将控制台输出的同时即时保存到文件

print函数中的file参数，file=f，输出到文件；file=sys.stdout，输出到终端；flush=True，即时刷新

4.用类实现

import sys
class Logger(object):
    def __init__(self, filename='default.log', stream=sys.stdout):
        self.terminal = stream
        self.log = open(filename, 'a')
    def write(self, message):
        self.terminal.write(message)
        self.log.write(message)
    def flush(self):
sys.stdout = Logger(stream=sys.stdout)
# now it works
print('print something')
print("output")