Merge sort program in python

# Merge sort function which uses recursive method to divide and sort the list
def mergeSort(alist):
    print("Splitting ",alist)
    if len(alist)>1:
        mid = len(alist)//2
        lefthalf = alist[:mid]
        righthalf = alist[mid:]

        mergeSort(lefthalf)
 print("after mergesort function of lefthalf")
        mergeSort(righthalf)
 print("after mergesort function of righthalf")
        i=0
        j=0
        k=0
        while i<len(lefthalf) and j<len(righthalf):
     print("inside first while")
            if lefthalf[i]<righthalf[j]:
                alist[k]=lefthalf[i]
                i=i+1
            else:
                alist[k]=righthalf[j]
                j=j+1
            k=k+1

        while i<len(lefthalf):
     print("inside second while")
            alist[k]=lefthalf[i]
            i=i+1
            k=k+1

        while j<len(righthalf):
     print("inside third while")
            alist[k]=righthalf[j]
            j=j+1
            k=k+1
    print("Merging ",alist)

# example array to be sorted
alist = [54,26,93,17,77,31,44,55,20]
# Calling mergesort function
mergeSort(alist)
# printing the sorted array
print(alist)