merge sort in Python

mergesort.py

def mergesort(array, start, end):
    if end - start < 2:
        #
        # Do nothing.
        #
        return
    half = (end-start)/2
    #
    # Divide and conquer step.
    #
    mergesort(array, start, start+half)
    mergesort(array, start+half, end)
    #
    # Merge using temporary storage.
    #
    tmp = list()
    left = start
    right = start+half
    for i in range(end-start):
        if left == start+half:
            assert right < end
            advance_left = False
        elif right == end:
            assert left < start+half
            advance_left = True
        else:
            advance_left = array[left] < array[right]
        if advance_left:
            tmp.append(array[left])
            left += 1
        else:
            tmp.append(array[right])
            right += 1
    #
    # Copy temporary storage back into input array.
    #
    for i,val in enumerate(tmp):
        array[start+i] = val

if __name__ == "__main__":
    array = range(100)
    import random
    random.shuffle(array)
    mergesort(array, 0, len(array))
    print array == sorted(array)