From 2a339fe69cbca2a871b5b2425caee417d462a928 Mon Sep 17 00:00:00 2001
From: Steven Silvester <steven.silvester@ieee.org>
Date: Mon, 4 Aug 2014 07:40:37 -0500
Subject: [PATCH] Improve import behavior for rag fallback

---
 skimage/graph/rag.py | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/skimage/graph/rag.py b/skimage/graph/rag.py
index fc4a1aaa..7c1b7f07 100644
--- a/skimage/graph/rag.py
+++ b/skimage/graph/rag.py
@@ -1,11 +1,13 @@
 try:
     import networkx as nx
 except ImportError:
+    msg = "Graph functions require networkx, which is not installed"
     class nx:
         class Graph:
-            pass
+            def __init__(self, *args, **kwargs):
+                raise ImportError(msg)
     import warnings
-    warnings.warn('Region Adjacency Graph (RAG) features require networkx')
+    warnings.warn(msg)
 import numpy as np
 from scipy.ndimage import filters
 from scipy import ndimage as nd