How to get sliding window of a values for each element in both direction (forward, backward)?

Question

I have a list of values like this,

lst = [1, 2, 3, 4, 5, 6, 7, 8]

Desired Output:

window size = 3
    1  # first element in the list
    forward = [2, 3, 4]
    backward = []

    2  # second element in the list
    forward = [3, 4, 5]
    backward = [1]

    3  # third element in the list
    forward = [4, 5, 6]
    backward = [1, 2]

    4  # fourth element in the list
    forward = [5, 6, 7]
    backward = [1, 2, 3]

    5  # fifth element in the list
    forward = [6, 7, 8]
    backward = [2, 3, 4]

    6  # sixth element in the list
    forward = [7, 8]
    backward = [3, 4, 5]

    7  # seventh element in the list
    forward = [8]
    backward = [4, 5, 6]

    8  # eight element in the list
    forward = []
    backward = [5, 6, 7]

Lets assume a window size of 4, now my desired output:

for each_element in the list, I want 4 values in-front and 4 values backward ignoring the current value.

I was able to use this to get sliding window of values but this also not giving me the correct required output.

import more_itertools
list(more_itertools.windowed([1, 2, 3, 4, 5, 6, 7, 8], n=3))

Answer 1

Code:

arr = [1, 2, 3, 4, 5, 6, 7, 8]
window = 3
for backward, current in enumerate(range(len(arr)), start = 0-window):
    if backward < 0:
        backward = 0
    print(arr[current+1:current+1+window], arr[backward:current])

Output:

[2, 3, 4], []
[3, 4, 5], [1]
[4, 5, 6], [1, 2]
[5, 6, 7], [1, 2, 3]
[6, 7, 8], [2, 3, 4]
[7, 8], [3, 4, 5]
[8], [4, 5, 6]
[], [5, 6, 7]

One Liner:

print(dict([(e, (lst[i+1:i+4], lst[max(i-3,0):i])) for i,e in enumerate(last)]))

Output:

{1: ([2, 3, 4], []),
 2: ([3, 4, 5], [1]),
 3: ([4, 5, 6], [1, 2]),
 4: ([5, 6, 7], [1, 2, 3]),
 5: ([6, 7, 8], [2, 3, 4]),
 6: ([7, 8], [3, 4, 5]),
 7: ([8], [4, 5, 6]),
 8: ([], [5, 6, 7])}

Credit: thanks to suggestions from @FeRD and @Androbin, the solution now looks better

Answer 2

This should get you started:

from dataclasses import dataclass
from typing import List

@dataclass
class Window:
  index: int
  backward: List[int]
  forward: List[int]

def window(iterable, window_size, index):
  backward = iterable[max(0, index - window_size):index]
  forward = iterable[index + 1:index + 1 + window_size]
  return Window(index, backward, forward)

>>> window([1,2,3,4,5,6], 3, 0)
Window(index=0, backward=[], forward=[2, 3, 4])
>>> window([1,2,3,4,5,6], 3, 5)
Window(index=5, backward=[3, 4, 5], forward=[])

I would also suggest adding some checks whether the index and window size make sense.

If you are stuck with an older Python version that doesn't have dataclasses yet, you can use Named Tuples instead.