}; this will be the
 130 return value of the \method{onecmd()} method.  The return value of
 131 this method will be used as the new value for the internal flag which
 132 corresponds to \var{stop}; returning false will cause interpretation
 133 to continue.
 134 \end{methoddesc}
 135
 136 \begin{methoddesc}{preloop}{}
 137 Hook method executed once when \method{cmdloop()} is called.  This
 138 method is a stub in \class{Cmd}; it exists to be overridden by
 139 subclasses.
 140 \end{methoddesc}
 141
 142 \begin{methoddesc}{postloop}{}
 143 Hook method executed once when \method{cmdloop()} is about to return.
 144 This method is a stub in \class{Cmd}; it exists to be overridden by
 145 subclasses.
 146 \end{methoddesc}
 147
 148 Instances of \class{Cmd} subclasses have some public instance variables:
 149
 150 \begin{memberdesc}{prompt}
 151 The prompt issued to solicit input.
 152 \end{memberdesc}
 153
 154 \begin{memberdesc}{identchars}
 155 The string of characters accepted for the command prefix.
 156 \end{memberdesc}
 157
 158 \begin{memberdesc}{lastcmd}
 159 The last nonempty command prefix seen.
 160 \end{memberdesc}
 161
 162 \begin{memberdesc}{intro}
 163 A string to issue as an intro or banner.  May be overridden by giving
 164 the \method{cmdloop()} method an argument.
 165 \end{memberdesc}
 166
 167 \begin{memberdesc}{doc_header}
 168 The header to issue if the help output has a section for documented
 169 commands.
 170 \end{memberdesc}
 171
 172 \begin{memberdesc}{misc_header}
 173 The header to issue if the help output has a section for miscellaneous
 174 help topics (that is, there are \method{help_*()} methods without
 175 corresponding \method{do_*()} methods).
 176 \end{memberdesc}
 177
 178 \begin{memberdesc}{undoc_header}
 179 The header to issue if the help output has a section for undocumented
 180 commands (that is, there are \method{do_*()} methods without
 181 corresponding \method{help_*()} methods).
 182 \end{memberdesc}
 183
 184 \begin{memberdesc}{ruler}
 185 The character used to draw separator lines under the help-message
 186 headers.  If empty, no ruler line is drawn.  It defaults to
 187 \character{=}.
 188 \end{memberdesc}
 189
 190 \begin{memberdesc}{use_rawinput}
 191 A flag, defaulting to true.  If true, \method{cmdloop()} uses
 192 \function{raw_input()} to display a prompt and read the next command;
 193 if false, \method{sys.stdout.write()} and
 194 \method{sys.stdin.readline()} are used. (This means that by
 195 importing \refmodule{readline}, on systems that support it, the
 196 interpreter will automatically support \program{Emacs}-like line editing
 197 and command-history keystrokes.)
 198 \end{memberdesc}