Determining if stdout for a Python process is redirected

Question

I've noticed that curl can tell whether or not I'm redirecting its output (in which case it puts up a progress bar).

Is there a reasonable way to do this in a Python script? So:

$ python my_script.py
Not redirected

and

$ python my_script.py > output.txt  
Redirected!

score 45 · Answer 1 · edited Apr 04 '23 at 12:44

45

import sys

if sys.stdout.isatty():
    print("Not redirected")
else:
    sys.stderr.write("Redirected!\n")

edited Apr 04 '23 at 12:44

Neuron

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answered Oct 03 '09 at 01:11

nosklo

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4

There is a catch on Windows: if a stream is redirected to nul, `isatty` still returns True. – user87690 Oct 08 '15 at 20:04

score 15 · Accepted Answer · edited Apr 04 '23 at 12:44

15

Actually, what you want to do here is find out if stdin and stdout are the same thing.

$ cat test.py
import os
print(os.fstat(0) == os.fstat(1))
$ python test.py
True
$ python test.py > f
$ cat f
False
$

The longer but more traditional version of the are they the same file test just compares st_ino and st_dev. Typically, on windows these are faked up with a hash of something so that this exact design pattern will work.

edited Apr 04 '23 at 12:44

Neuron

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answered Oct 03 '09 at 01:31

DigitalRoss

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Since this works on Windows I think it's what I want. Note that this approach *won't* work if I do: $ echo "blah" | python test.py – Jeffrey Harris Oct 04 '09 at 18:49
1

This tells you if either stdin OR stdout have been redirected. `echo foo | python test.py` will also result in "False" as an output. – CDahn Feb 04 '22 at 21:05

score 4 · Answer 3 · answered Oct 03 '09 at 00:55

4

Look at

os.isatty(fd)

(I don't think this works on Windows, however)

answered Oct 03 '09 at 00:55

Jim Garrison

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Determining if stdout for a Python process is redirected

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