make elements of a list twice or more.

[Peter Otten]

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]

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...