diff --git a/Lib/string.py b/Lib/string.py
index ef0334c..0f4ede2 100644
--- a/Lib/string.py
+++ b/Lib/string.py
@@ -236,12 +236,14 @@
 
     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 --git a/Lib/test/test_string.py b/Lib/test/test_string.py
index a352ee3..34d1dcf 100644
--- a/Lib/test/test_string.py
+++ b/Lib/test/test_string.py
@@ -26,6 +26,18 @@
         self.assertEqual(string.capwords('\taBc\tDeF\t'), 'Abc Def')
         self.assertEqual(string.capwords('\taBc\tDeF\t', '\t'), '\tAbc\tDef\t')
 
+    def test_conversion_specifiers(self):
+        fmt = string.Formatter()
+        self.assertEqual(fmt.format("-{arg!r}-", arg='test'), "-'test'-")
+        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')
+        self.assertEqual(fmt.format("{0!a}",  string.ascii_letters),
+            "'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'")
+        self.assertEqual(fmt.format("{0!a}",  chr(255)), "'\\xff'")
+        self.assertEqual(fmt.format("{0!a}",  chr(256)), "'\\u0100'")
+
     def test_formatter(self):
         fmt = string.Formatter()
         self.assertEqual(fmt.format("foo"), "foo")