def group_select(selects, length=None, depth=None):
    """
    Given a list of key tuples to select, groups them into sensible
    chunks to avoid duplicating indexing operations.
    """
    if length == None and depth == None:
        length = depth = len(selects[0])
    getter = operator.itemgetter(depth-length)
    if length > 1:
        selects = sorted(selects, key=getter)
        grouped_selects = defaultdict(dict)
        for k, v in itertools.groupby(selects, getter):
            grouped_selects[k] = group_select(list(v), length-1, depth)
        return grouped_selects
    else:
        return list(selects)
Given a list of key tuples to select, groups them into sensible
    chunks to avoid duplicating indexing operations.