Index: Objects/unicodeobject.c
===================================================================
--- Objects/unicodeobject.c	(revision 76582)
+++ Objects/unicodeobject.c	(working copy)
@@ -8809,8 +8809,6 @@
             case 'F':
             case 'g':
             case 'G':
-                if (c == 'F')
-                    c = 'f';
                 temp = formatfloat(v, flags, prec, c);
                 if (temp == NULL)
                     goto onError;
Index: Objects/stringobject.c
===================================================================
--- Objects/stringobject.c	(revision 76582)
+++ Objects/stringobject.c	(working copy)
@@ -4966,8 +4966,6 @@
 			case 'F':
 			case 'g':
 			case 'G':
-				if (c == 'F')
-					c = 'f';
 				temp = formatfloat(v, flags, prec, c);
 				if (temp == NULL)
 					goto error;
Index: Lib/test/formatfloat_testcases.txt
===================================================================
--- Lib/test/formatfloat_testcases.txt	(revision 76582)
+++ Lib/test/formatfloat_testcases.txt	(working copy)
@@ -81,6 +81,14 @@
 %f 0.0000005001 -> 0.000001
 %f 0.0000004999 -> 0.000000
 
+-- nans and infinities
+%f nan -> nan
+%f inf -> inf
+%f -infinity -> -inf
+%F nan -> NAN
+%F infinity -> INF
+%F -inf -> -INF
+
 -- 'e' code formatting with explicit precision (>= 0). Output should
 -- always have exactly the number of places after the point that were
 -- requested.
@@ -202,6 +210,14 @@
 %#.1e 123.4 -> 1.2e+02
 %#.2e 0.0001357 -> 1.36e-04
 
+-- nans and infinities
+%e nan -> nan
+%e inf -> inf
+%e -infinity -> -inf
+%E nan -> NAN
+%E infinity -> INF
+%E -inf -> -INF
+
 -- 'g' code formatting.
 
 -- zeros
@@ -314,6 +330,14 @@
 %#.5g 234.56 -> 234.56
 %#.6g 234.56 -> 234.560
 
+-- nans and infinities
+%g nan -> nan
+%g inf -> inf
+%g -infinity -> -inf
+%G nan -> NAN
+%G infinity -> INF
+%G -inf -> -INF
+
 -- for repr formatting see the separate test_short_repr test in
 -- test_float.py.  Not all platforms use short repr for floats.
 
Index: Lib/test/test_float.py
===================================================================
--- Lib/test/test_float.py	(revision 76582)
+++ Lib/test/test_float.py	(working copy)
@@ -321,8 +321,10 @@
 
             lhs, rhs = map(str.strip, line.split('->'))
             fmt, arg = lhs.split()
-            self.assertEqual(fmt % float(arg), rhs)
-            self.assertEqual(fmt % -float(arg), '-' + rhs)
+            arg = float(arg)
+            self.assertEqual(fmt % arg, rhs)
+            if not math.isnan(arg) and copysign(1.0, arg) > 0.0:
+                self.assertEqual(fmt % -arg, '-' + rhs)
 
     def test_issue5864(self):
         self.assertEquals(format(123.456, '.4'), '123.5')