I need floor division for the remaining computation, so that part I'm happy with.(Actually, when I start to use divmod I have only non-negative numbers)

>>> bwi = 5
>>> np.arange(-bwi // 2, bwi // 2 + 1)
array([-3, -2, -1,  0,  1,  2])
>>> np.arange(-(bwi // 2), bwi // 2 + 1)
array([-2, -1,  0,  1,  2])

the bug hunting was: Why is the window asymmetric?

What threw me off is the operator precedence, what Robert said about operator precedence is kind of obvious ex-post, but the case where it matters doesn't show up often enough to automatically think about it, and the familiar float analogy doesn't apply.

e.g. I avoid remembering some rules by using explicit, defensive parenthesis

>>> 1.5**(-2)

it's commutative

>>> --2 / -3
-0.6666666666666666
>>> ---2 / 3
-0.6666666666666666
>>> ---(2 / 3)
-0.6666666666666666

Josef