diff -r 4fae1004fc58 argparse.py
--- a/argparse.py	Sat Aug 20 10:31:36 2011 +0200
+++ b/argparse.py	Sat Aug 20 10:34:08 2011 +0200
@@ -1071,14 +1071,18 @@
         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)
+            setattr(namespace, self.dest, p)
 
-        # select the parser
-        try:
-            parser = self._name_parser_map[parser_name]
-        except KeyError:
+        if not parser:
             tup = parser_name, ', '.join(self._name_parser_map)
             msg = _('unknown parser %r (choices: %s)' % tup)
             raise ArgumentError(self, msg)
@@ -2239,10 +2243,16 @@
 
     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:
-            tup = value, ', '.join(map(repr, action.choices))
-            msg = _('invalid choice: %r (choose from %s)') % tup
-            raise ArgumentError(action, msg)
+        if action.choices is not None:
+            ac = [ax for ax in action.choices.keys() if ax.startswith(value)]
+            if len(ac) == 0:
+                tup = value, ', '.join(map(repr, action.choices))
+                msg = _('invalid choice: %r (choose from %s)') % tup
+                raise ArgumentError(action, msg)
+            elif len(ac) > 1:
+                tup = value, ', '.join(ac)
+                msg = _('ambigous choice: %r (choose from %s)') % tup
+                raise ArgumentError(action, msg)
 
     # =======================
     # Help-formatting methods