class Slice:
    def __init__(self, istart, iend, jstart, jend):
        self.istart = istart
        self.iend   = iend
        self.jstart = jstart
        self.jend   = jend

    def __getitem__(self, pos):
        return np.s_[self.jstart+pos[0]:self.jend+pos[0],\
                     self.istart+pos[1]:self.iend+pos[1]]

    def __call__(self, j, i):
        return np.s_[self.jstart+j:self.jend+j,\
                     self.istart+i:self.iend+i]

offs = Slice(istart, iend, jstart, jend)

tend_3 = np.zeros_like(scalar)
tend_3[ offs(0,0) ] = -u[ offs(0,0) ] * (scalar[ offs(0,+1) ] - scalar[ offs(0,-1) ]) / (2*dxy) \
                      -v[ offs(0,0) ] * (scalar[ offs(+1,0) ] - scalar[ offs(-1,0) ]) / (2*dxy)
# OR:
tend_3[ offs(0,0) ] = -u[ offs[0,0] ] * (scalar[ offs[0,+1] ] - scalar[ offs[0,-1] ]) / (2*dxy) \
                      -v[ offs[0,0] ] * (scalar[ offs[+1,0] ] - scalar[ offs[-1,0] ]) / (2*dxy)