Python output buffering

By Enrico Zini

Here's a little toy program that displays a message like a split-flap display:

#!/usr/bin/python3 import sys
import time def display(line: str): cur = '0' * len(line) while True: print(cur, end="\r") if cur == line: break time.sleep(0.09) cur = "".join(chr(min(ord(c) + 1, ord(oc))) for c, oc in zip(cur, line)) print() message = " ".join(sys.argv[1:])
display(message.upper())

This only works if the script's stdout is unbuffered. Pipe the output through cat, and you get a long wait, and then the final string, without the animation.

What is happening is that since the output is not going to a terminal, optimizations kick in that buffer the output and send it in bigger chunks, to make processing bulk I/O more efficient.

I haven't found a good introductory explanation of buffering in Python's documentation. The details seem to be scattered in the io module documentation and they mostly assume that one is already familiar with concepts like unbuffered, line-buffered or block-buffered. The libc documentation has a good quick introduction that one can read to get up to speed.

Controlling buffering in Python

In Python, one can force a buffer flush with the flush() method of the output file descriptor, like sys.stdout.flush(), to make sure pending buffered output gets sent.

Python's print() function also supports flush=True as an optional argument:

 print(cur, end="\r", flush=True)

If one wants to change the default buffering for a file descriptor, since Python 3.7 there's a convenient reconfigure() method, which can reconfigure line buffering only:

sys.stdout.reconfigure(line_buffering=True)

Otherwise, the technique is to reassign sys.stdout to something that has the behaviour one wants (code from this StackOverflow thread):

import io
# Python 3, open as binary, then wrap in a TextIOWrapper with write-through.
sys.stdout = io.TextIOWrapper(open(sys.stdout.fileno(), 'wb', 0), write_through=True)

If one needs all this to implement a progressbar, one should make sure to have a look at the progressbar module first.