added non in-place merge

doc/examples/plot_rag.py

@@ -75,7 +75,7 @@ display(g, "Original Graph")
 g.merge_nodes(1, 3)
 display(g, "Merged with default (min)")
 
-gc.merge_nodes(1, 3, weight_func=max_edge)
-display(gc, "Merged with max")
+gc.merge_nodes(1, 3, weight_func=max_edge, in_place=False)
+display(gc, "Merged with max without in_place")
 
 plt.show()

skimage/graph/rag.py

@@ -59,9 +59,9 @@ class RAG(nx.Graph):
     `networx.Graph <http://networkx.github.io/documentation/latest/reference/classes.graph.html>`_
     """
 
-    def merge_nodes(self, src, dst, weight_func=min_weight, extra_arguments=[],
-                    extra_keywords={}):
-        """Merge node `src` into `dst`.
+    def merge_nodes(self, src, dst, weight_func=min_weight, in_place=True,
+                    extra_arguments=[], extra_keywords={}):
+        """Merge node `src` and `dst`.
 
         The new combined node is adjacent to all the neighbors of `src`
         and `dst`. `weight_func` is called to decide the weight of edges
@@ -78,24 +78,44 @@ class RAG(nx.Graph):
             **extra_keywords)`. `src`, `dst` and `n` are IDs of vertices in the
             RAG object which is in turn a subclass of
             `networkx.Graph`.
+        in_place : bool
+            If set to `True`, the merged node has the id `dst`, else merged
+            node has a new id which is returned.
         extra_arguments : sequence, optional
             The sequence of extra positional arguments passed to
             `weight_func`.
         extra_keywords : dictionary, optional
             The dict of keyword arguments passed to the `weight_func`.
 
+        Returns
+        -------
+        id : int
+            The id of the new node if `in_place` is `True`.
         """
         src_nbrs = set(self.neighbors(src))
         dst_nbrs = set(self.neighbors(dst))
         neighbors = (src_nbrs & dst_nbrs) - set([src, dst])
+        if not in_place:
+            new = self.number_of_nodes() + 1
+            self.add_node(new)
 
         for neighbor in neighbors:
             w = weight_func(self, src, dst, neighbor, *extra_arguments,
                             **extra_keywords)
-            self.add_edge(neighbor, dst, weight=w)
+            if in_place:
+                self.add_edge(neighbor, dst, weight=w)
+            else:
+                self.add_edge(neighbor, new, weight=w)
 
-        self.node[dst]['labels'] += self.node[src]['labels']
-        self.remove_node(src)
+        if in_place:
+            self.node[dst]['labels'] += self.node[src]['labels']
+            self.remove_node(src)
+        else:
+            self.node[new]['labels'] = (self.node[src]['labels'] +
+                                        self.node[dst]['labels'])
+            self.remove_node(src)
+            self.remove_node(dst)
+            return new
 
 
 def _add_edge_filter(values, graph):

skimage/graph/tests/test_rag.py

@@ -41,8 +41,8 @@ def test_rag_merge():
     assert gc.edge[1][2]['weight'] == 20
     assert gc.edge[2][3]['weight'] == 40
 
-    g.merge_nodes(1, 4)
-    g.merge_nodes(2, 3)
+    g.merge_nodes(1, 4, in_place=True)
+    g.merge_nodes(2, 3, in_place=True)
     g.merge_nodes(3, 4)
     assert sorted(g.node[4]['labels']) == list(range(5))
     assert g.edges() == []