Getting a reversed slice of an array that includes the base element

Question

I'm using numpy arrays. I'm trying to calculate an array that contains the products of elements spreading out from some center. My code curently looks like this, where psi is a numpy.array():

up=psi[position:position+width]
pre_down=psi[position-width+1:position+1]
down=pre_down[::-1]
ac=up*down

Is there a more elegant, pythonic way to make the array "down"? Something like

down=psi[position:position-width:-1]

doesn't work when position-width is 0.

Answer 1

When you need to slice down to element 0, you can use None for the end of the slice.

end = position - width - 1 if position > width else None
down = psi[position:end:-1]

Answer 2

You can use the or keyword to replace the value 0 with None, giving you the base element.

down=psi[position:position-width or None:-1]