Purely functional merge sort in python

merge_sort.py

def merge(chunk1, chunk2):
    if len(chunk1) == 0:
        return chunk2
    elif len(chunk2) == 0:
        return chunk1
    else:
        c1 = chunk1[0]
        c2 = chunk2[0]
        if c1 < c2:
            return [c1] + merge(chunk1[1:], chunk2)
        else:
            return [c2] + merge(chunk1, chunk2[1:])

def partition(l, i):
    return [l[0:i], l[i:len(l)]]

def merge_sort(l):
    if len(l) == 1:
        return l
    else:
        a, b = partition(l, len(l)/2)
        return merge(merge_sort(a), merge_sort(b))

merge_sort([5,4,5,1,6,7,4])