in place merge sort

from random import randrange

arr = [1]

def generate_random():
    for i in range(0,20):
        arr.append(randrange(0,20))

def shift(s, e):
    end = arr[e]
    for i in range(e-1, s-1, -1):
        arr[i+1] = arr[i]
    arr[s] = end

# def while_shift():
#     value = arr[start2]
#     index = start2

#     # Shift all the elements between element 1
#     # element 2, right by 1.
#     while (index != start):
#         arr[index] = arr[index - 1]
#         index -= 1

#     arr[start] = value

def merge(p1, p2, end):

    print("Pointer 1: "  + str(p1) + ", Pointer 2 " + str(p2))

    # If the direct merge is already sorted (already sorted in the previous stack)
    if (arr[p2-1] <= arr[p2]):
        return

    # Two pointers to maintain start
    # of both arrays to merge
    while (p1 <= p2-1 and p2 <= end):

        # If element 1 is in right place
        if (arr[p1] <= arr[p2]):
            p1 += 1
        else:
            shift(p1, p2)

            # Update all the pointers
            p1 += 1
            p2 += 1


def merge_sort(s, e):
    if s < e:
        m = (e - s) // 2 + s

        # first half (s,m)
        merge_sort(s,m)
        # second half (m+1,e)
        merge_sort(m+1,e)

        merge(s,m+1,e)

def main():
    generate_random()
    print(arr)
    if len(arr) >= 1:
        merge_sort(0,len(arr)-1)
    print(arr)

if __name__ == '__main__':
    main()