Python - stdin - how to recognize the source of the input?

Question

I am looking for a way how to determine whether stdin input comes from another application by piping or not.

Let say that I have a program that either accepts the input data from piped stdin (when you pipe it from another application - grep, tail, ...) or it uses a default data file. I don't want the user to fill in manually the data when prompted because there was no stdin piped.

My simple code example looks like this:

from sys import stdin

for line in stdin:
    print line

When I run the script using this:

echo "data" | python example.py

I get

data

and the script ends.

While if I run the script in the following way,

python example.py

it prompts user to fill in the input and it waits.

Therefore I am looking for something like follows to avoid the prompt when no data are piped.

from sys import stdin

if is_stdin_piped():
    for line in stdin:
        print line
else:
    print "default"

Is something like this possible? Thanks

Joni · Accepted Answer · 2014-11-22 19:10:13Z

5

If you use input redirection, the standard input is not connected to a terminal like it usually is. You can check if a file descriptor is connected to a terminal or not with the isatty function:

import os

def is_stdin_piped():
    return not os.isatty(0)

For extra safety use sys.stdin.fileno() instead of 0.

Update: To check if stdin is redirected from a file (rather than another program, such as an IDE or a shell pipeline) you can use fstat:

import os, stat

def is_stdin_a_file():
    status = os.fstat(0)
    return stat.S_ISREG(status.st_mode)

edited Nov 22, 2014 at 19:10

answered Nov 22, 2014 at 18:39

Joni

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2 Comments

Marek Over a year ago

It looks good and it does almost what I need. When I run it from the command line, everythin is working well. However, when I run it in PyDev IDE (therefore no piping) the is_stdin_piped() returns as it was piped. Any idea how to fix this? Thanks

Joni Over a year ago

Your IDE uses a pipe without you knowing it. But hold on - there's a way to check if the input is redirected from a file.

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