diff -r 9f464eff218a Lib/string.py
--- a/Lib/string.py	Wed Oct 31 12:03:48 2012 +0200
+++ b/Lib/string.py	Wed Oct 31 13:11:02 2012 +0000
@@ -169,7 +169,8 @@
         self.check_unused_args(used_args, args, kwargs)
         return result
 
-    def _vformat(self, format_string, args, kwargs, used_args, recursion_depth):
+    def _vformat(self, format_string, args, kwargs, used_args, recursion_depth,
+                 auto_arg_index=0):
         if recursion_depth < 0:
             raise ValueError('Max string recursion exceeded')
         result = []
@@ -185,6 +186,23 @@
                 # this is some markup, find the object and do
                 #  the formatting
 
+                # handle arg indexing when empty field_names are given.
+                if field_name == '':
+                    if auto_arg_index is False:
+                        raise ValueError('cannot switch from manual field '
+                                         'specification to automatic field '
+                                         'numbering')
+                    field_name = str(auto_arg_index)
+                    auto_arg_index += 1
+                elif field_name.isdigit():
+                    if auto_arg_index:
+                        raise ValueError('cannot switch from manual field '
+                                         'specification to automatic field '
+                                         'numbering')
+                    # disable auto arg incrementing, if it gets
+                    # used later on, then an exception will be raised
+                    auto_arg_index = False
+                     
                 # given the field_name, find the object it references
                 #  and the argument it came from
                 obj, arg_used = self.get_field(field_name, args, kwargs)
@@ -195,7 +213,8 @@
 
                 # expand the format spec, if needed
                 format_spec = self._vformat(format_spec, args, kwargs,
-                                            used_args, recursion_depth-1)
+                                            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))
diff -r 9f464eff218a Lib/test/test_string.py
--- a/Lib/test/test_string.py	Wed Oct 31 12:03:48 2012 +0200
+++ b/Lib/test/test_string.py	Wed Oct 31 13:11:02 2012 +0000
@@ -31,7 +31,16 @@
         self.assertEqual(fmt.format("foo"), "foo")
         self.assertEqual(fmt.format("foo{0}", "bar"), "foobar")
         self.assertEqual(fmt.format("foo{1}{0}-{1}", "bar", 6), "foo6bar-6")
-
+        self.assertEqual(fmt.format("foo{}{}", "bar", 6), "foobar6")
+        self.assertEqual(fmt.format("foo{1}{num}{1}", None, "bar", num=6), 
+                         "foo{1}{num}{1}".format(None, "bar", num=6))
+        self.assertEqual(fmt.format("{:^{}}", 'bar', 6), ' bar  ')
+        self.assertEqual(fmt.format("{:^{pad}}{}", 'foo', 'bar', pad=6), 
+                         "{:^{pad}}{}".format('foo', 'bar', pad=6))
+        
+        with self.assertRaises(ValueError):
+            fmt.format("foo{1}{}", "bar", 6)
+    
     def test_conversion_specifiers(self):
         fmt = string.Formatter()
         self.assertEqual(fmt.format("-{arg!r}-", arg='test'), "-'test'-")
@@ -82,7 +91,6 @@
         fmt = NamespaceFormatter({'greeting':'hello'})
         self.assertEqual(fmt.format("{greeting}, world!"), 'hello, world!')
 
-
     def test_override_format_field(self):
         class CallFormatter(string.Formatter):
             def format_field(self, value, format_spec):
@@ -91,7 +99,6 @@
         fmt = CallFormatter()
         self.assertEqual(fmt.format('*{0}*', lambda : 'result'), '*result*')
 
-
     def test_override_convert_field(self):
         class XFormatter(string.Formatter):
             def convert_field(self, value, conversion):
@@ -102,7 +109,6 @@
         fmt = XFormatter()
         self.assertEqual(fmt.format("{0!r}:{0!x}", 'foo', 'foo'), "'foo':None")
 
-
     def test_override_parse(self):
         class BarFormatter(string.Formatter):
             # returns an iterable that contains tuples of the form:
@@ -148,10 +154,19 @@
         with self.assertRaises(ValueError) as err:
             fmt._vformat("{i}", args, kwargs, set(), -1)
         self.assertIn("recursion", str(err.exception))
-
+        
+    def test_formatter_with_dates(self):
+        from datetime import datetime
+        dt = datetime(year=1989, month=12, day=1)
+        fmt = string.Formatter()
+        self.assertEqual(fmt.format("{name}: born {:%U weeks into %Y}", 
+                                    dt, name='Python'), 
+                         "Python: born 48 weeks into 1989")
+        
 
 def test_main():
     support.run_unittest(ModuleTest)
 
+
 if __name__ == "__main__":
     test_main()