popen - read from subprocess output python

Question

I am running a subprocess using 'Popen'. I need to block till this subprocess finishes and then read its output.

p = Popen(command, stdin=PIPE, stdout=PIPE, stderr=PIPE, encoding="utf-8")
p.communicate():
output = p.stdout.readline()
print(output)

I get an error that

ValueError: I/O operation on closed file.

How can I read the output after the subprocess finishes, I do not want to use poll() though as the subprocess takes time and I would need to wait for its completion anyway.

Any particular reason you're not using subprocess.run() or the legacy subprocess.check_output()? You should avoid Popen if you can precisely because it's tricky to get right. — tripleee
@jasonharper, p.communicate() returns bound method Popen.communicate of <subprocess.Popen object at 0x105e84e10, not the output of the subprocess code — marc
output, error = p.communicate() should work, that output looks like you are just printing p.communicate (without the parentheses). — cdarke

cdarke cdarke · Accepted Answer · 2018-08-02T18:44:07

This should work:

p = Popen(command, stdin=PIPE, stdout=PIPE, stderr=PIPE, encoding="utf-8")
output, error = p.communicate()

print(output)
if error:
    print('error:', error, file=sys.stderr)

However, subprocess.run() is preferred these days:

p = subprocess.run(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

print("output:", p.stdout)

if proc.stderr:
    print("error:", p.stderr, file=sys.stderr)

popen - read from subprocess output python

2 Answers