If your incoming value was a dataset, the example above would be adjusted like this:
pydset = system.dataset.toPyDataSet(value)
returned_rows = []
for row in pydset:
row_dict = {}
row_dict['country'] = row['country']
row_dict['city'] = row['city']
population = row['population']
if int(population) < 3000000:
population = {'value': population, 'style': {'backgroundColor': '#00FF00'}}
row_dict['population'] = population
returned_rows.append(row_dict)
return returned_rows