[Python-Dev] Re: Single- vs. Multi-pass iterability
Guido van Rossum
Thu, 18 Jul 2002 16:08:54 -0400
> 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/)