python 报错I/O问题

python 报错 ValueError: I/O operation on closed file.

找了一下午原因不知道为什么 有知道的吗，谢谢评论

import datetime
# 处理时间循
f_out =open('/pyt/num.txt','r+')
a = f_out.read()
a = int(a) + 1
#print(a)
f_out.seek(0)
while a >= 30:
           f_out.write('30');
           f_out.close();
           with open('/pyt/num.txt',mode='r+') as f:
                t = f.read()
                t = t.replace('30','0')
                f.seek(0)
                f.write(t)
                f.truncate()
else:
    f_out.write(str(a))
f_out.close()

# 处理时间加多少天
src_day = '20211129'
src_day = datetime.datetime.strptime(src_day, '%Y%m%d')
out_dt = src_day + datetime.timedelta(days = a)
print (out_dt)

第10行已经将f_out.close()关闭，第19行又调用write()写入。把第10行去掉