Peek inside iterator (is there a PEP about this?)

[Luis Zarrabeitia]

On Wednesday 01 October 2008 01:14:14 pm Peter Otten wrote:
> Luis Zarrabeitia wrote:
> > a = iter([1,2,3,4,5]) # assume you got the iterator from a function and
> > b = iter([1,2,3])     # these two are just examples.
>
> Can you provide a concrete use case?

I'd like to... but I've refactored away all the examples I had, as soon as I 
realized that I didn't know which one was the shortest sequence to put it 
first.

But, it went something like this:

===
def do_stuff(tasks, params):
    params = iter(params)
    for task in tasks:
        for partial_task, param in zip(task, params):
            pass #blah blah, do stuff here.
        print "task completed"
===

Unfortunately that's not the real example (as it is, it shows very bad 
programming), but imagine if params and/or tasks were streams beyond your 
control (a data stream and a control stream). Note that I wouldn't like a 
task or param to be wasted.

I didn't like the idea of changing both the 'iter' and the 'zip' (changing 
only one of them wouldn't have worked).

> > Will this iterator yield any value? Like with most iterables, a construct
> >
> > if iterator:
> >    # do something
>
> I don't think this has a chance. By adding a __len__ to some iterators R.
> Hettinger once managed to break GvR's code. The BDFL was not amused.

Ouch :D
But, no no no. Adding a __len__ to iterators makes little sense (specially 
in my example), and adding an optional __len__ that some iterators have and 
some don't (the one that can't know their own lengths) would break too many 
things, and still, wouldn't solve the problem of knowing if there is a next 
element. A __nonzero__() that would move the iterator forward and cache the 
result, with a next() that would check the cache before advancing, would be 
closer to what I'd like.

> > if any(iterator):
> >    # do something ... but the first true value was already consumed and
> >    # cannot be reused. "Any" cannot peek inside the iterator without
> >    # consuming the value.
>
> for item in iflter(bool, iterator):
>    # do something
>    break

It is not, but (feel free to consider this silly) I don't like breaks. In this 
case, you would have to read until the end of the block to know that what you 
wanted was an if (if you are lucky you may figure out that you wanted to 
simulate an if test).

(Well, I use breaks sometimes, but most of them are because I need to test if 
an iterator is empty or not)

> Personally I think that Python's choice of EAFP over LBYL is a good one,
> but one that cannot easily be reconciled with having peekable iterators. If
> I were in charge I'd rather simplify the iterator protocol (scrap send()
> and yield expressions) than making it more complex.

Oh, I defend EAFP strongly. On my university LBYL is preferred, so whenever I 
teach python, I have to give strong examples of why I like EAFP.

When the iterator is empty means that there is something wrong, I wouldn't 
think of using "if iterator:". That would be masquerading what should be an 
exception. However, if "iterator is empty" is meaningful, that case should go 
in an "else" clause, rather than "except". Consider if you need to find the 
first non-empty iterator from a list (and then sending it to another 
function - can't test for emptiness with a "for" there, or one could lose the 
first element)

On Wednesday 01 October 2008 04:14:09 pm Terry Reedy wrote:
> Interesting observation.  Iterators are intended for 'iterate through
> once and discard' usages.  To zip a long sequence with several short
> sequences, either use itertools.chain(short sequences) or put the short
> sequences as the first zip arg.

I guess that the use of the word 'rewind' wasn't right. To 'push back' an item 
into the iterator would be an ugly hack to not being able to know if it was 
there in the first place.

Putting the short sequences first wont help a lot when you cannot know which 
sequence is shorter, and chaining all of them could be hard to read at best - 
look at the do_stuff function at the beginning of this message.

> > Knowing if the iteration will end or not, and/or accessing the
> > next value, without consuming it? 

> No, it is not possible to do that for some iterators. For example, this 
> code:
(...)
> if you peeked the iterator in advance, the result would be different 
> compared to the result when you actually need it.

But that's one of the cases where one should know what is doing. Both C# and 
Java have iterators that let you know if they are finished before consuming 
the item. (I didn't mean to compare, and I like java's more than C#, as 
java's iterator also promote the 'use once' design).

This may be dreaming, but if the default iter() constructor returned an object 
with a .has_next() or a __nonzero__() attribute (if __iter__() had 
a 'has_next', no problem, if it didn't, just wrap it - no problem with 
backwards compatibility), functions like zip or self-made zips could make use 
of it... Common cases could be solved by having the has_next compute the next 
one and save it until the 'next()', and weird cases like your time() example 
could define the has_next() as true.

I think that my only objection to this is (beside the similarities with java) 
is that it could promote a LBYL style.

-- 
Luis Zarrabeitia (aka Kyrie)
Fac. de Matemática y Computación, UH.
http://profesores.matcom.uh.cu/~kyrie