merge sort in python

merge.py

def ms(n):
    if len(n)>1:
        mid=len(n)//2
        lefthalf=n[:mid]
        righthalf=n[mid:]
        ms(lefthalf)
        ms(righthalf)
        i=j=k=0
        while i<len(lefthalf) and j<len(righthalf):
            if lefthalf[i]<righthalf[j]:
                n[k]=lefthalf[i]
                i+=1
                k+=1
            else:
                n[k]=righthalf[j]
                j+=1
                k+=1
        while i<len(lefthalf):
            n[k]=lefthalf[i]
            i+=1
            k+=1
        while j<len(righthalf):
            n[k]=righthalf[j]
            j+=1
            k+=1
    return n
a=[9,1,5,3,4,3]
print(ms(a))