call __float__ on str subclasses #5759

tests by R. David Murray

Lib/test/test_float.py

@@ -82,11 +82,23 @@ class GeneralFloatCases(unittest.TestCase):
             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 [

Misc/NEWS

@@ -12,6 +12,8 @@ What's New in Python 2.7 alpha 1
 Core and Builtins
 -----------------
 
+- Issue #5759: float() didn't call __float__ on str subclasses.
+
 - Issue #5704: the "-3" command-line option now implies "-t".
 
 - Issue #2170: refactored xml.dom.minidom.normalize, increasing both

Objects/floatobject.c

@@ -1630,7 +1630,9 @@ float_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 		return float_subtype_new(type, args, kwds); /* Wimp out */
 	if (!PyArg_ParseTupleAndKeywords(args, kwds, "|O:float", kwlist, &x))
 		return NULL;
-	if (PyString_Check(x))
+	/* If it's a string, but not a string subclass, use
+	   PyFloat_FromString. */
+	if (PyString_CheckExact(x))
 		return PyFloat_FromString(x, NULL);
 	return PyNumber_Float(x);
 }