Good Style in Python Objects

Question

Most of my programming prior to Python was in C++ or Matlab. I don\'t have a degree in CS (almost completed a PhD in physics), but have done some courses and a good amount o

Answer 1

One option is to use dictionaries to store the information you need about a data item, rather than attributes on the item directly. For instance, rather than referring to d.size you could refer to size[d] (where size is a dict instance). This requires that your data items be hashable, but they don't need to allow attributes to be assigned on them.

Here's a straightforward translation of your current code to use this style:

class UnionFind:
    def __init__(self,data):
        self.data = data
        self.size = {d:1 for d in data}
        self.leader = {d:d for d in data}
        self.next = {d:None for d in data}
        self.last = {d:d for d in data}

    def find(self,element):
        return self.leader[element]

    def union(self,leader1,leader2):
        if self.size[leader1] >= self.size[leader2]:
            newleader = leader1
            oldleader = leader2
        else:
            newleader = leader2
            oldleader = leader1

        self.size[newleader] = self.size[leader1] + self.size[leader2]

        d = oldleader
        while d != None:
            self.leader[d] = newleader
            d = self.next[d]

        self.next[self.last[newleader]] = oldleader
        self.last[newleader] = self.last[oldleader]

A minimal test case:

>>> uf = UnionFind(list(range(100)))
>>> uf.find(10)
10
>>> uf.find(20)
20
>>> uf.union(10,20)
>>> uf.find(10)
10
>>> uf.find(20)
10

Beyond this, you could also consider changing your implementation a bit to require less initialization. Here's a version that doesn't do any initialization (it doesn't even need to know the set of data it's going to work on). It uses path compression and union-by-rank rather than always maintaining an up-to-date leader value for all members of a set. It should be asymptotically faster than your current code, especially if you're doing a lot of unions:

class UnionFind:
    def __init__(self):
        self.rank = {}
        self.parent = {}

    def find(self, element):
        if element not in self.parent: # leader elements are not in `parent` dict
            return element
        leader = self.find(self.parent[element]) # search recursively
        self.parent[element] = leader # compress path by saving leader as parent
        return leader

    def union(self, leader1, leader2):
        rank1 = self.rank.get(leader1,1)
        rank2 = self.rank.get(leader2,1)

        if rank1 > rank2: # union by rank
            self.parent[leader2] = leader1
        elif rank2 > rank1:
            self.parent[leader1] = leader2
        else: # ranks are equal
            self.parent[leader2] = leader1 # favor leader1 arbitrarily
            self.rank[leader1] = rank1+1 # increment rank