Why does bash sometime not flush output of a python program to a file

I have a crontab job calling a python script and outputting to a file:

python run.py &> current_date.log

now sometimes when I do

tail -f current_date.log

I see the file filling up with the output, but other times the log file exists, but stays empty for a long time. I am sure that the python script is printing stuff right after it starts running, and the log file is created. Any ideas why d开发者_开发问答oes it stay empty some of the time?

Python buffers output when it detects that it is not writing to a tty, and so your log file may not receive any output right away. You can configure your script to flush output or you can invoke python with the -u argument to get unbuffered output.

$ python -h

...

-u     : unbuffered binary stdout and stderr (also PYTHONUNBUFFERED=x)
         see man page for details on internal buffering relating to '-u'

...

The problem is actually Python (not bash) and is by design. Python buffers output by default. Run python with -u to prevent buffering.

Another suggestion is to create a class (or special function) which calls flush() right after the write to the log file.