Output of a python script running as unit is out of order while shells seems unnafected

Question

How would I get all outputs of systemd managed python2 script instantly into journal, just like it appears in terminal

I'm on centos 7, but seems to be same on arch

according to systemd manual, stdout and stderr should automatically go to journal

systemd config

#DefaultStandardOutput=journal
#DefaultStandardError=inherit

Here is my unit /usr/lib/systemd/system/tick.service

[Unit]
Description=Tick

[Service]
Type=simple
ExecStart=/home/stack/tick

script

#!/usr/bin/python
import time
import sys

print "Tick"
sys.stderr.write('First error\n')
time.sleep(10)
sys.stdout.write("Tick\n")
sys.stderr.write('Second error\n')
time.sleep(10)

and here is the output from journal, notice the time and order

Oct 30 14:30:41 controller systemd[1]: Starting Tick...
Oct 30 14:30:41 controller systemd[1]: Started Tick.
Oct 30 14:30:41 controller tick[5020]: First error
Oct 30 14:30:51 controller tick[5020]: Second error
Oct 30 14:31:01 controller tick[5020]: Tick
Oct 30 14:31:01 controller tick[5020]: Tick

With python3 even stderr is delayed until process ends

I tried similar c program, and seems to be same

but this does not seem to affect bash script

#!/bin/bash
echo "Tick"
echo "First error" 1>&2
sleep 10
echo "Tick"
echo "Second error" 1>&2
sleep 10

journal

Oct 30 14:41:36 controller systemd[1]: Starting Tick...
Oct 30 14:41:36 controller systemd[1]: Started Tick.
Oct 30 14:41:36 controller tick[5084]: Tick
Oct 30 14:41:36 controller tick[5084]: First error
Oct 30 14:41:46 controller tick[5084]: Tick
Oct 30 14:41:46 controller tick[5084]: Second error

Answer 1

Stdout is being buffered, probably because of systemd redirect

Adding sys.stdout.flush() after write or PYTHONUNBUFFERED environment will solve this problem

See also the Stack Overflow question Python output buffering.