Merge sort with python

最近在看《算法导论》，同时又在学习Python，所以想到用PYTHON实现看过的算法，来练手。

PYTHON帮忙看看有没有需要改进的写法

# Suppose the index of current item is i,
# then the parent index is int( (i-1)/2 ),
# the left child index is 2*i+1,
# the right child index is 2*i+2

heap_size = int()
def heapsort(A):
    global heap_size
    heap_size = 0
    build_max_heap(A)
    for i  in range(len(A)-1, 0, -1):
        # exchange A[0], A[i]
        A[0], A[i] = A[i], A[0]
        heap_size = heap_size-1
        max_heapify(A, 0);

def build_max_heap(A):
    global heap_size
    heap_size = len(A)
    for i in range( int( (len(A)-2)/2 ),-1,-1 ):
        max_heapify(A, i)

def max_heapify(A, i):
    global heap_size
    l = 2*i+1
    r = 2*i+2
    if l <= heap_size-1 and A[l] > A[i]:
        largest = l
    else:
        largest = i
    if r <= heap_size-1 and A[r] > A[largest]:
        largest = r
    if largest != i:
        A[larget], A[i] = A[i], A[largest]
        max_heapify(A, i)