copy Python programs of small

#!/usr/local/bin/python3
import sys

#将old文件复制为new
def copy(old, new):
    sf = open(old)
    df = open(new, 'w')
    for line in sf:
        df.write(line)
    df.close

try:
    source_file=sys.argv[1]     #源文件
    des_file=sys.argv[2]        #目标文件
except IndexError:
    print('Error！正确用法:%s 原文件名 拷贝后的文件名' % sys.argv[0])
else:
    copy(source_file,des_file)
    print('copy success!')