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Re: Idioms combining 'next(items)' and 'for item in items:'

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  Re: Idioms combining 'next(items)' and 'for item in items:' Peter Otten <__peter__@web.de> - 2011-09-11 15:41 +0200

#13126 — Re: Idioms combining 'next(items)' and 'for item in items:'

Terry Reedy wrote:

> 3. Process the items of an iterable in pairs.
> 
> items = iter(iterable)
> for first in items:
>      second = next(items)
>      <process first and second>
> 
> This time, StopIteration is raised for an odd number of items. Catch and 
> process as desired. One possibility is to raise ValueError("Iterable 
> must have an even number of items").
 
Another way is zip-based iteration:

(a) silently drop the odd item

items = iter(iterable)
for first, second in zip(items, items): # itertools.izip in 2.x
   ...

(b) add a fill value

for first, second in itertools.zip_longest(items, items):
    ...

(c) raise an exception

Unfortunately there is no zip_exc() that guarantees that all iterables are 
of the same "length", but I've written a recipe some time ago

http://code.activestate.com/recipes/497006-zip_exc-a-lazy-zip-that-ensures-
that-all-iterables/

that achieves near-C speed.

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