Inputs not working using subprocess with Python

Question

I am trying to run AVL (Athena Vortex Lattice) with Python. It's a software that runs on a separate terminal window, exactly like XFOIL. The user writes commands and presses enter to execute.

I am using the subprocess module to run the software, and the terminal window opens as expected. However, I can't get my Python program to write commands into the opened window. I don't get any error, but nothing is written to AVL, and it stays in its initial state.

The command I am trying to write to AVL is LOAD followed by an absolute path to an input file.

This is the first time I'm using the subprocess module, so I may just have forgotten an essential option somewhere. I'm running the program with Python 3.9 on macOS.

What could be wrong?

import os.path
import subprocess as sp

AVL_FOLDER_PATH = '~/Desktop/AVL_VLM'
AVL_EXE_NAME = 'avl335'
avl_path = os.path.join(AVL_FOLDER_PATH, AVL_EXE_NAME)
avl_open_cmd = 'open ' + avl_path

avl_ps = sp.Popen([avl_open_cmd], stdin=sp.PIPE, stdout=None, stderr=None, shell=True)

def avl_command(cmd):
    cmd += '\n'
    cmd = cmd.encode('ascii')
    avl_ps.stdin.write(cmd)

avl_command('LOAD')
avl_command('/Users/vianneydubois/Desktop/AVL_VLM/test_gen.avl')

shell=True is probably wrong here, but then your command needs to be ['open', avl_path]. Perhaps see also Actual meaning of shell=True in subprocess... and probably get Python to expand the tilde in the path for you. — tripleee
– tripleee, Commented Jun 1, 2021 at 17:48
From what I can tell from the AVL docs, the LOAD command and the filename should be on the same line, separated by a space. You're putting them on different lines entirely. — jasonharper
– jasonharper, Commented Jun 1, 2021 at 17:53
The LOAD command and filename can be on different lines, it works when I do it manually and I have already seen automated scripts using a command per line, so this is not where the problem is I think... — vianndub
– vianndub, Commented Jun 1, 2021 at 17:55
Why do you need the open at all here? If avl335 is actually the name of your executable, then just run that. open is a MacOS thing that tries to find the executable associated with a given data file. — Tim Roberts
– Tim Roberts, Commented Jun 1, 2021 at 18:36
plus you can't have spaces in your command line. Open and your path need to be part of the list. — eatmeimadanish
– eatmeimadanish, Commented Jun 1, 2021 at 18:41

Cyril Jouve · Accepted Answer · 2021-06-01 19:10:16Z

2

input/output is buffered by default (https://docs.python.org/3/library/subprocess.html#subprocess.Popen)

you can set bufsize=0 in Popen:

0 means unbuffered (read and write are one system call and can return short)

(https://docs.python.org/3/library/subprocess.html#subprocess.Popen)

as an alternative, you could flush the buffer manually after write:

avl_ps.stdin.write(cmd)
avl_ps.stdin.flush()

answered Jun 1, 2021 at 19:10

Cyril Jouve

1,0607 silver badges15 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

vianndub Over a year ago

I added a .flush()after my command writing, and it is almost working... The new problem is, when I add more commands, the program stops during the execution, at a random position. What I mean is that if I run the Python program multiple times, the AVL execution is stopped before it is finished, an it is always at a different time, sometimes it is stopped right at the beginning, sometimes it is during the computation, after a few commands have been successfully written

eatmeimadanish · Accepted Answer · 2021-06-01 21:23:52Z

0

Is this really what you are intending?

import os.path
import subprocess as sp

AVL_FOLDER_PATH = '~/Desktop/AVL_VLM'
AVL_EXE_NAME = 'avl335'
avl_path = os.path.join(AVL_FOLDER_PATH, AVL_EXE_NAME)


def avl_command(cmd):
    return sp.run(cmd, capture_output=True, shell=True)

ret = avl_command(['open ', avl_path])
ret = avl_command(['LOAD','/Users/vianneydubois/Desktop/AVL_VLM/test_gen.avl'])

or do you mean:

ret = avl_command('open' + avl_path + '; LOAD /Users/vianneydubois/Desktop/AVL_VLM/test_gen.avl')
print(ret.stdout.decode())

edited Jun 1, 2021 at 21:23

answered Jun 1, 2021 at 18:50

eatmeimadanish

3,9231 gold badge16 silver badges21 bronze badges

4 Comments

vianndub Over a year ago

I tried your code, removed the open command as advised above by Tim Roberts, but the result is the same, AVL runs in terminal, but the LOAD and following commands are not written in AVL

eatmeimadanish Over a year ago

What is the LOAD command doing, does that take inputs? Or is this all part of the initial open command?

vianndub Over a year ago

The LOAD command asks the user to write an input file name. You can either write LOAD myfile.avl or separately LOAD then myfile.avl. Then the program reads the input file.

eatmeimadanish Over a year ago

So my first option is what you want. You are wanting this in the same session?

Collectives™ on Stack Overflow

Inputs not working using subprocess with Python

2 Answers 2

1 Comment

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related