[Python-Dev] Re: Single- vs. Multi-pass iterability

[Guido van Rossum]

> Maybe the reasons behind having __iter__() returning itself should be
> clearly expressed in the PEP, too.  On this list, Tim gave one recently,
> Guido gives another here, but unless I missed it, the PEP gives none.
> Usually, PEPs explain the reasons behind the choices.

Ping added this to the PEP:

    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.

(I could do without the last sentence, since this expresses a value
judgement rather than fact -- not a good thing to have in a PEP's
"specification" section.)

--Guido van Rossum (home page: http://www.python.org/~guido/)