diff -r f6fcff683866 Doc/reference/datamodel.rst
--- a/Doc/reference/datamodel.rst	Wed Oct 17 18:08:40 2012 -0400
+++ b/Doc/reference/datamodel.rst	Wed Oct 17 20:15:03 2012 -0400
@@ -1799,10 +1799,13 @@
       builtin: len
       single: __bool__() (object method)
 
-   Called to implement the built-in function :func:`len`.  Should return the length
-   of the object, an integer ``>=`` 0.  Also, an object that doesn't define a
-   :meth:`__bool__` method and whose :meth:`__len__` method returns zero is
-   considered to be false in a Boolean context.
+   Called to implement the built-in function :func:`len`.  Should return the
+   length of the object, an integer ``>=`` 0.  Also, an object that doesn't
+   define a :meth:`__bool__` method and whose :meth:`__len__` method returns
+   zero is considered to be false in a Boolean context. A dynamic object with
+   a :meth:`__len__` method should report accurately. If it cannot, it should
+   raise a :exc:`TypeError`.  For more details on :meth:`__len__` see the
+   docstring in the test file :source:`Lib/test/test_iterlen.py`.
 
 
 .. method:: object.__length_hint__(self)