partition.py

def partition_items(items, num_top_items, key=lambda x: x):
    if items and 0 < num_top_items < len(items):
        pivot = items[0]
        smaller = []
        larger = []

        for i in items[1:]:
            if key(i) < key(pivot):
                smaller.append(i)
            else:
                larger.append(i)

        smaller = partition_items(smaller, num_top_items, key)
        larger = partition_items(larger, num_top_items - len(smaller) - 1, key)
        items = smaller + [pivot] + larger

    return items