Why are "broken iterators" broken?

[Steven D'Aprano]

According to the Python docs, once an iterator raises StopIteration, it 
should continue to raise StopIteration forever. Iterators that fail to 
behave in this fashion are deemed to be "broken":

http://docs.python.org/lib/typeiter.html

I don't understand the reasoning behind this. As I understand it, an 
iterator is something like a stream. There's no constraint that once a 
stream is empty it must remain empty forever.

Can somebody explain why "broken iterators" are broken?


-- 
Steven