friends.py

class Friends:
    def __init__(self, connections):
        self.connections = [c for c in connections]

    def add(self, connection):
        if connection not in self.connections:
            self.connections.append(connection)
            return True
        else:
            return False

    def remove(self, connection):
        if connection in self.connections:
            self.connections.remove(connection)
            return True
        else:
            return False

    def names(self):
        return set([name for c in self.connections for name in c])       

    def connected(self, name):
        ans = []
        for c in self.connections:
            if name in c:
                ans.append(list(c - set([name]))[0])
                print(name, c, c - set([name]))
        return set(ans)



if __name__ == '__main__':
    #These "asserts" using only for self-checking and not necessary for auto-testing
    letter_friends = Friends(({"a", "b"}, {"b", "c"}, {"c", "a"}, {"a", "c"}))
    digit_friends = Friends([{"1", "2"}, {"3", "1"}])
    assert letter_friends.add({"c", "d"}) is True, "Add"
    assert letter_friends.add({"c", "d"}) is False, "Add again"
    assert letter_friends.remove({"c", "d"}) is True, "Remove"
    assert digit_friends.remove({"c", "d"}) is False, "Remove non exists"
    assert letter_friends.names() == {"a", "b", "c"}, "Names"
    assert letter_friends.connected("d") == set(), "Non connected name"
    assert letter_friends.connected("a") == {"b", "c"}, "Connected name"
    f = Friends(({"nikola", "sophia"}, {"stephen", "robot"}, {"sophia", "pilot"}))
    assert f.connected("nikola") == {'sophia'}
    f = Friends(({"nikola", "sophia"}, {"stephen", "robot"}, {"sophia", "pilot"}))
    assert f.connected("sophia") == {'pilot', 'nikola'}
    print('test all good')