rate_graph.py

class RateGraph(object):
    def __init__(self, rates):
        'Initialize the graph from an iterable of (start, end, rate) tuples.'
        self.graph = {}
        for orig, dest, rate in rates:
            self.add_conversion(orig, dest, rate)

    
    def add_conversion(self, orig, dest, rate):
        'Insert a conversion into the graph.'
        if orig not in self.graph:
            self.graph[orig] = {}
        self.graph[orig][dest] = rate


    def get_neighbors(self, node):
        'Returns an iterable of the nodes neighboring the given node.'
        if node not in self.graph:
            return None
        return self.graph[node].items()
        

    def get_nodes(self):
        'Returns an iterable of all the nodes in the graph.'
        return self.graph.keys()

The comment in line 10 contains spoilers ;)

whoooooooops, thanks for pointing that out

fixed