def snap(origin, point, grid):
    def singlesnap( (o, p, d) ):
        p = p - o
        p_d = p % d
        if p_d < d / 2.:
            return p - p_d + o
        else:
            return p - p_d + o + d

    return map(singlesnap, zip(origin, point, grid))

origin = [0] * 3
point = [
  3,
  7,
  32,
]
grid = [5] * 3

print snap(origin, point, grid)

[5, 5, 30]