make elements of a list twice or more.

[Joshua Landau]

On 7 August 2013 17:59, Peter Otten <__peter__ at web.de> wrote:
> liuerfire Wang wrote:
>
>> Here is a list x = [b, a, c] (a, b, c are elements of x. Each of them are
>> different type).  Now I wanna generate a new list as [b, b, a, a, c, c].
>>
>> I know we can do like that:
>>
>> tmp = []
>> for i in x:
>>     tmp.append(i)
>>     tmp.append(i)
>>
>> However, I wander is there a more beautiful way to do it, like [i for i in
>> x]?
>
> Using itertools:
>
>>>> items
> [b, a, c]
>>>> from itertools import chain, tee, repeat
>
>>>> list(chain.from_iterable(zip(*tee(items))))
> [b, b, a, a, c, c]
>
> Also using itertools:
>
>>>> list(chain.from_iterable(repeat(item, 2) for item in items))
> [b, b, a, a, c, c]

list(chain.from_iterable([item, item] for item in items))
?


I'm actually posting to point out
http://www.python.org/dev/peps/pep-0448/ would let you write:

    [*(item, item) for item in items]

which I think is totz rad and beats out OP's

    [item for item in items for _ in range(2)]

in readability, succinctness and obviousness.


PS: For jokes, you can also painfully do:

    list((yield item) or item for item in items)


> For lists only, should be fast:
>
>>>> result = 2*len(items)*[None]
>>>> result[::2] = result[1::2] = items
>>>> result
> [b, b, a, a, c, c]
>
> But I would call none of these beautiful...

Au contraire, that is marvelous (I'd still avoid it, though).