A class may wish to define __len__() despite that it 
doesn't know how many objects iterating an instance 
may produce.  The docs should give advice for this 
case.  They should also explicitly point out that the 
result of __len__() is generally taken to be a _hint_ 
(functions like map(), tuple(), etc., may preallocate 
space based on __len__()'s result, but adjust to the 
actual number of objects produced).

Tail end of a related thread:

<http://mail.zope.org/pipermail/zope3-dev/2005-
September/015599.html>