Issue #25034: Merge from 3.5.

Lib/string.py

@@ -183,7 +183,7 @@ class Formatter:
 
     def vformat(self, format_string, args, kwargs):
         used_args = set()
-        result = self._vformat(format_string, args, kwargs, used_args, 2)
+        result, _ = self._vformat(format_string, args, kwargs, used_args, 2)
         self.check_unused_args(used_args, args, kwargs)
         return result
 
@@ -230,14 +230,15 @@ class Formatter:
                 obj = self.convert_field(obj, conversion)
 
                 # expand the format spec, if needed
-                format_spec = self._vformat(format_spec, args, kwargs,
-                                            used_args, recursion_depth-1,
-                                            auto_arg_index=auto_arg_index)
+                format_spec, auto_arg_index = self._vformat(
+                    format_spec, args, kwargs,
+                    used_args, recursion_depth-1,
+                    auto_arg_index=auto_arg_index)
 
                 # format the object and append to the result
                 result.append(self.format_field(obj, format_spec))
 
-        return ''.join(result)
+        return ''.join(result), auto_arg_index
 
 
     def get_value(self, key, args, kwargs):

Lib/test/test_string.py

@@ -58,6 +58,8 @@ class ModuleTest(unittest.TestCase):
                          'foo{1}{num}{1}'.format(None, 'bar', num=6))
         self.assertEqual(fmt.format('{:^{}}', 'bar', 6),
                          '{:^{}}'.format('bar', 6))
+        self.assertEqual(fmt.format('{:^{}} {}', 'bar', 6, 'X'),
+                         '{:^{}} {}'.format('bar', 6, 'X'))
         self.assertEqual(fmt.format('{:^{pad}}{}', 'foo', 'bar', pad=6),
                          '{:^{pad}}{}'.format('foo', 'bar', pad=6))
 

Misc/ACKS

@@ -1013,6 +1013,7 @@ Tony Nelson
 Trent Nelson
 Chad Netzer
 Max Neunhöffer
+Anthon van der Neut
 George Neville-Neil
 Hieu Nguyen
 Johannes Nicolai

Misc/NEWS

@@ -190,6 +190,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #25034: Fix string.Formatter problem with auto-numbering and
+  nested format_specs. Patch by Anthon van der Neut.
+
 - Issue #25233: Rewrite the guts of asyncio.Queue to be more understandable and correct.
 
 - Issue #23600: Default implementation of tzinfo.fromutc() was returning