--- a/numerics/conformity.py	Fri Jul 21 12:50:17 2017 -0700
+++ b/numerics/conformity.py	Fri Jul 21 12:58:40 2017 -0700
@@ -6,6 +6,12 @@
 class NonConformantRowLengths(Exception):
     """nested arrays have different lengths"""
 
+    _msg_tmpl = "Different lengths to array_mean: {}"
+
+    def __init__(self, lengths):
+        self.lengths = lengths
+        Exception.__init__(self,
+                           self._msg_tmpl.format(' '.join([str(l) for l in lengths])))
 
 def ensure_row_length(data):
     """
@@ -13,7 +19,7 @@
     If so, return that length.
     If not, raise NonConformantArrayLengths
     """
-    lengths = [len(i) for i in data]
-    if len(set(lengths)) != 1:
-        raise NonConformantRowLengths("Different lengths to array_mean: {}".format(' '.join(lengths)))
+    lengths = set([len(i) for i in data])
+    if len(lengths) != 1:
+        raise NonConformantRowLengths(lengths)
     return lengths.pop()

--- a/tests/test_conformity.py	Fri Jul 21 12:50:17 2017 -0700
+++ b/tests/test_conformity.py	Fri Jul 21 12:58:40 2017 -0700
@@ -13,24 +13,24 @@
     def test_equal_lengths(self):
 
         data = [[1,2,3],
-                [4,5,6]
+                [4,5,6],
                 [7,8,9]]
 
         assert conformity.ensure_row_length(data) == 3
 
     def test_nonequal_lengths(self):
         data = [[1,2,3],
-                [4,5,6]
+                [4,5,6],
                 [7,8,9, 10]  # oops!
                 ]
 
         e = None
         try:
             conformity.ensure_row_length(data)
-        except conformity.NonformantRowLengths as e:
+        except conformity.NonConformantRowLengths as e:
             pass
         assert e is not None
-        assert isinstance(e, NonConformantRowLengths)
+        assert isinstance(e, conformity.NonConformantRowLengths)
 
 if __name__ == '__main__':
     unittest.main()