linux tee is not working with python?

Question

I made a python script which communicates with a web server using an infinite loop. I want to log every communication data to a file and also monitor them from terminal at same time. so I used tee command like this.

python client.py | tee logfile

however, I got nothing from terminal nor logfile. the python script is working fine. what is happening here? am I missing something?

some advice would be appreciated. thank you in advance.

Answer 1

From man python:

   -u     Force stdin, stdout and stderr to  be  totally  unbuffered.   On  systems
          where it matters, also put stdin, stdout and stderr in binary mode.  Note
          that there is internal buffering in xreadlines(), readlines()  and  file-
          object  iterators  ("for  line  in sys.stdin") which is not influenced by
          this option.  To work around this, you will want to use  "sys.stdin.read‐
          line()" inside a "while 1:" loop.

So what you can do is:

/usr/bin/python -u client.py >> logfile 2>&1

Or using tee:

python -u client.py | tee logfile

Answer 2

Instead of making it fully unbuffered you can make it linebuffered as it is normally with sys.stdout.reconfigure(line_buffering=True) (after import sys of course).

This was added in 3.7, docs: https://docs.python.org/3/library/io.html#io.TextIOWrapper.reconfigure