Index: Lib/test/test_float.py
===================================================================
--- Lib/test/test_float.py	(revision 71579)
+++ Lib/test/test_float.py	(working copy)
@@ -78,11 +78,23 @@
             def __float__(self):
                 return 42
 
+        #Issue 5759: __float__ not called on str subclasses
+        #is on unicode subclasses).
+        class FooBytes(bytes):
+            def __float__(self):
+                return float(bytes(self))+1
+
+        class FooStr(str):
+            def __float__(self):
+                return float(str(self))+1
+
         self.assertAlmostEqual(float(Foo0()), 42.)
         self.assertAlmostEqual(float(Foo1()), 42.)
         self.assertAlmostEqual(float(Foo2()), 42.)
         self.assertAlmostEqual(float(Foo3(21)), 42.)
         self.assertRaises(TypeError, float, Foo4(42))
+        self.assertAlmostEqual(float(FooBytes(b'8')), 9.)
+        self.assertAlmostEqual(float(FooStr('8')), 9.)
 
     def test_floatasratio(self):
         for f, ratio in [