diff -r 787d7587abb5 Lib/test/test_float.py
--- a/Lib/test/test_float.py	Thu Jan 01 05:04:41 2009 +0100
+++ b/Lib/test/test_float.py	Thu Jan 01 16:49:22 2009 +0900
@@ -285,6 +285,14 @@
         self.assertEqual(str(float("+infinity")), "inf")
         self.assertEqual(str(float("-infinity")), "-inf")
 
+        for f in ['%f', '%e', '%g']:
+            self.assertEqual(f % (float("inf")), "inf")
+            self.assertEqual(f % (float("+inf")), "inf")
+            self.assertEqual(f % (float("-inf")), "-inf")
+            self.assertEqual(f % (float("infinity")), "inf")
+            self.assertEqual(f % (float("+infinity")), "inf")
+            self.assertEqual(f % (float("-infinity")), "-inf")
+
         self.assertRaises(ValueError, float, "info")
         self.assertRaises(ValueError, float, "+info")
         self.assertRaises(ValueError, float, "-info")
@@ -319,6 +327,11 @@
         self.assertEqual(str(float("nan")), "nan")
         self.assertEqual(str(float("+nan")), "nan")
         self.assertEqual(str(float("-nan")), "nan")
+
+        for f in ['%f', '%g', '%e']:
+            self.assertEqual(f % (float("nan")), "nan")
+            self.assertEqual(f % (float("+nan")), "nan")
+            self.assertEqual(f % (float("-nan")), "nan")
 
         self.assertRaises(ValueError, float, "nana")
         self.assertRaises(ValueError, float, "+nana")
diff -r 787d7587abb5 Objects/stringobject.c
--- a/Objects/stringobject.c	Thu Jan 01 05:04:41 2009 +0100
+++ b/Objects/stringobject.c	Thu Jan 01 16:49:22 2009 +0900
@@ -4336,6 +4336,9 @@
 	   worst case length = 3 + 10 (len of INT_MAX) + 1 = 14 (use 20)*/
 	char fmt[20];
 	double x;
+        register char* cp;
+        int i;
+
 	x = PyFloat_AsDouble(v);
 	if (x == -1.0 && PyErr_Occurred()) {
 		PyErr_Format(PyExc_TypeError, "float argument required, "
@@ -4373,6 +4376,47 @@
 		      (flags&F_ALT) ? "#" : "",
 		      prec, type);
         PyOS_ascii_formatd(buf, buflen, fmt, x);
+
+        /* Handle nan and inf */
+	cp = buf;
+	if (*cp == '-')
+		cp++;
+	for (; *cp != '\0'; cp++) {
+		/* Any non-digit means it's not an integer;
+		   this takes care of NAN and INF as well. */
+		if (!isdigit(Py_CHARMASK(*cp)))
+			break;
+	}
+
+	/* Checking the next three chars should be more than enough to
+	 * detect inf or nan, even on Windows. We check for inf or nan
+	 * at last because they are rare cases.
+	 */
+	for (i=0; *cp != '\0' && i<3; cp++, i++) {
+		if (isdigit(Py_CHARMASK(*cp)) || *cp == '.')
+			continue;
+		/* found something that is neither a digit nor point
+		 * it might be a NaN or INF
+		 */
+#ifdef Py_NAN
+		if (Py_IS_NAN(x)) {
+                        /* we now buf it is a least 4 bytes long because of the
+                         * previous size check a few lines above */
+			strcpy(buf, "nan");
+		}
+                else
+#endif
+		if (Py_IS_INFINITY(x)) {
+			cp = buf;
+			if (*cp == '-')
+				cp++;
+                        /* we now buf is a least 5 (sign + 'inf' + '\0') bytes
+                         * long because of the previous size check a few lines
+                         * above*/
+			strcpy(cp, "inf");
+		}
+		break;
+	}
 	return (int)strlen(buf);
 }