Velocity Reviews > Re: unpacking first few items of iterable

# Re: unpacking first few items of iterable

Terry Reedy
Guest
Posts: n/a

 12-13-2012
On 12/13/2012 3:09 PM, MRAB wrote:
> On 2012-12-13 19:37, Daniel Fetchinson wrote:
>> Hi folks, I swear I used to know this but can't find it anywhere:
>>
>> What's the standard idiom for unpacking the first few items of an
>> iterable whose total length is unknown?

An hinted by some of the answers, this is not a complete specification.

>> Something like
>>
>> a, b, c, _ = myiterable
>>
>> where _ could eat up a variable number of items, in case I'm only
>> interested in the first 3 items?

The literal answer given by demian, a,b,c,*_=iterable, has some good
uses but fails on an infinite iterable, and otherwise exhausts iterators
and creates a potentially long sequence that, by the specification, is
not needed. Mitya's alternative of slicing, seq[:3] requires a directly
sliceable sequence rather than just an iterable.

> You could do this:
>
> from itertools import islice
> a, b, c = islice(myiterable, 3)

This works for any iterable and the only discarded temporary is a
sequence of three items (needed so either all bindings work or none are

If you want to bind a default values if iterable has less than 3 values,
one way is

>>> a,b,c = itertools.islice(itertools.chain(itertools.islice( (1,2),

3), [None]*3), 3)
>>> a,b,c

(1, 2, None)

Perhaps clearer is

>>> a,b,c = [None]*3
>>> it = iter((1,2))
>>> try:

a = next(it)
b = next(it)
c = next(it)
except StopIteration:
pass

>>> a,b,c

(1, 2, None)

This has the advantage that if iterable has more than 3 items, 'it' is
available to iterate over the rest. This is the standard idiom for
removing a couple of special items before iterating over the remainder
(when one does not want the remainder as a concrete list).

--
Terry Jan Reedy