import sys
savedStdout = sys.stdout  #保存标准输出流
print_log = open("printlog.txt","w")
sys.stdout = print_log
print('This message is for file!')
# 中间print的内容都被输出到printlog.txt    
sys.stdout = savedStdout  #恢复标准输出流
print ('This message is for screen!')
print_log.close()

方法二