[Python-ideas] copyable iterators, named loops, continue a expression per indentation and a few functional things

Talin talin at acm.org
Sun Dec 24 19:07:43 CET 2006

Calvin Spealman wrote:
> On 12/24/06, Talin <talin at acm.org> wrote:
>> Josiah Carlson wrote:
>>> Mathias Panzenböck <grosser.meister.morti at gmx.net> wrote:
>>>> Copyable iterators
>>>> ------------------
>>>> There are a few ways how you could implement this. However, it only makes sense for iterators, not
>>>> for generators!
>>> -1 on the entire proposal.  All iterators (and generators) are already
>>> copyable.  It's called list().
>> Actually, what he wants to be able to do is to 'tee' an iterator.
>> However, we already have a 'tee' function in itertools that does just
>> that. (And it works on generators too!)
>> -- Talin
> If I am understanding the proposal correctly, which I think I am,
> itertools.tee is not the same as what this is proposing. itertools.tee
> simply iterates the original and caches the results to be given when
> iterating over the two tees created for it. This is not the same as
> copying an iterator. Copying an iterator would create a new iterator
> of the same state as the original. For example, with generators the
> copy would be a new generator object with the same state as the
> original and iterating over the copy would execute the same code as
> the first.
> def g():
>     i = 0
>     while True:
>         print i
>         yield i
>         i = i + 1
> If you "copy" this with itertools.tee, you only see i printed once for
> each iteration of the generator, not each iteration of the tees. With
> a real copying of iterators, if I copied this, iterating over the
> copies would always produce an output of that copies value of i.

The OP already stated that the proposal is not meant to apply to 
generators, and rightly so, because there's no way to copy all of the 
mutable state that a generator might access.

However, I think that the idea of copying 'iterators but not generators' 
is too restrictive, since externally you might not be able to tell the 
difference. That's why I suggested tee, since it works on both. No, you 
don't get the same side effects, but that's not what the OP was asking for.

-- Talin

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