问为什么我们要在python的一个函数中多次打开文件？

EN

a = open("prime_numbers.txt", 'r+')
b = open('happy_numbers.txt','r+')
c = a.read().split()
d = b.read().split()
def overlap(list1,list2):
    e= open('final_number_list.txt','r+')
    for i in list1:
       for j in list2:
           if i == j:
             e.write(i+'\n')
           else:continue
    e = open('final_number_list.txt', 'r+')
    print(e.read().split())
overlap(c,d)