I didn't add the whole thing verbatim, because the tone doesn't fit: it was written with the intent of motivating a change to the protocol, rather than describing what the protocol is. Presumably we don't want the PEP to say "__iter__ is a red herring".
There's a bunch of issues flying around here, which i'll try to explain better in a separate posting. But i wanted to take care of Guido's request first. I have toned down and abridged my text somewhat, and strengthened the requirement for __iter__(). Here is what the "API specification" section now says:
Classes can define how they are iterated over by defining an __iter__() method; this should take no additional arguments and return a valid iterator object. A class that wants to be an iterator should implement two methods: a next() method that behaves as described above, and an __iter__() method that returns self. The two methods correspond to two distinct protocols: 1. An object can be iterated over with "for" if it implements __iter__() or __getitem__(). 2. An object can function as an iterator if it implements next(). Container-like objects usually support protocol 1. Iterators are currently required to support both protocols. The semantics of iteration come only from protocol 2; protocol 1 is present to make iterators behave like sequences. But the analogy is weak -- unlike ordinary sequences, iterators are "sequences" that are destroyed by the act of looking at their elements.
Find up to here.
Consequently, whenever any Python programmer says "for x in y", he or she must be sure of whether this is going to destroy y.
I don't understand why this is here. *Why* is it important to know whether this is going to destroy y?
--Guido van Rossum (home page: http://www.python.org/~guido/)