Allow translators to reorder placeholders in localizable messages from

argparse (#10528).

There is no unit test; I checked with xgettext that no more warnings
were emitted.  Steven approved the change.

Lib/argparse.py

@@ -1079,8 +1079,9 @@ class _SubParsersAction(Action):
         try:
             parser = self._name_parser_map[parser_name]
         except KeyError:
-            tup = parser_name, ', '.join(self._name_parser_map)
-            msg = _('unknown parser %r (choices: %s)') % tup
+            args = {'parser_name': parser_name,
+                    'choices': ', '.join(self._name_parser_map)}
+            msg = _('unknown parser %(parser_name)r (choices: %(choices)s)') % args
             raise ArgumentError(self, msg)
 
         # parse all the remaining options into the namespace
@@ -1380,10 +1381,11 @@ class _ActionsContainer(object):
         for option_string in args:
             # error on strings that don't start with an appropriate prefix
             if not option_string[0] in self.prefix_chars:
-                msg = _('invalid option string %r: '
-                        'must start with a character %r')
-                tup = option_string, self.prefix_chars
-                raise ValueError(msg % tup)
+                args = {'option': option_string,
+                        'prefix_chars': self.prefix_chars}
+                msg = _('invalid option string %(option)r: '
+                        'must start with a character %(prefix_chars)r')
+                raise ValueError(msg % args)
 
             # strings starting with two prefix characters are long options
             option_strings.append(option_string)
@@ -2049,8 +2051,9 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
         if len(option_tuples) > 1:
             options = ', '.join([option_string
                 for action, option_string, explicit_arg in option_tuples])
-            tup = arg_string, options
-            self.error(_('ambiguous option: %s could match %s') % tup)
+            args = {'option': arg_string, 'matches': options}
+            msg = _('ambiguous option: %(option)s could match %(matches)s')
+            self.error(msg % args)
 
         # if exactly one action matched, this segmentation is good,
         # so return the parsed action
@@ -2229,8 +2232,9 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
         # TypeErrors or ValueErrors also indicate errors
         except (TypeError, ValueError):
             name = getattr(action.type, '__name__', repr(action.type))
-            msg = _('invalid %s value: %r')
-            raise ArgumentError(action, msg % (name, arg_string))
+            args = {'type': name, 'value': arg_string}
+            msg = _('invalid %(type)s value: %(value)r')
+            raise ArgumentError(action, msg % args)
 
         # return the converted value
         return result
@@ -2238,9 +2242,10 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
     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)
+            args = {'value': value,
+                    'choices': ', '.join(map(repr, action.choices))}
+            msg = _('invalid choice: %(value)r (choose from %(choices)s)')
+            raise ArgumentError(action, msg % args)
 
     # =======================
     # Help-formatting methods
@@ -2332,4 +2337,5 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
         should either exit or raise an exception.
         """
         self.print_usage(_sys.stderr)
-        self.exit(2, _('%s: error: %s\n') % (self.prog, message))
+        args = {'prog': self.prog, 'message': message}
+        self.exit(2, _('%(prog)s: error: %(message)s\n') % args)

Misc/NEWS

@@ -35,6 +35,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #10528: Allow translators to reorder placeholders in localizable
+  messages from argparse.
+
 - Issue #10497: Fix incorrect use of gettext in argparse.
 
 - Issue #10478: Reentrant calls inside buffered IO objects (for example by