Since we are talking about namedtuple and implementation, I just noticed: In [22]: Point = namedtuple('Point', ['x', 'y']) In [23]: p = Point(2,3) In [24]: p.x = 5 --------------------------------------------------------------------------- AttributeError Traceback (most recent call last) <ipython-input-24-328d3fab3e30> in <module>() ----> 1 p.x = 5 AttributeError: can't set attribute OK -- that makes sense. but then, if you try: In [25]: p.z = 5 --------------------------------------------------------------------------- AttributeError Traceback (most recent call last) <ipython-input-25-625ed954d865> in <module>() ----> 1 p.z = 5 AttributeError: 'Point' object has no attribute 'z' I think this should be a different message -- key here is that you can't set a new attribute, not that one doesn't exist. Maybe: "AttributeError: can't set new attribute" -CHB -- Christopher Barker, Ph.D. Oceanographer Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception Chris.Barker@noaa.gov