I've refactored how the merged-dictionary (all_classes) below is created, but I'm wondering if it can be more efficient. I have a dictionary of dictionaries, like this: groups_and_classes = {'group_1': {'class_A': [1, 2, 3], 'class_B': [1, 3, 5, 7], 'class_c': [1, 2], # ...many more items like this }, 'group_2': {'class_A': [11, 12, 13], 'class_C': [5, 6, 7, 8, 9] }, # ...and many more items like this } A function creates a new object from groups_and_classes like this (the function to create this is called often): all_classes = {'class_A': [1, 2, 3, 11, 12, 13], 'class_B': [1, 3, 5, 7, 9], 'class_C': [1, 2, 5, 6, 7, 8, 9] } Right now, there is a loop that does this: all_classes = {} for group in groups_and_classes.values(): for c, vals in group.iteritems(): for v in vals: if all_classes.has_key(c): if v not in all_classes[c]: all_classes[c].append(v) else: all_classes[c] = [v] So far, I changed the code to use a set instead of a list since the order of the list doesn't matter and the values need to be unique: all_classes = {} for group in groups_and_classes.values(): for c, vals in group.iteritems(): try: all_classes[c].update(set(vals)) except KeyError: all_classes[c] = set(vals) This is a little nicer, and I didn't have to convert the sets to lists because of how all_classes is used in the code. Question: Is there a more efficient way of creating all_classes (aside from building it at the same time groups_and_classes is built, and changing everywhere this function is called)?