user defined list sorting function using divide-and-conquer

mergesort.py

def merge(X,left,middle,right):                   #Assume that left and right part are sorted
    A=X[left:middle+1]
    B=X[middle+1:right+1]
    a,b=len(A),len(B)
    i,j=0,0                                       #i for A, j for B
    while (i!=a and j!=b):                        #compare elements of both part until one reaches end
        if A[i]<=B[j]:                            #right one is bigger
            X[left+i+j]=A[i]                      #Modify X i+j th element as left one
            i+=1
        else:
            X[left+i+j]=B[j]
            j+=1
    if i!=a:                                      #end of B
        for l in range (i,a):
            X[left+l+j]=A[l]                      #add leftover of A
    else:
        for l in range (j,b):
            X[left+l+i]=B[l]


def mergesort(A, left, right):                    #Modifier Function for 'A"
    if right>left:                                #if len(A[left:right+1])>1. target has more then two values
        middle = left + (right-left)/2
        mergesort(A,left,middle)                  #sort left part
        mergesort(A,middle+1,right)               #sort right part
        merge(A,left,middle,right)                #merge sorted both part into one.

A=[4,3,2,1]
mergesort(A,0,3)