Address comments from @cjosey on #890

openmc/cell.py

@@ -103,7 +103,7 @@ class Cell(object):
         self._rotation_matrix = None
         self._temperature = None
         self._translation = None
-        self._paths = []
+        self._paths = None
         self._num_instances = None
         self._volume = None
         self._atoms = None
@@ -215,7 +215,7 @@ class Cell(object):
 
     @property
     def paths(self):
-        if not self._paths:
+        if self._paths is None:
             raise ValueError('Cell instance paths have not been determined. '
                              'Call the Geometry.determine_paths() method.')
         return self._paths
@@ -230,6 +230,10 @@ class Cell(object):
 
     @property
     def num_instances(self):
+        if self._num_instances is None:
+            raise ValueError(
+                'Number of cell instances have not been determined. Call the '
+                'Geometry.determine_paths() method.')
         return self._num_instances
 
     @id.setter

openmc/geometry.py

@@ -491,7 +491,7 @@ class Geometry(object):
 
         Parameters
         ----------
-        instances_only : bool
+        instances_only : bool, optional
             If true, this method will only determine the number of instances of
             each cell and material.
 

openmc/material.py

@@ -97,7 +97,7 @@ class Material(object):
         self._density = None
         self._density_units = ''
         self._depletable = False
-        self._paths = []
+        self._paths = None
         self._num_instances = None
         self._volume = None
         self._atoms = {}
@@ -210,13 +210,17 @@ class Material(object):
 
     @property
     def paths(self):
-        if not self._paths:
+        if self._paths is None:
             raise ValueError('Material instance paths have not been determined. '
                              'Call the Geometry.determine_paths() method.')
         return self._paths
 
     @property
     def num_instances(self):
+        if self._num_instances is None:
+            raise ValueError(
+                'Number of material instances have not been determined. Call '
+                'the Geometry.determine_paths() method.')
         return self._num_instances
 
     @property