def __init__(self, city_distances, *args, **kwargs):
super().__init__(*args, **kwargs)<EOL>self.city_distances = city_distances<EOL>self.max_distance = max([max(subdict.values()) for subdict in city_distances.values()])<EOL>self.num_cities = len(self.city_distances)<EOL>
city_distances:  2-deep mapping of city_id -> city_id -> distance