Send input to python subprocess without waiting for result

Question

I'm trying to write some basic tests for a piece of code that normally accepts input endlessly through stdin until given a specific exit command.

I want to check if the program crashes on being given some input string (after some amount of time to account for processing), but can't seem to figure out how to send data and not be stuck waiting for output which I don't care about.

My current code looks like this (using cat as an example of the program):

myproc = subprocess.Popen(['cat'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)

myproc.communicate(input=inputdata.encode("utf-8"))
time.sleep(0.1)

if myproc.poll() != None:
    print("not running")
else:
    print("still running")

How can I modify this to allow the program to proceed to the polling instead of hanging after the communicate() call?

Answer 1

You are using the wrong tool here with communicate which waits for the end of the program. You should simply feed the standard input of the subprocess:

myproc = subprocess.Popen(['cat'], stdin=subprocess.PIPE, stdout=subprocess.PIPE,
                          stderr=subprocess.PIPE)
myproc.stdin.write(inputdata.encode("utf-8"))

time.sleep(0.1)

if myproc.poll() != None:
    print("not running")
else:
    print("still running")

But beware: you cannot be sure that the output pipes will contain anything before the end of the subprocess...