python stdout to file does not respond in real time

Question

Write a simple python script test.py:

  import time
  print "begin"
  time.sleep(10)
  print "stop"

In bash, run

python test.py > log.txt

What I observe is that both "begin" and "stop" appear in log.txt at the same time, after 10 seconds.

Is this expected behavior?

Answer 1

You need to flush the buffer after printing.

import sys
import time

print "begin"
sys.stdout.flush()

time.sleep(10)

print "end"
sys.stdout.flush()

Or in Python 3:

# This can also be done in Python 2.6+ with
# from __future__ import print_function

import time

print("begin", flush=True)
time.sleep(10)
print("end", flush=True)

Answer 2

您是否尝试使用-u选项调用python，“强制stdin，stdout和stderr完全无缓冲”=>

python -u test.py > log.txt

Answer 3

This is because the actual writing happens only when a buffer is full, or when the file is closed. When the program ends (after 10s), the buffer is emptied, the data gets written and the file closed.