More graceful handling of empty partitions.

Author: stefanv

brainx/modularity.py

@@ -71,6 +71,9 @@ def __init__(self, graph, index):
         self.num_nodes = graph.number_of_nodes()
         self.num_edges = graph.number_of_edges()
 
+        if self.num_edges == 0:
+            raise ValueError("TODO: Cannot create a graph partition of only one node.")
+
         # Store the nodes as a set, needed for many operations
         self._node_set = set(graph.nodes())
 
@@ -1339,11 +1342,12 @@ def _divide_partition(p, max_div=np.inf):
             only `pp` is returned.
 
         """
-        if max_div <= 0:
+        p = np.asarray(p)
+
+        if max_div <= 0 or p.size == 1:
             return [p]
 
         # Construct the subgraph modularity matrix
-        p = np.asarray(p)
         A_ = A[p, p[:, None]]
         k_ = np.sum(A_, axis=0)
         M_ = np.sum(k_)

brainx/tests/test_modularity.py

@@ -762,3 +762,13 @@ def test_adjust_partition():
 
     npt.assert_(p0 > 0.38)
     npt.assert_(p1 > 0.42)
+
+
+def test_empty_graphpartition():
+    g = nx.Graph()
+    g.add_node(1)
+    npt.assert_raises(ValueError, mod.GraphPartition, g, {1: g.nodes()})
+
+
+if __name__ == "__main__":
+    npt.run_module_suite()