Location descriptor is not always there, and will return (None, None)…

… if it isnt. I think that makes more sense than raising a cryptic exception.

genologics/descriptors.py

@@ -164,7 +164,7 @@ def __init__(self, instance, *args, **kwargs):
 
  @property
  def rootnode(self):
- if not self._rootnode:
+ if self._rootnode is None:
  self._rootnode = self.instance.root
  for rootkey in self.rootkeys:
  self._rootnode = self._rootnode.find(rootkey)
@@ -553,6 +553,8 @@ def __get__(self, instance, cls):
  from genologics.entities import Container
  instance.get()
  node = instance.root.find(self.tag)
+ if node is None:
+ return (None,None)
  uri = node.find('container').attrib['uri']
  return Container(instance.lims, uri=uri), node.find('value').text