diff --git a/python/tests/test_chunk_layout.py b/python/tests/test_chunk_layout.py
index 204a94c61..be69167d9 100644
--- a/python/tests/test_chunk_layout.py
+++ b/python/tests/test_chunk_layout.py
@@ -2,36 +2,43 @@
 import copy
 import unittest
 
-process_ids = []
-chunk_areas = []
 
 def traverse_tree(bp=None,min_corner=None,max_corner=None):
-    if ((min_corner.x > max_corner.x) or (min_corner.y > max_corner.y)):
-        raise RuntimeError("min_corner/max_corner have been incorrectly defined.")
-
-    ## reached a leaf
-    if (bp.left is None and bp.right is None):
-        process_ids.append(bp.proc_id)
-        chunk_area = (max_corner.x-min_corner.x)*(max_corner.y-min_corner.y)
-        chunk_areas.append(chunk_area)
-
-    ## traverse the left branch
-    if (bp.left is not None):
-        new_max_corner = copy.deepcopy(max_corner)
-        if bp.split_dir == mp.X:
-            new_max_corner.x = bp.split_pos
-        else:
-            new_max_corner.y = bp.split_pos
-        traverse_tree(bp.left,min_corner,new_max_corner)
-
-    ## traverse the right branch                                      
-    if (bp.right is not None):
-        new_min_corner = copy.deepcopy(min_corner)
-        if bp.split_dir == mp.X:
-            new_min_corner.x = bp.split_pos
-        else:
-            new_min_corner.y = bp.split_pos
-        traverse_tree(bp.right,new_min_corner,max_corner)
+
+    process_ids = []
+    chunk_areas = []
+
+    def _traverse_tree(bp=None,min_corner=None,max_corner=None):
+        if ((min_corner.x > max_corner.x) or (min_corner.y > max_corner.y)):
+            raise RuntimeError("min_corner/max_corner have been incorrectly defined.")
+
+        ## reached a leaf
+        if (bp.left is None and bp.right is None):
+            process_ids.append(bp.proc_id)
+            chunk_area = (max_corner.x-min_corner.x)*(max_corner.y-min_corner.y)
+            chunk_areas.append(chunk_area)
+
+        ## traverse the left branch
+        if (bp.left is not None):
+            new_max_corner = copy.deepcopy(max_corner)
+            if bp.split_dir == mp.X:
+                new_max_corner.x = bp.split_pos
+            else:
+                new_max_corner.y = bp.split_pos
+            _traverse_tree(bp.left,min_corner,new_max_corner)
+
+        ## traverse the right branch
+        if (bp.right is not None):
+            new_min_corner = copy.deepcopy(min_corner)
+            if bp.split_dir == mp.X:
+                new_min_corner.x = bp.split_pos
+            else:
+                new_min_corner.y = bp.split_pos
+            _traverse_tree(bp.right,new_min_corner,max_corner)
+
+    _traverse_tree(bp=bp, min_corner=min_corner, max_corner=max_corner)
+
+    return process_ids, chunk_areas
 
 
 class TestChunkLayoutBinaryPartition(unittest.TestCase):
@@ -49,7 +56,7 @@ def test_chunk_layout_binary_partition(self):
         owners = sim.structure.get_chunk_owners()
         areas = [ v.surroundings().full_volume() for v in sim.structure.get_chunk_volumes() ]
 
-        traverse_tree(chunk_layout,-0.5*cell_size,0.5*cell_size)
+        process_ids, chunk_areas = traverse_tree(chunk_layout,-0.5*cell_size,0.5*cell_size)
 
         self.assertListEqual([int(f) for f in owners],[f % mp.count_processors() for f in process_ids])
         self.assertListEqual(areas,chunk_areas)
@@ -64,7 +71,8 @@ def test_meep_default_chunk_layout(self):
         areas = [ v.surroundings().full_volume() for v in sim.structure.get_chunk_volumes() ]
 
         chunk_layout = sim.chunk_layout
-        traverse_tree(chunk_layout,-0.5*cell_size,0.5*cell_size)
+        
+        process_ids, chunk_areas = traverse_tree(chunk_layout,-0.5*cell_size,0.5*cell_size)
 
         self.assertListEqual([int(f) for f in owners],[f % mp.count_processors() for f in process_ids])
         self.assertListEqual(areas,chunk_areas)