Question

I have this function for checking which way we are facing when traveling through a coordinate system, by looking at how the values in a tuple increase, or decrease in comparison to the next value in a list of waypoints. Looking at the code, it feels convoluted and clumsy:

a.facing = self.direction(a.travel_list[0], a.travel_list[1])

def direction(self, start, end):
    s_width = start[0]
    s_height = start[1]
    e_width = end[0]
    e_height = end[1]
    # check directions
    if s_height < e_height:
        if s_width < e_width:
            return 'right'
        elif s_width > e_width:
            return 'up'
        else:
            return 'up_right'

    elif s_height > e_height:
        if s_width < e_width:
            return 'down'
        elif s_width > e_width:
            return 'left'
        else:
            return 'down_left'

    elif s_height == e_height and s_width < e_width:
        return 'down_right'
    else:
        return 'up_left'

The return values are adjusted to be rotated one step clockwise. My question is, how can we change the code to make the function shorter and more efficient?

Edit: Note that movement can only happen in the 8 directions specified.

Answer 1

Use a dictionary based on the cmp() return values:

def direction(self, start, end):
    return table[cmp(start[0], end[0]), cmp(start[1], end[1])]

Build the dictionary to summarize your current logic:

table = {
    (-1, -1): 'right',
    (-1, 1):  'up',
    (-1, 0):  'up_right',
       ...
}

If you’re using Python 3, you will need define your own cmp() function as:

cmp = lambda x, y:  -1 if x < y else 1 if x > y else 0