The deficiency noticed by Terry has been at least partially corrected in the len() docs
https://docs.python.org/2/library/functions.html#len
Return the length (the number of items) of an object. The argument may be a sequence (such as a string, bytes, tuple, list, or range) or a collection (such as a dictionary, set, or frozen set).

It doesn't mention the fact that the collection has to implement __len__, but I suppose that might be obvious to experienced Python users (isn't obvious to a beginner like me, though). So perhaps this might be a welcome clarification. 

The issue regarding OverflowError raised by Rostyslav still remains unresolved.