From https://www.educative.io/edpresso/merge-sort-in-python

mergesort.py

class Counter(object):
    value = 0

def mergeSort(myList):
    if len(myList) > 1:
        depth.value += 1

        mid = (len(myList) + 1) // 2
        left = myList[:mid]
        right = myList[mid:]
        print(f"left = {left}, right = {right}")

        # Recursive call on each half
        mergeSort(left)
        mergeSort(right)

        # Two iterators for traversing the two halves
        i = 0
        j = 0
        
        # Iterator for the main list
        k = 0
        
        while i < len(left) and j < len(right):
            merge.value += 1
            print(f"comparing position {i} ({left[i]}) from left and {j} ({right[j]}) from right")
            if left[i] < right[j]:
                # The value from the left half has been used
                print(f"putting left {left[i]} at position {k}")
                myList[k] = left[i]
                # Move the iterator forward
                i += 1
            else:
                # The value from the left half has been used
                print(f"putting right {right[j]} at position {k}")
                myList[k] = right[j]
                j += 1
            # Move to the next slot
            k += 1
            print(f"now, i, j, k = {i}, {j}, {k}")

        # For all the remaining values
        while i < len(left):
            print(f"adding remaining left {left[i]} to position {k}")
            myList[k] = left[i]
            i += 1
            k += 1

        while j < len(right):
            print(f"adding remaining left {right[j]} to position {k}")
            myList[k]=right[j]
            j += 1
            k += 1
    else:
        print("list len is <= 1")

depth = Counter()
merge = Counter()

myList = [83,20,9,50,115,61,17]
mergeSort(myList)
print(myList)