Improved by Eric Raymond.

Lib/cmd.py

 # A generic class to build line-oriented command interpreters
+#
+# Interpreters constructed with this class obey the following conventions:
+#
+# 1. End of file on input is processed as the command 'EOF'.
+# 2. A command is parsed out of each line by collecting the prefix composed
+#    of characters in the identchars member.
+# 3. A command `foo' is dispatched to a method 'do_foo()'; the do_ method
+#    is passed a single argument consisting of the remainder of the line.
+# 4. Typing an empty line repeats the last command.  (Actually, it calls the
+#    method `emptyline', which may be overridden in a subclass.)
+# 5. There is a predefined `help' method.  Given an argument `topic', it
+#    calls the command `help_topic'.  With no arguments, it lists all topics
+#    with defined help_ functions, broken into up to three topics; documented
+#    commands, miscellaneous help topics, and undocumented commands.
+# 6. The command '?' is a synonym for `help'.  The command '!' is a synonym
+#    for `shell', if a do_shell method exists.
+#
+# The `default' method may be overridden to intercept commands for which there
+# is no do_ method.
+#
+# The data member `self.ruler' sets the character used to draw separator lines
+# in the help messages.  If empty, no ruler line is drawn.  It defaults to "=".
+#
+# If the value of `self.intro' is nonempty when the cmdloop method is called,
+# it is printed out on interpreter startup.  This value may be overridden
+# via an optional argument to the cmdloop() method.
+#
+# The data members `self.doc_header', `self.misc_header', and
+# `self.undoc_header' set the headers used for the help function's
+# listings of documented functions, miscellaneous topics, and undocumented
+# functions respectively.
+#
+# These interpreters use raw_input; thus, if the readline module is loaded,
+# they automatically support Emacs-like command history and editing features.
+#
 
 import string
 import sys
@@ -8,26 +43,57 @@ PROMPT = '(Cmd) '
 IDENTCHARS = string.letters + string.digits + '_'
 
 class Cmd:
+	prompt = PROMPT
+	identchars = IDENTCHARS
+	ruler = '='
+	lastcmd = ''
+	intro = None
+	doc_header = "Documented commands (type help <topic>):"
+	misc_header = "Miscellaneous help topics:"
+	undoc_header = "Undocumented commands:"
 
-	def __init__(self):
-		self.prompt = PROMPT
-		self.identchars = IDENTCHARS
-		self.lastcmd = ''
-
-	def cmdloop(self):
+	def cmdloop(self, intro=None):
+		self.preloop()
+		if intro != None:
+			self.intro = intro
+		if self.intro:
+			print self.intro
 		stop = None
 		while not stop:
 			try:
 				line = raw_input(self.prompt)
 			except EOFError:
 				line = 'EOF'
+			self.precmd()
 			stop = self.onecmd(line)
+			self.postcmd()
+		self.postloop()
+
+	def precmd(self):
+		pass
+
+	def postcmd(self):
+		pass
+
+	def preloop(self):
+		pass
+
+	def postloop(self):
+		pass
 
 	def onecmd(self, line):
 		line = string.strip(line)
-		if not line:
-			line = self.lastcmd
+		if line == '?':
+			line = 'help'
+		elif line == '!':
+			if hasattr(self, 'do_shell'):
+				line = 'shell'
 			else:
+				self.default(line)
+				return
+		elif not line:
+			self.emptyline()
+			return
 		self.lastcmd = line
 		i, n = 0, len(line)
 		while i < n and line[i] in self.identchars: i = i+1
@@ -41,6 +107,9 @@ class Cmd:
 				return self.default(line)
 			return func(arg)
 
+	def emptyline(self):
+		return self.onecmd(self.lastcmd)
+
 	def default(self, line):
 		print '*** Unknown syntax:', line
 
@@ -70,17 +139,15 @@ class Cmd:
 					else:
 						cmds_undoc.append(cmd)
 			print 
-			self.print_topics("Documented commands (type help " \
-		                          "<topic>):",cmds_doc, 15, 80)
-			self.print_topics("Miscellaneous help topics:",
-		                          help.keys(), 15, 80)
-			self.print_topics("Undocumented commands:", 
-		                          cmds_undoc, 15, 80)
+			self.print_topics(self.doc_header,   cmds_doc,   15,80)
+			self.print_topics(self.misc_header,  help.keys(),15,80)
+			self.print_topics(self.undoc_header, cmds_undoc, 15,80)
 
 	def print_topics(self, header, cmds, cmdlen, maxcol):
 		if cmds:
 			print header;
-			print "="*len(header)
+			if self.ruler:
+			    print self.ruler * len(header)
 			(cmds_per_line,junk)=divmod(maxcol,cmdlen)
 			col=cmds_per_line
 			for cmd in cmds: