Python Merge Sort

#KEY:Visualize the control flow (recursion) and the return value; range checking about the merge process

import pdb

def print_array(arr):
    for a in arr:
        print a,

def merge(a,b):
    i,j = 0,0
    merge_array = []
    merge_array_cursor = 0
    while merge_array_cursor < len(a)+len(b):
        if i < len(a):
            if j < len(b):
                if a[i]< b[j]:
                    merge_array.append(a[i])
                    i += 1
                else:
                    merge_array.append(b[j])
                    j += 1
            elif not (j < len(b)):
                merge_array.append(a[i])
                i += 1
        elif not (i < len(a)):
            if j < len(b):
                merge_array.append(b[j])
                j += 1
            elif not(j < len(b)):
                pass   
        
        
        merge_array_cursor += 1
        
    return merge_array

def merge_sort(array):
    if len(array) == 1:
        #pdb.set_trace()
        return array

    new_length = len(array)/2
    sub_array1 = merge_sort(array[:new_length])
    sub_array2 = merge_sort(array[new_length:])

    return merge(sub_array1, sub_array2)

if __name__ == "__main__":
    array = [9,-1,1,2,1,3,6,8,7,0]
    answer = merge_sort(array) #stupid bug: forgot to take 
                               # the return value from  merge_sort(array) 
    print "answer:"
    print_array(answer)