[Python-ideas] __iter__ implies __contains__?

Guido van Rossum guido at python.org
Mon Oct 3 16:44:51 CEST 2011

On Mon, Oct 3, 2011 at 12:22 AM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> Guido van Rossum wrote:
>> And I can assure you that this was no coincidence, mistake or
>> accident. It was done very deliberately.
> I know, and I understand why it seemed like a good idea at
> the time. It's just that my own experiences since then have
> led me to think that a different choice might have worked
> out better.
> Consuming an iterator is something you really don't want to
> do accidentally, just like you don't want to accidentally
> do anything else that changes the internal state of an
> object. The current design makes it all too easy to do
> just that.
> Passing non-reiterable objects around is not something that
> I think should be encouraged. Ideally, one would hardly ever
> see a bare iterator -- they should be like virtual particles,
> coming into existence when needed, performing their function
> and then disappearing before anyone notices they're there.
> I think Py3 is heading in the right direction with things
> like dict.keys() returning iterable views instead of iterators.
> Generally, we should strive to make reiterables easier to
> obtain and non-reiterables harder to obtain.

Actually I think there is at least *some* trend in the opposite
direction -- we now have a much more refined library (and even
vocabulary) for "iterator algebra" than before iter() was introduced,
and a subgroup of the community who can easily whip out clever ways to
do things by combining iterators in new ways.

> Maybe when we've been doing that for long enough, we'll be
> in a position to make fallback to iteration work only for
> reiterables without breaking too much code.

Maybe if we had introduced new syntax to iterate over a single-use
iterable from the start (*requiring* to use one form or the other
depending on whether iterating over a reiterable or not), we would
live in a slightly better world now, but I don't think there will ever
be a time when it's easy to introduce that distinction.

--Guido van Rossum (python.org/~guido)

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