请写一个python逻辑，计算一个文件中的大写字母数量

import os
os.chdir(r'C:\Users\Administrator\Desktop')#os.chdir切换到指定目录
with open('a.txt') as today:
    count=0
    for i in today.read():
        if i.isupper():
            count+=1
print(count)

#11