Producing multiple items in a list comprehension

[inhahe]

"Joel Koltner" <zapwireDASHgroups at yahoo.com> wrote in message 
news:5RiZj.165543$ng7.151222 at en-nntp-05.dc1.easynews.com...
> Is there an easy way to get a list comprehension to produce a flat list 
> of, say, [x,2*x] for each input argument?
>
> E.g., I'd like to do something like:
>
> [ [x,2*x] for x in range(4) ]
>
> ...and receive
>
> [ 0,0,1,2,2,4,3,6]
>
> ...but of course you really get a list of lists:
>
> [[0, 0], [1, 2], [2, 4], [3, 6]]
>
> I'm aware I can use any of the standard "flatten" bits of code to turn 
> this back into what I want, but I was hoping there's some way to avoid the 
> "lists of lists" generation in the first place?
>
> A slightly similar problem: If I want to "merge," say, list1=[1,2,3] with 
> list2=[4,5,6] to obtain [1,4,2,5,3,6], is there some clever way with "zip" 
> to do so?
>
> Thanks,
> ---Joel
>

i figured out a solution

sum([x,2*x] for x in range(4)],[]) #not tested
sum(zip(list1,list2),()) #not tested

(you did say you knew of ways to flatten, but i dunno if you knew of that 
way or not)

as an aside, i wish that sum didn't have to take the second parameter. it's 
kind of superfluous.  it can just use the first item as the initial value 
instead of 0 when no initial value is specified.   it would be a little 
complex to do without putting a conditional in your main loop and slowing it 
down, but it could be done.