A quick implementation of heapsort

heapsort.py

parent_index = lambda x: (x - 1) / 2
left_child_index = lambda x: 2 * x + 1

def heapsort(arr):
    """
    sorts the given list using heapsort
    """
    heapify(arr)
    print_heap(arr)
    sort_heap(arr)
    
def heapify(arr):
    """
    turns list arr into a heap
    """
    cnt = len(arr)
    for current in range(parent_index(cnt - 1), -1, -1):
        sift_down(arr, current, cnt)

def sort_heap(arr):
    """
    turns the heap into a sorted list
    """
    end = len(arr)
    while end > 0:
        arr[0], arr[end - 1] = arr[end - 1], arr[0]
        end -= 1
        sift_down(arr, 0, end)
        print_heap(arr[:end])

def sift_down(arr, root, end):
    """
    fix the heap
    """
    while left_child_index(root) < end:
        left = left_child_index(root)
        swap = root
        if arr[left] > arr[swap]:
            swap = left
        if left + 1 < end and arr[left + 1] > arr[swap]:
            swap = left + 1
        if swap == root:
            return
        arr[root], arr[swap] = arr[swap], arr[root]
        root = swap

a = [6,4,7,1,2,5,8,9,3]

from math import log
def print_heap(heap):
    start = 0
    end = 1
    print('heap')
    while start < len(heap):
        spaces = ' ' * ((len(heap) * 2) / (end - start + 1))
        print(spaces + spaces.join(str(i) for i in heap[start:end]))
        start = end
        end = (end * 2) + 1
    print('end of heap')

heapsort(a)
print(a)