Python in batch jobs

By default, print in Python is buffered.  When running Python in a batch job in SLURM you may see output less often than you would when running interactively.  This is because the output is being buffered - the print statements are collected until there is a large amount to print,  then the messages are all printed at once.  For debugging or checking that a Python script is producing the correct output, you may want to switch off buffering.
Switch off buffering
For a single python script you can use the -u option, e.g. 
python -u my_script.py
The -u stands for "unbuffered".  You can use the environment variable PYTHONUNBUFFERED to set unbuffered I/O for your whole batch script. 
#!/bin/bash
#SBATCH -n 1
​
export PYTHONUNBUFFERED=TRUE
python my_script.py
There is some performance penalty for having unbuffered print statements, so you may want to reduce the number of print statements, or run buffered for production runs. 
Last modified 3yr ago