More for SF bug [#460020] bug or feature: unicode() and subclasses

Repair float constructor to return a true float when passed a subclass
instance.  New PyFloat_CheckExact macro.

Include/floatobject.h

@@ -19,6 +19,7 @@ typedef struct {
 extern DL_IMPORT(PyTypeObject) PyFloat_Type;
 
 #define PyFloat_Check(op) PyObject_TypeCheck(op, &PyFloat_Type)
+#define PyFloat_CheckExact(op) ((op)->ob_type == &PyFloat_Type)
 
 /* Return Python float from string PyObject.  Second argument ignored on
    input, and, if non-NULL, NULL is stored into *junk (this tried to serve a

Lib/test/test_descr.py

@@ -1372,7 +1372,7 @@ def inherits():
     verify(repr(precfloat(1.1)) == "1.1")
     a = precfloat(12345)
     #XXX verify(float(a) == 12345.0)
-    #XXX verify(float(a).__class__ is float)
+    verify(float(a).__class__ is float)
 
     class madtuple(tuple):
         _rev = None

Objects/abstract.c

@@ -909,10 +909,14 @@ PyNumber_Float(PyObject *o)
 
 	if (o == NULL)
 		return null_error();
-	if (PyFloat_Check(o)) {
+	if (PyFloat_CheckExact(o)) {
 		Py_INCREF(o);
 		return o;
 	}
+	if (PyFloat_Check(o)) {
+		PyFloatObject *po = (PyFloatObject *)o;
+		return PyFloat_FromDouble(po->ob_fval);
+	}
 	if (!PyString_Check(o)) {
 		m = o->ob_type->tp_as_number;
 		if (m && m->nb_float)