Getting output archives while using subprocess module

Question

I'm new in this world of python. Recently I have been asked to make an interface between XFoil (an aerodynamics program) and python. After researching a little bit, I found the subprocess module. As the documentation says it's used to "Spawn new processes, connect to their input/output/error pipes, and obtain their return codes."

The problem is that I need some output archives that XFoil creates while its running. If I close the program, the archives are accesible, but if I try to open or read them while the subprocess is still opened it gives me the following error (Although I can see the archive in the folder):

OSError: save not found.

Here the code:

import subprocess
import numpy as np 
import os

process = subprocess.Popen(['<xfoil_path>'], stdin=subprocess.PIPE, universal_newlines=True, creationflags = subprocess.CREATE_NEW_PROCESS_GROUP)
airfoil_path = '<path to airfoil>'
process.stdin.write(f'\nload\n{airfoil_path}')
process.stdin.write('\n\n\noper\nalfa\n2\ncpwr\nsave\n')
process.stdin.tell()
print(os.listdir())
c = np.loadtxt('save', skiprows=1)
print(c)
process.stdin.write('\n\n\noper\nalfa\n3\ncpwr\nsave2\n')

stdin.tell is used to get this output archives, but they are not accesible.

Someone knows why this could be happening?

score 0 · Accepted Answer · answered Dec 21 '21 at 09:45

Why do you imagine process.stdin.tell() should "get this output archives"? It retrieves the file pointer's position.

I'm imagining that the actual problem here is that the subprocess doesn't write the files immediately. Maybe just time.sleep(1) before you try to open them, or figure out a way for it to tell you when it's done writing (some OSes let you tell whether another process has a file open for writing, but I have no idea whether this is possible on Windows, let alone reliable).

Sleeping for an arbitrary time is obviously not very robust; you can't predict how long it takes for the subprocess to write out the files. But if that solves your immediate problem, at least you have some idea of what caused it and maybe where to look next.

As an aside, maybe look into the input= keyword parameter for subprocess.run(). If having the subprocess run in parallel is not crucial, that might be more pleasant as well as more robust.

(Converted into an answer from a comment thread.)

Getting output archives while using subprocess module

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