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[Python-Dev] RE: cloning iterators again

Guido van Rossum guido at python.org
Mon Oct 27 10:34:59 EST 2003 
> I see!  So you want to be more prudent here than an ordinary copy
> would be, and also disallow alternatives to __copy__ such as
> __getinitargs__ or __getstate__/__setstate__ ...?  Could you give
> an example of an iterator class, which is "accidentally" copyable, but
> "shouldn't" be for purposes of tee only?

We discussed this before: if the state representing the iterator's
position is a mutable object, copy.copy() will not copy this mutable
object, so the two would share their state (or, more likely, part of
their state).  The example would be a tree iterator using a stack,
represented as a list.


> Yes, it IS worth pointing out that the idiom for using tee must
> always be
>     a, b = tee(c)
> and c is not to be used afterwards -- or equivalently
>     a, b = tee(a)
> when, as common, there are no other references to a (even
> indirectly e.g. via somebody holding on to a ref to a.next).  Hmmm,
> I wonder if that should go in my PEP, though, since it's more about tee
> than about copy...?

I think Raymond should add this to the tee() docs in big bold print.

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