Allow that an edge ID may occur multiple times in a graph.

For example:

Foo id:1
Bar p:1
Baz p:1

Would only store the ID for the last added edge (Foo,Baz) and forget
about the ID of 1 for (Foo,Bar).

Also make sure that add_edge does not add an edge twice, otherwise the
edge ID could be accidentally overwritten.

Graph.py

@@ -25,15 +25,15 @@ class DirectedGraph(object):
 
         The p_id is the id of the edge, if the client wishes to maintain this.
         """
-        if not self.has_node(p_from):
-            self.add_node(p_from)
+        if not self.has_edge(p_from, p_to):
+            if not self.has_node(p_from):
+                self.add_node(p_from)
 
-        if not self.has_node(p_to):
-            self.add_node(p_to)
+            if not self.has_node(p_to):
+                self.add_node(p_to)
 
-        self._edges[p_from].add(p_to)
-        if p_id:
-            self._edge_numbers[p_id] = (p_from, p_to)
+            self._edges[p_from].add(p_to)
+            self._edge_numbers[(p_from, p_to)] = p_id
 
     def has_path(self, p_from, p_to):
         """
@@ -125,7 +125,14 @@ class DirectedGraph(object):
         """
         Returns True if the client registered an edge with the given id.
         """
-        return p_id in self._edge_numbers
+        result = False
+
+        for edge_id in self._edge_numbers.itervalues():
+            if edge_id == p_id:
+                result = True
+                break
+
+        return result
 
     def edge_id(self, p_from, p_to):
         """
@@ -133,14 +140,10 @@ class DirectedGraph(object):
 
         Returns None if the edge does not exist or has no value assigned.
         """
-        result = None
-
-        for key, value in self._edge_numbers.iteritems():
-            if value == (p_from, p_to):
-                result = key
-                break
-
-        return result
+        try:
+            return self._edge_numbers[(p_from, p_to)]
+        except KeyError:
+            return None
 
     def remove_edge(self, p_from, p_to, remove_unconnected_nodes=True):
         """
@@ -152,9 +155,10 @@ class DirectedGraph(object):
         if self.has_edge(p_from, p_to):
             self._edges[p_from].remove(p_to)
 
-        edge_id = self.edge_id(p_from, p_to)
-        if edge_id:
-            del self._edge_numbers[edge_id]
+        try:
+            del self._edge_numbers[(p_from, p_to)]
+        except KeyError:
+            return None
 
         if remove_unconnected_nodes:
             if self.is_isolated(p_from):

test/GraphTest.py

@@ -7,7 +7,7 @@ class GraphTest(unittest.TestCase):
         self.graph = Graph.DirectedGraph()
 
         self.graph.add_edge(1, 2, 1)
-        self.graph.add_edge(2, 4)
+        self.graph.add_edge(2, 4, "Test")
         self.graph.add_edge(4, 3)
         self.graph.add_edge(4, 6)
         self.graph.add_edge(6, 2)
@@ -26,9 +26,17 @@ class GraphTest(unittest.TestCase):
         for i in range(1, 7):
             self.assertTrue(self.graph.has_node(i))
 
+    def test_has_edge_ids(self):
+        self.assertTrue(self.graph.has_edge_id(1))
+        self.assertTrue(self.graph.has_edge_id("Test"))
+        self.assertFalse(self.graph.has_edge_id("1"))
+
     def test_incoming_neighbors1(self):
         self.assertEquals(self.graph.incoming_neighbors(1), set())
 
+    def test_edge_id_of_nonexistent_edge(self):
+        self.assertFalse(self.graph.edge_id(1, 6))
+
     def test_incoming_neighbors2(self):
         self.assertEquals(self.graph.incoming_neighbors(2), set([1, 6]))
 
@@ -127,8 +135,17 @@ class GraphTest(unittest.TestCase):
         # the one and only edge must be removed now
         self.assertFalse(self.graph.has_edge(1, 3))
 
+    def test_add_double_edge_with_id(self):
+        self.graph.add_edge(1, 3, "Dummy")
+        self.assertFalse(self.graph.has_edge_id("Dummy"))
+
+        self.graph.remove_edge(1, 3)
+
+        # 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'
+        out = 'digraph g {\n  1\n  1 -> 2 [label="1"]\n  1 -> 3\n  2\n  2 -> 4 [label="Test"]\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):