ftp .retrbinary() 提示write() argument must be str, not bytes

python3里面write 给open函数添加了encoding新参数，默认‘utf-8',因此文件句柄write和read要求传入的是Unicode字符实例，二不接收二进制byte实例，而retrbinary()的write写入方式必须是二进制byte实例，所以文件句柄fp=open(local_file,'wb'), ftp.retrbinary('RETR'+each,fp.write,bufsize)

如python2里可以用’w',不会报错：

with open('/home/tom.bin','w') as f:

       f.write(os.urandom(10)）

而python3需写成(python2也可)：

with open('/home/tom.bin','wb') as f:

       f.write(os.urandom(10))