Question

I have a simple graph and want to create a method “get_edge” that will take two vertices as arguments and return an edge between them if it exists, and None otherwise. Here’s a snippet of what I’ve tried. It doesn’t work because it currently creates an object instead of checking if there is one in existence already. What’s the simplest way to write get_edge()?

def add_edge(self, e):
    """Adds and edge to the graph by adding an entry in both directions.

    If there is already an edge connecting these Vertices, the
    new edge replaces it.
    """
    v, w = e
    self[v][w] = e
    self[w][v] = e

def get_edge(self, v1, v2):
    try:
        Edge(v1, v2)
        print 'Edge exists'
    except:
        print 'Edge does not exist'
        return None

Answer 1

I suspect you want something like:

def get_edge(self, v1, v2):
    try:
        e = self[v1][v2] # order shouldn't matter
        print("edge exists")
        return e
    except KeyError:
        print("edge does not exist")
        return None

I’m assuming your class is derived from dict or has a __getitem__ method that works and will raise a KeyError if you ask for a key that doesn’t exist. If you don’t need the print statements (that is, they’re just for debugging), you can do away with the e variable and just return the result directly.