假设有一个英文文本文件，编写一个程序读取其内容并将里面的大写字母变成小写字母，小写字母变成大写字母

[code]def transform(filename):
file = filename + ".txt"
f = open(file,"r+")
fn = open("{}转化后.txt".format(filename),"w")
for line in f:
for word in line:
if word in "abcdefghijklmnopqrstuvwxyz":
word = word.upper()
elif word in "ABCDEFGHIJKLMNOPQRSTUVWXYZ":
word = word.lower()
fn.write(word)
f.close()
fn.close()

def main():
filename = input("请输入文件名称：")
transform(filename)

main()