On Tue, May 25, 2010 at 1:01 PM, Ian Kelly <ian.g.ke...@gmail.com> wrote:
> On Tue, May 25, 2010 at 1:10 PM, <python-list-requ...@python.org> wrote:
>> ---------- Forwarded message ----------
>> From: Terry Reedy <tjre...@udel.edu>
>> To: python-l...@python.org
>> Date: Tue, 25 May 2010 13:09:23 -0400
>> Subject: Re: Generator expressions vs. comprehensions
>> On 5/25/2010 3:08 AM, Peter Otten wrote:
>>>
>>> Michele Simionato wrote:
>>
>>> I think not turning the list-comp into syntactic sugar for list(genexp) in
>>> py3 is a missed opportunity.
>>
>> Implementing it that way was tried but was much slower than the current
>> implementation. If one uses StopIteration as it is intended to be used (and
>> is so documented), then, I believe, they are equivalent. There was a
>> conscious decision to not slow comprehensions for the many to cater to the
>> very few.
>
> I thought that I was using it as intended. The full function that I
> was working with when I ran into the problem was:
>
> def tuples(iterable, n=2):
> """Make an iterator that returns elements from iterable in tuples of n. If
> the number of elements from the iterable is not a multiple of n, any
> trailing elements will be truncated.
>
> tuples('ABCDEFG', n=2) --> ('A', 'B') ('C', 'D') ('E', 'F')
> """
> iterator = iter(iterable)
> while True:
> yield tuple(iterator.next() for i in xrange(n))
>
> The intention being that if iterator.next() raised a StopIteration, it
> would propagate out and signal no further values for the tuples
> generator. Instead, the generator expression results in empty tuples
> once the iterator has run out, and the tuples generator never runs
> out. This has since been fixed by replacing the generator expression
> with a for loop, but if you don't mind my asking, how does this
> violate the documented usage pattern? Is the recommendation to use an
> explicit try-except around every call of iterator.next()?
>
> Off-topic, does anybody know of a better name for this function?
truncating_grouper() ?
from itertools import izip
def truncating_grouper(n, iterable):
"truncating_grouper(3, 'ABCDEFG') --> ABC DEF"
args = [iter(iterable)] * n
return izip(*args)
Implementation adapted from itertools's docs's "Recipes" section.
Also, avoid replying to digests in the future; it messes up
conversation threading.
Cheers,
Chris
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