Merge Sort Python

merge_sort.py

def msort(L):

    if len(L) == 1:
        return L

    m = len(L)/2
    return merge(msort(L[:m]), msort(L[m:]))

def merge(left, right):

    BIG = max(left[-1], right[-1]) + 1
    left.append(BIG)
    right.append(BIG)

    L = []
    cl = 0
    cr = 0

    for i in range(len(left) + len(right) - 2):
        if left[cl] < right[cr]:
            L.append(left[cl])
            cl += 1
        else:
            L.append(right[cr])
            cr += 1

    return L

print msort([2,1,4,3,6,5,8,7])