diff -r b5ccdf7c032a Doc/library/argparse.rst
--- a/Doc/library/argparse.rst	Sun Aug 21 00:39:18 2011 +0200
+++ b/Doc/library/argparse.rst	Sun Aug 21 18:21:59 2011 +0200
@@ -1498,6 +1498,17 @@
      >>> parser.parse_args(['co', 'bar'])
      Namespace(foo='bar')
 
+   Subparsers now support abbreviation when used.
+   So for example the following three calls all can be used
+   - as long as the choosen abbreviation is unambiguous:
+   
+     >>> parser.parse_args(['checkout', 'bar'])
+     Namespace(foo='bar')
+     >>> parser.parse_args(['check', 'bar'])
+     Namespace(foo='bar')
+     >>> parser.parse_args(['che', 'bar'])
+     Namespace(foo='bar')
+
    One particularly effective way of handling sub-commands is to combine the use
    of the :meth:`add_subparsers` method with calls to :meth:`set_defaults` so
    that each subparser knows which Python function it should execute.  For
diff -r b5ccdf7c032a Lib/argparse.py
--- a/Lib/argparse.py	Sun Aug 21 00:39:18 2011 +0200
+++ b/Lib/argparse.py	Sun Aug 21 18:21:59 2011 +0200
@@ -1108,14 +1108,18 @@
         parser_name = values[0]
         arg_strings = values[1:]
 
+        # select the parser
+        parser = None
+        for p in self._name_parser_map:
+            if p.startswith(parser_name):
+                parser = self._name_parser_map[p]
+                break
+
         # set the parser name if requested
         if self.dest is not SUPPRESS:
-            setattr(namespace, self.dest, parser_name)
-
-        # select the parser
-        try:
-            parser = self._name_parser_map[parser_name]
-        except KeyError:
+            setattr(namespace, self.dest, p)
+
+        if not parser:
             args = {'parser_name': parser_name,
                     'choices': ', '.join(self._name_parser_map)}
             msg = _('unknown parser %(parser_name)r (choices: %(choices)s)') % args
@@ -2286,11 +2290,18 @@
 
     def _check_value(self, action, value):
         # converted value must be one of the choices (if specified)
-        if action.choices is not None and value not in action.choices:
-            args = {'value': value,
-                    'choices': ', '.join(map(repr, action.choices))}
-            msg = _('invalid choice: %(value)r (choose from %(choices)s)')
-            raise ArgumentError(action, msg % args)
+        if action.choices is not None:
+            ac = [ax for ax in action.choices if str(ax).startswith(str(value))]
+            if len(ac) == 0:
+                args = {'value': value,
+                        'choices': ', '.join(map(repr, action.choices))}
+                msg = _('invalid choice: %(value)r (choose from %(choices)s)')
+                raise ArgumentError(action, msg % args)
+            elif len(ac) > 1:
+                args = {'value': value,
+                        'choices': ', '.join(ac)}
+                msg = _('ambiguous choice: %(value)r could match %(choices)s')
+                raise ArgumentError(action, msg % args)
 
     # =======================
     # Help-formatting methods