@pelson - are you happy with this example as it is?

Western Sahara shouldn't really be in the results when sorting by population (pop_est = -99.0)
>>> [c.attributes['pop_est'] for c in countries_by_pop]
[-99.0, 140.0, 3140.0, 3802.0, 57600.0]
 
Also, the example seems to be non-trivial - three of the smallest countries are actually not countries...
>>> [c.attributes['sovereignt'] for c in countries_by_pop]
['Western Sahara', 'France', 'United Kingdom', 'Antarctica', 'Denmark']