How do I write to a Python subprocess' stdin?


I’m trying to write a Python script that starts a subprocess, and writes to the subprocess stdin. I’d also like to be able to determine an action to be taken if the subprocess crashes.

The process I’m trying to start is a program called nuke which has its own built-in version of Python which I’d like to be able to submit commands to, and then tell it to quit after the commands execute. So far I’ve worked out that if I start Python on the command prompt like and then start nuke as a subprocess then I can type in commands to nuke, but I’d like to be able to put this all in a script so that the master Python program can start nuke and then write to its standard input (and thus into its built-in version of Python) and tell it to do snazzy things, so I wrote a script that starts nuke like this:["C:/Program Files/Nuke6.3v5/Nuke6.3", "-t", "E:/NukeTest/test.nk"])

Then nothing happens because nuke is waiting for user input. How would I now write to standard input?

I’m doing this because I’m running a plugin with nuke that causes it to crash intermittently when rendering multiple frames. So I’d like this script to be able to start nuke, tell it to do something and then if it crashes, try again. So if there is a way to catch a crash and still be OK then that’d be great.

Asked By: jonathan topf



You can provide a file-like object to the stdin argument of

The documentation for the Popen object applies here.

To capture the output, you should instead use subprocess.check_output(), which takes similar arguments. From the documentation:

>>> subprocess.check_output(
...     "ls non_existent_file; exit 0",
...     stderr=subprocess.STDOUT,
...     shell=True)
'ls: non_existent_file: No such file or directoryn'
Answered By: user626998

It might be better to use communicate:

from subprocess import Popen, PIPE, STDOUT
p = Popen(['myapp'], stdout=PIPE, stdin=PIPE, stderr=PIPE)
stdout_data = p.communicate(input='data_to_write')[0]

“Better”, because of this warning:

Use communicate() rather than .stdin.write, or to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process.

Answered By: jro

To clarify some points:

As jro has mentioned, the right way is to use subprocess.communicate.

Yet, when feeding the stdin using subprocess.communicate with input, you need to initiate the subprocess with stdin=subprocess.PIPE according to the docs.

Note that if you want to send data to the process’s stdin, you need to create the Popen object with stdin=PIPE. Similarly, to get anything other than None in the result tuple, you need to give stdout=PIPE and/or stderr=PIPE too.

Also qed has mentioned in the comments that for Python 3.4 you need to encode the string, meaning you need to pass Bytes to the input rather than a string. This is not entirely true. According to the docs, if the streams were opened in text mode, the input should be a string (source is the same page).

If streams were opened in text mode, input must be a string. Otherwise, it must be bytes.

So, if the streams were not opened explicitly in text mode, then something like below should work:

import subprocess
command = ['myapp', '--arg1', 'value_for_arg1']
p = subprocess.Popen(command, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
output = p.communicate(input='some data'.encode())[0]

I’ve left the stderr value above deliberately as STDOUT as an example.

That being said, sometimes you might want the output of another process rather than building it up from scratch. Let’s say you want to run the equivalent of echo -n 'CATCHnme' | grep -i catch | wc -m. This should normally return the number characters in ‘CATCH’ plus a newline character, which results in 6. The point of the echo here is to feed the CATCHnme data to grep. So we can feed the data to grep with stdin in the Python subprocess chain as a variable, and then pass the stdout as a PIPE to the wc process’ stdin (in the meantime, get rid of the extra newline character):

import subprocess

what_to_catch = 'catch'
what_to_feed = 'CATCHnme'

# We create the first subprocess, note that we need stdin=PIPE and stdout=PIPE
p1 = subprocess.Popen(['grep', '-i', what_to_catch], stdin=subprocess.PIPE, stdout=subprocess.PIPE)

# We immediately run the first subprocess and get the result
# Note that we encode the data, otherwise we'd get a TypeError
p1_out = p1.communicate(input=what_to_feed.encode())[0]

# Well the result includes an 'n' at the end, 
# if we want to get rid of it in a VERY hacky way
p1_out = p1_out.decode().strip().encode()

# We create the second subprocess, note that we need stdin=PIPE
p2 = subprocess.Popen(['wc', '-m'], stdin=subprocess.PIPE, stdout=subprocess.PIPE)

# We run the second subprocess feeding it with the first subprocess' output.
# We decode the output to convert to a string
# We still have a 'n', so we strip that out
output = p2.communicate(input=p1_out)[0].decode().strip()

This is somewhat different than the response here, where you pipe two processes directly without adding data directly in Python.

Hope that helps someone out.

Answered By: eaydin

Since subprocess 3.5, there is the function, which provides a convenient way to initialize and interact with Popen() objects. run() takes an optional input argument, through which you can pass things to stdin (like you would using Popen.communicate(), but all in one go).

Adapting jro‘s example to use run() would look like:

import subprocess
p =['myapp'], input='data_to_write', capture_output=True, text=True)

After execution, p will be a CompletedProcess object. By setting capture_output to True, we make available a p.stdout attribute which gives us access to the output, if we care about it. text=True tells it to work with regular strings rather than bytes. If you want, you might also add the argument check=True to make it throw an error if the exit status (accessible regardless via p.returncode) isn’t 0.

This is the "modern"/quick and easy way to do to this.

Answered By: L0tad

One can write data to the subprocess object on-the-fly, instead of collecting all the input in a string beforehand to pass through the communicate() method.

This example sends a list of animals names to the Unix utility sort, and sends the output to standard output.

import sys, subprocess
p = subprocess.Popen('sort', stdin=subprocess.PIPE, stdout=sys.stdout)
for v in ('dog','cat','mouse','cow','mule','chicken','bear','robin'):
    p.stdin.write( v.encode() + b'n' )

Note that writing to the process is done via p.stdin.write(v.encode()). I tried using
print(v.encode(), file=p.stdin), but that failed with the message TypeError: a bytes-like object is required, not 'str'. I haven’t figured out how to get print() to work with this.

Answered By: Jon Rifkin
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