[Python-ideas] Fwd: unpacking generalisations for list comprehension

[Sven R. Kunze]

On 12.10.2016 21:38, אלעזר wrote:
>
> What is the intuition behind [1, *x, 5]? The starred expression is 
> replaced with a comma-separated sequence of its elements.
>
> The trailing comma Nick referred to is there, with the rule that [1,, 
> 5] is the same as [1, 5].
>

I have to admit that I have my problems with this "comma-separated 
sequence" idea. For me, lists are just collections of items. There are 
no commas involved. I also think that thinking about commas here 
complicates the matter.


What * does, it basically plugs in the items from the starred expression 
into its surroundings:

[*[1,2,3]] = [1,2,3]

Let's plug in two lists into its surrounding list:

[*[1,2,3], *[1,2,3]] = [1,2,3,1,2,3]

So, as the thing goes, it looks like as if * could just work anywhere 
inside those brackets:

[*[1,2,3] for _ in range(3)] = [*[1,2,3], *[1,2,3], *[1,2,3]] = 
[1,2,3,1,2,3,1,2,3]


I have difficulties to understand the problem of understanding the 
syntax. The * and ** variants just flow naturally whereas the "chain" 
equivalent is bit "meh".

Cheers,
Sven