[andreasvc]

I'm puzzled by this example:

    sums = [s for s in [0] for x in data for s in [s + x]]

Why would you do "for s in" twice? Is that intentional? It would make more sense to me if the variables would have been different. And why would you want to add 0 to numbers?! Curious about a real world use case for this.

[duckerude]

I wouldn't endorse that code, but it does make sense. You can read it like this:

  sums = []
  s = 0
  for x in data:
      s = s + x
      sums.append(s)

`for s in [0]` assigns 0 to `s`, as an initial value. `for s in [s + x]` adds `x` to `s`. Both instances of `s` are the same variable, there's no shadowing going on.

[andreasvc]

Ah, I see. That really doesn't deserve to be called an idiom, it's a clever hack. But it's nice to know about it. It seems less ugly than the walrus operator to me, and it doesn't leak the variable outside of the comprehension.

[megaman821]

It is a nested loop. I don't find it very readable but the single line formatting makes it worse.

[nurettin]

    s
    for s in [0] # for every s of 0 (there is only one)
    for x in data # for every x in data
    for s in [s + x] # s is s + x. 

so s = 0 + x[0], then s = (0 + x[0]) + x[1], then (0 + x[0] + x[1]) + x[2] it results in an array of rolling sums.

Edit: someone already answered, I am blind.