Fixed version of the heap implementation from https://www.programiz.com/dsa/heap-data-structure

heap.py

# Max-Heap data structure in Python
import math

def siftDown(arr, n, i):
    largest = i
    l = 2 * i + 1
    r = 2 * i + 2

    if l < n and arr[i] < arr[l]:
        largest = l

    if r < n and arr[largest] < arr[r]:
        largest = r

    if largest != i:
        arr[i], arr[largest] = arr[largest], arr[i]
        siftDown(arr, n, largest)


def siftUp(arr, n, i):
    if i <= 0:
        return
    parent = (i - 1) // 2
    if arr[parent] < arr[i]:
        arr[i], arr[parent] = arr[parent], arr[i]
        siftUp(arr, n, parent)

def insert(array, newNum):
    size = len(array)
    if size == 0:
        array.append(newNum)
    else:
        array.append(newNum)
        siftUp(array, len(array), len(array)-1)

def deleteNode(array, num):
    size = len(array)
    i = 0
    for i in range(0, size):
        if num == array[i]:
            break

    array[i], array[size - 1] = array[size - 1], array[i]

    array = array[:size-1]
    siftDown(array, len(array), i)
    return array

def printHeap(array):
    size = len(array)
    height = math.ceil(math.log(size, 2))
    for h in range(0, height):
        start = 2**h
        end = 2**(h+1)
        print(array[start-1:end-1])

def heapify(array):
    size = len(array)
    for i in range(size // 2 - 1, -1, -1):
        siftDown(array, size, i)

arr = []

insert(arr, 9)
print("Max-Heap array: " + str(arr))
insert(arr, 4)
print("Max-Heap array: " + str(arr))
insert(arr, 5)
print("Max-Heap array: " + str(arr))
insert(arr, 1)
print("Max-Heap array: " + str(arr))
insert(arr, 3)
print("Max-Heap array: " + str(arr))
insert(arr, 2)
print("Max-Heap array: " + str(arr))
insert(arr, 7)

print("Max-Heap array: " + str(arr))

print("printHeap:")
printHeap(arr)

arr = deleteNode(arr, 9)
print("After deleting element 9: " + str(arr))

print("heapify:")
arr = [1,2,3,4,5,7,9]
heapify(arr)
printHeap(arr)