diff -r 80a50b7ad88f Lib/string.py
--- a/Lib/string.py	Mon Feb 13 20:28:46 2012 +0200
+++ b/Lib/string.py	Mon Feb 13 20:54:03 2012 +0100
@@ -195,7 +195,7 @@
 
                 # expand the format spec, if needed
                 format_spec = self._vformat(format_spec, args, kwargs,
-                                            used_args, recursion_depth-1)
+                                            used_args, recursion_depth - 1)
 
                 # format the object and append to the result
                 result.append(self.format_field(obj, format_spec))
@@ -220,12 +220,14 @@
 
     def convert_field(self, value, conversion):
         # do any conversion on the resulting object
-        if conversion == 'r':
-            return repr(value)
+        if conversion is None:
+            return value
         elif conversion == 's':
             return str(value)
-        elif conversion is None:
-            return value
+        elif conversion == 'r':
+            return repr(value)
+        elif conversion == 'a':
+            return ascii(value)
         raise ValueError("Unknown conversion specifier {0!s}".format(conversion))
 
 
diff -r 80a50b7ad88f Lib/test/test_string.py
--- a/Lib/test/test_string.py	Mon Feb 13 20:28:46 2012 +0200
+++ b/Lib/test/test_string.py	Mon Feb 13 20:54:03 2012 +0100
@@ -38,6 +38,9 @@
         self.assertEqual(fmt.format("{0!s}", 'test'), 'test')
         self.assertRaises(ValueError, fmt.format, "{0!h}", 'test')
 
+        # issue13579
+        self.assertEqual(fmt.format("{0!a}", 42), '42')
+
     def test_name_lookup(self):
         fmt = string.Formatter()
         class AnyAttr: