Index: Lib/test/test_float.py
===================================================================
--- Lib/test/test_float.py	(revision 71594)
+++ Lib/test/test_float.py	(working copy)
@@ -82,11 +82,23 @@
             def __float__(self):
                 return 42
 
+        #Issue 5759: __float__ not called on str subclasses  (though it
+        #is on unicode subclasses).
+        class FooStr(str):
+            def __float__(self):
+                return float(str(self))+1
+
+        class FooUnicode(unicode):
+            def __float__(self):
+                return float(unicode(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(FooUnicode('8')), 9.)
+        self.assertAlmostEqual(float(FooStr('8')), 9.)
 
     def test_floatasratio(self):
         for f, ratio in [