gistfile1.py

>>> class IdentifierList(list):
...     def by_namespace(self):
...         return None
... 
>>> class Locality():
...     @property
...     def identifiers(self):
...         return IdentifierList(self._identifiers)
...     @identifiers.setter
...     def identifiers(self, value):
...         self._identifiers = value
... 
>>> l = Locality()
>>> l.identifiers = ["hello"]  # assigning a list?
>>> l.identifiers.append("hello") # don't you want something more like this?
>>> l.identifiers
['hello']
>>> l.identifiers.by_namespace
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'list' object has no attribute 'by_namespace'
>>> type(l.identifiers)
<type 'list'>