diff -r 422a0175bbf5 Doc/reference/datamodel.rst
--- a/Doc/reference/datamodel.rst	Fri Oct 19 23:59:35 2012 +1000
+++ b/Doc/reference/datamodel.rst	Sat Oct 20 00:56:00 2012 -0400
@@ -1875,7 +1875,10 @@
    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:`__nonzero__` method and whose :meth:`__len__` method returns zero is
-   considered to be false in a Boolean context.
+   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.__getitem__(self, key)