[Python-ideas] Add lookahead iterator (peeker) to itertools

[Joao S. O. Bueno]

Hi all!

What is the problem with iterttols.tee even?
It is almots the samething - although much more flexible, than what is
proposed on this thread.


>>> from itertools import tee
>>> my_iter, peek = tee(iter(range(3)), 2)
>>> next(peek)
0
>>> next(my_iter)
0
>>>

  js
 -><-

On 25 February 2013 09:03, Terry Reedy <tjreedy at udel.edu> wrote:
> On 2/25/2013 5:08 AM, Paul Moore wrote:
>>
>> On 25 February 2013 09:51, Wolfgang Maier
>> <wolfgang.maier at biologie.uni-freiburg.de> wrote:
>>>
>>> Terry Reedy <tjreedy at ...> writes:
>>>
>>>> class lookahead():
>>>>       "Wrap iterator with lookahead to both peek and test exhausted"
>>>
>>> ...
>>>
>>> +1 That's a nice tool that I'd love to have in itertools.
>>
>>
>> It's not a bad idea, but I don't like the fact that as written it
>> turns a finite iterator into an infinite one (returning an endless
>> sequence of sentinels after  the underlying iterator is exhausted).
>
>
> This is a bug in this re-write. The corrected .__next__
>
>     def __next__(self):
>         if self:
>
>             ret = self.peek
>             self._set_peek()
>             return ret
>         else:
>             raise StopIteration()
>
> passes the test with this addition
>
>     try:
>         next(it)
>         assert False, "Next should have raised StopIteration"
>     except StopIteration:
>         pass
>
>>>> list(lookahead('abc')) == list('abc')
>
> True
>
> --
> Terry Jan Reedy
>
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