diff -r 694110bc91d8 -r ed0fce615582 Lib/argparse.py
--- a/Lib/argparse.py	Sat Jan 07 18:34:24 2012 +0100
+++ b/Lib/argparse.py	Sat Jan 07 12:36:35 2012 -0800
@@ -99,6 +99,7 @@
 ZERO_OR_MORE = '*'
 ONE_OR_MORE = '+'
 PARSER = 'A...'
+_OPTIONAL_PARSER = 'A...?'
 REMAINDER = '...'
 _UNRECOGNIZED_ARGS_ATTR = '_unrecognized_args'
 
@@ -579,7 +580,7 @@
             result = '%s [%s ...]' % get_metavar(2)
         elif action.nargs == REMAINDER:
             result = '...'
-        elif action.nargs == PARSER:
+        elif action.nargs in [ PARSER, _OPTIONAL_PARSER ]:
             result = '%s ...' % get_metavar(1)
         else:
             formats = ['%s' for _ in range(action.nargs)]
@@ -1059,6 +1060,7 @@
                  parser_class,
                  dest=SUPPRESS,
                  help=None,
+                 default=None,
                  metavar=None):
 
         self._prog_prefix = prog
@@ -1066,10 +1068,15 @@
         self._name_parser_map = _collections.OrderedDict()
         self._choices_actions = []
 
+        nargs = PARSER
+        if default is not None:
+          nargs = _OPTIONAL_PARSER
+
         super(_SubParsersAction, self).__init__(
             option_strings=option_strings,
             dest=dest,
-            nargs=PARSER,
+            nargs=nargs,
+            default=default,
             choices=self._name_parser_map,
             help=help,
             metavar=metavar)
@@ -2190,6 +2197,11 @@
         elif nargs == PARSER:
             nargs_pattern = '(-*A[-AO]*)'
 
+        # allow one optional argument followed by any number of options or
+        # arguments
+        elif nargs == _OPTIONAL_PARSER:
+            nargs_pattern = '(-*A?[-AO]*)?'
+
         # all others should be integers
         else:
             nargs_pattern = '(-*%s-*)' % '-*'.join('A' * nargs)
@@ -2207,7 +2219,8 @@
     # ========================
     def _get_values(self, action, arg_strings):
         # for everything but PARSER args, strip out '--'
-        if action.nargs not in [PARSER, REMAINDER]:
+        #if action.nargs not in [PARSER, REMAINDER]:
+        if action.nargs not in [_OPTIONAL_PARSER, PARSER, REMAINDER]:
             arg_strings = [s for s in arg_strings if s != '--']
 
         # optional argument produces a default when not present
@@ -2219,7 +2232,6 @@
             if isinstance(value, str):
                 value = self._get_value(action, value)
                 self._check_value(action, value)
-
         # when nargs='*' on a positional, if there were no command-line
         # args, use the default if it is anything other than None
         elif (not arg_strings and action.nargs == ZERO_OR_MORE and
@@ -2241,8 +2253,11 @@
             value = [self._get_value(action, v) for v in arg_strings]
 
         # PARSER arguments convert all values, but check only the first
-        elif action.nargs == PARSER:
+        elif action.nargs in [ PARSER, _OPTIONAL_PARSER ]:
             value = [self._get_value(action, v) for v in arg_strings]
+            if (not arg_strings and action.default is not None
+               and action.nargs == _OPTIONAL_PARSER):
+              value = [action.default]
             self._check_value(action, value[0])
 
         # all other types of nargs produce a list