python - Reversed array slice including the first element

You can omit the second index when slicing if you want your reversed interval to end at index 0.

a = [1, 2, 3, 4]
a[1::-1] # [2, 1]

In general whenever your final index is zero, you want to replace it by None, otherwise you want to decrement it.

Due to indexing arithmetic, we must treat those cases separately to be consistent with the usual slicing behavior. This can be done neatly with a ternary expression.

def reversed_interval(lst, i=None, j=None):
    return lst[j:i - 1 if i else None:-1]
reversed_interval([1, 2, 3, 4], 0, 1) # [2, 1]