Add dot output to the Graph class.

Graph.py

@@ -181,3 +181,25 @@ class DirectedGraph(object):
         for edge in removals:
             self.remove_edge(edge[0], edge[1])
 
+    def dot(self, p_print_labels=True):
+        """ Prints the graph in Dot format. """
+        out = 'digraph g {\n'
+
+        for from_node, neighbors in self._edges.iteritems():
+            out += "  %s\n" % from_node
+
+            for neighbor in neighbors:
+                out += "  %s -> %s" % (from_node, neighbor)
+
+                edge_id = self.edge_id(from_node, neighbor)
+                if edge_id and p_print_labels:
+                    out += ' [label="%s"]' % edge_id
+
+                out += "\n"
+
+        out += '}\n'
+        return out
+
+    def __str__(self):
+        """ Returns the graph in Dot format. """
+        return self.dot()

test/GraphTest.py

@@ -126,3 +126,11 @@ class GraphTest(unittest.TestCase):
 
         # the one and only edge must be removed now
         self.assertFalse(self.graph.has_edge(1, 3))
+
+    def test_str_output(self):
+        out = 'digraph g {\n  1\n  1 -> 2 [label="1"]\n  1 -> 3\n  2\n  2 -> 4\n  3\n  3 -> 5\n  4\n  4 -> 3\n  4 -> 6\n  5\n  6\n  6 -> 2\n}\n'
+        self.assertEquals(str(self.graph), out)
+
+    def test_dot_output_without_labels(self):
+        out = 'digraph g {\n  1\n  1 -> 2\n  1 -> 3\n  2\n  2 -> 4\n  3\n  3 -> 5\n  4\n  4 -> 3\n  4 -> 6\n  5\n  6\n  6 -> 2\n}\n'
+        self.assertEquals(self.graph.dot(False), out)