Python3 Disjoint Set imlementation

disjoint.py

from collections.abc import MutableMapping


class DisjointSet(MutableMapping):
    """Working disjoint set implementatin in Python - 
    
    check https://en.wikipedia.org/wiki/Disjoint-set_data_structure
    """
    def __init__(self, initial):
        self.data = dict()
        self.index = dict()
        for item in initial:
            self.add(item)

    def add(self, item):
        if not item in self:
            # Create a new singleton subset
            self.data[item] = item
            self.index[item] = {item,}

    def __getitem__(self, item):
        return self.data[item]

    def __setitem__(self, item, value):
        raise NotImplementedError("Use 'add' to add new elements")

    def __delitem__(self, item):
        raise NotImplementedError("Not implemented")

    def __iter__(self):
        yield from self.data

    def __len__(self):
        return len(self.data)

    find = __getitem__

    def join(self, key1, key2):
        if len(self.index[key1]) > len(self.index[key2]):
            source = key2; target = key1
        else:
            source = key1; target = key2

        for item in self.index[source]:
            self.index[target].add(item)
            self.data[item] = target
        del self.index[source]