arr = ['hello' ',' , 'how', 'is' , 'your', 'day', 'going', '?' , '#', 'HelloWorld']

N = 0
for index to arr:
    if arr[N] == '#':
        arr[N] = (' ')
        arr[N+1] = (' ')
    N += 1

arr = ['a', 'b', '#', 'aa']
indices = [idx for idx, elt in enumerate(arr) if elt == '#']

for idx in indices:
    if idx != len(arr): arr[idx+1] = ' '  # Check if not at the end of the list
    arr[idx] = ' '

for i in range(len(arr)):
    if arr[i] == '#':
        arr[i] = ' '
        if i < len(arr)-2:
            arr[i+1] = ' '

arr = ['hello' ',' , 'how', 'is' , 'your', 'day', 'going', '?' , '#', 'HelloWorld']

for index in range(0, len(arr)):
    if arr[index] == '#':
        arr[index:index+2] = ['', '']
print (arr)

['hello,', 'how', 'is', 'your', 'day', 'going', '?', '', '']
[Finished in 0.133s]

arr = ['hello' ',' , 'how', 'is' , 'your', 'day', 'going', '?' , '#', 'HelloWorld', '#']

for index in range(0, len(arr)):
    if arr[index] == '#':
        arr[index:index+2] = ['', '']
print (arr)

['hello,', 'how', 'is', 'your', 'day', 'going', '?', '', '', '', '']
[Finished in 0.179s]