Thanks for your help. It worked.
Also, as a follow up question, how can I use the datasets.ClassLabel feature to specify a predefined set of classes which can have labels associated to them and be stored as integers in the dataset.
This field will be stored and retrieved as an integer value and two conversion methods, datasets.ClassLabel.str2int() and datasets.ClassLabel.int2str() can be used to convert from the label names to the associate integer value and vice-versa.