Question

I have some points in space where each point has an id. I also have a subset of these points in another group that have different id values.

How can I create a new type of id for both groups of points so that the points that have the same coordinates end up using the same id values?

I assume I need to generate hash codes using their coordinates which should give me the same id value for points that have the same coordinates, right?

I am confused how I could use it because the set of hashcodes is much smaller than float[3]. So not sure if I am on the right track.

Answer 1

I’m not completely sure what you mean here, but you could use __hash__ with a tuple:

class Point(object):
    def __init__(self,x,y,z):
        self.x = x
        self.y = y
        self.z = z

    def __hash__(self):
        return hash((self.x,self.y,self.z))

    def __eq__(self,other):
        return (self.x,self.y,self.z) == (other.x,other.y,other.z)

Now, objects which contain the same point all hash to the same value. As a side benefit, they can now be used as dictionary keys or in set objects a little bit more reasonably.

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Of course, if you’re going to write a class this simple, you might want to consider a collections.namedtuple instead. You could even subclass it (it’s all spelled out in the link). This has the advantage of the object being immutable — Mutating a hashable object just isn’t a nice thing to do ;-). The objects also have no __dict__ associated with them, so they’ll probably be a little easier on your memory if you’re creating 100M of them.