diff -r df2fdd42b375 Lib/test/test_unicode.py
--- a/Lib/test/test_unicode.py	Mon Aug 26 22:28:21 2013 +0200
+++ b/Lib/test/test_unicode.py	Tue Aug 27 19:11:51 2013 -0700
@@ -1117,20 +1117,41 @@ class UnicodeTest(string_tests.CommonTes
         INF = float('inf')
         self.assertEqual('%f' % NAN, 'nan')
         self.assertEqual('%F' % NAN, 'NAN')
         self.assertEqual('%f' % INF, 'inf')
         self.assertEqual('%F' % INF, 'INF')
 
         # PEP 393
         self.assertEqual('%.1s' % "a\xe9\u20ac", 'a')
         self.assertEqual('%.2s' % "a\xe9\u20ac", 'a\xe9')
 
+    def test_formatting_with_enum(self):
+        # issue18738
+        import enum
+        class Float(float, enum.Enum):
+            PI = 3.1415926
+        class Int(enum.IntEnum):
+            IDES = 15
+        class Str(str, enum.Enum):
+            ABC = 'abc'
+        # Testing Unicode formatting strings...
+        self.assertEqual("%s, %s" % (Str.ABC, Str.ABC), 'Str.ABC, Str.ABC')
+        self.assertEqual("%s, %s, %d, %i, %u, %f, %5.2f" % (Str.ABC, Str.ABC, Int.IDES, Int.IDES, Int.IDES, Float.PI, Float.PI), 'Str.ABC, Str.ABC, 15, 15, 15, 3.141593,  3.14')
+
+        # formatting jobs delegated from the string implementation:
+        self.assertEqual('...%(foo)s...' % {'foo':Str.ABC}, '...Str.ABC...')
+        self.assertEqual('...%(foo)s...' % {'foo':Int.IDES}, '...Int.IDES...')
+        self.assertEqual('...%(foo)i...' % {'foo':Int.IDES}, '...15...')
+        self.assertEqual('...%(foo)d...' % {'foo':Int.IDES}, '...15...')
+        self.assertEqual('...%(foo)u...' % {'foo':Int.IDES,'def':Float.PI},  '...15...')
+        self.assertEqual('...%(foo)f...' % {'foo':Float.PI,'def':123}, '...3.141593...')
+
     @support.cpython_only
     def test_formatting_huge_precision(self):
         from _testcapi import INT_MAX
         format_string = "%.{}f".format(INT_MAX + 1)
         with self.assertRaises(ValueError):
             result = format_string % 2.34
 
     def test_formatting_huge_width(self):
         format_string = "%{}f".format(sys.maxsize + 1)
         with self.assertRaises(ValueError):
diff -r df2fdd42b375 Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Mon Aug 26 22:28:21 2013 +0200
+++ b/Objects/unicodeobject.c	Tue Aug 27 19:11:51 2013 -0700
@@ -13560,24 +13560,22 @@ formatlong(PyObject *val, struct unicode
 
     assert(PyLong_Check(val));
 
     switch (type) {
     default:
         assert(!"'type' not in [diuoxX]");
     case 'd':
     case 'i':
     case 'u':
         /* Special-case boolean: we want 0/1 */
-        if (PyBool_Check(val))
-            result = PyNumber_ToBase(val, 10);
-        else
-            result = Py_TYPE(val)->tp_str(val);
+        /* on second thought, all int subclasses should behave this way */
+        result = PyNumber_ToBase(val, 10);
         break;
     case 'o':
         numnondigits = 2;
         result = PyNumber_ToBase(val, 8);
         break;
     case 'x':
     case 'X':
         numnondigits = 2;
         result = PyNumber_ToBase(val, 16);
         break;