Basic Graphviz input file generator in Python

Generating a Graphviz Dot input file is fairly simple, so I tend to code it up myself rather than use an existing library. However the need to normalise the node names complicates things a little bit. Here's a copy of the one I wrote today. Note that you may need to beef up the normalisation if your node names contain additional non-alphanumeric characters. (Note to future self: next time just autoincrement the node labels instead of trying to use a normalised form.)

class Graph:
    def __init__(self):
        self.lookup = {}
        self.edges = []
    def add_edge(self, x):
        if x in self.lookup: return
        nx = x
        for y in " -()[]":
            nx = nx.replace(y, "_") # normalise the label
        self.lookup[x] = nx
    def add(self, x, y):
        self.add_edge(x)
        self.add_edge(y)
        self.edges.append( (x, y) )
    def write(self):
        tmp = ["digraph {"]
        for x, y in self.edges:
            tmp.append( '%s -> %s;' % (self.lookup[x], self.lookup[y]))
        for x, y in self.lookup.iteritems():
            tmp.append('%s [label="%s"]' % (y, x))
        tmp.append("}")
        return "\n".join(tmp)